PYROMETRY 

THE  PAPERS  AND  DISCUSSION 

OF  A 

SYMPOSIUM  ON  PYROMETRY 

HELD  BY  THE] 

AMERICAN  INSTITUTE  OF 

«\ 

Mining  and  Metallurgical  Engineers 

AT  ITS  CHICAGO  MEETING,  SEPTEMBER,  1919, 

IN   COOPERATION   WITH    THE 

NATIONAL  RESEARCH  COUNCIL 

AND    THE 

NATIONAL  BUREAU  OF  STANDARDS 


NEW  YORK  CITY 
PUBLISHED    BY    THE    INSTITUTE 

AT   THE    OFFICE    OF   THE    SECRETARY 
1920 


COPYRIGHT,  1920,  BY  THE 
AMERICAN  INSTITUTE  OF  MINING  AND  METALLURGICAL  ENGINEERS 

(INCORPORATED) 


T  j  i  K    .\i  A  i  •  i .  i ;    i  •  i<  t-:  *  s    YORK:    i  •  A 


PREFACE 


At  the  Chicago  Meeting  of  the  Institute,  September,  1919,  there 
was  held  a  Symposium  on  Pyrometry  which  brought  out  a  collection  of 
papers  and  the  correlative  discussion  which  marked  an  epoch  in  this 
branch  of  metallurgical  technique.  To  preserve  to  the  profession  at 
large  as  well  as  to  our  own  members  this  unique  literature  on  this  subject, 
which  heretofore  has  been  distinguished  by  its  paucity,  is  the  object 
of  this  volume. 

Both  the  National  Research  Council  and  the  National  Bureau  of 
Standards  cooperated  with  the  Institute  in  this  symposium.  The 
Pyrometer  Committee  of  the  Council  was  formed  for  the  specific  purpose 
of  executing  certain  experimental  operations  which  are  very  properly 
described  in  the  report  of  the  Pyrometer  Committee.  Before  the  founda- 
tion of  the  Pyrometer  Committee  the  National  Bureau  of  Standards 
contemplated  organizing  a  symposium  on  this  subject  and  went  so  far 
as  to  complete  a  working  organization  to  that  end.  Members  of  the 
Bureau  of  Standards  contributed  one-third  of  the  total  number  of  papers 
in  this  volume. 

So  the  cooperation  between  the  Institute  and  these  two  national 
bodies  has  been  harmonious  and  complete  and  has  resulted  in  covering 
the  subject  with  a  degree  of  fullness  and  finality  seldom  achieved  in 
technical  publication. 

These  papers  have  all  been  published  in  the  monthly  Bulletin,  but 
will  not  be  included  in  the  Transactions  of  the  Institute. 


417684 


111 


CONTENTS 


PAGE 
Report  of  Pyrometer  Committee  of  National  Research  Council.  By  GEORGE 

K.  BURGESS  (With  Discussion) 3 

Temperature.  By  J.  S.  AMES 37 

Standard  Scale  of  Temperature.  By  C.  W.  WAIDNER,  E.  F.  MUELLER  and  PAUL 

D.  FOOTS  (With  Discussion) 46 

Metals  for  Pyrometer  Standardization.  By  C.  W.  WAIDNER  and  GEO.  K. 

BURGESS 61 

Fundamental  Laws  of  Pyrometry.  By  C.  E.  MENDENHALL 63 

Present  Status  of  Radiation  Constants.  By  W.  W.  COBLENTZ 72 

Thermoelectric  Pyrometry.  By  PAUL  D.  FOOTE,  T.  R.  HARRISON  and  C.  O. 

FAIRCHILD  (With  Discussion) 74 

Potentiometers  for  Thermoelement  Work.  By  WALTER  P.  WHITE  (With  Dis- 
cussion)    137 

Self-checking  Galvanometer  Pyrometer.  By  H.  F.  PORTER  (With  Discussion) .  149 
Some  Factors  Affecting  Usefulness  of  Base-metal  Thermocouples.  By  O.  L. 

KOWALKE  (With  Discussion) 154 

Tables  and  Curves  for  Use  in  Measuring  Temperatures  with  Thermocouples. 

By  L.  H.  ADAMS 165 

Reference  Standard  for  Base-metal  Thermocouples.  By  N.  E.  BONN 179 

Alloys  Suitable  for  Thermocouples  and  Base-metal  Thermoelectric  Practice. 

By  J.  M.  LOHR 181 

Recent  Improvements  in  Pyrometry.  By  R.  P.  BROWN  (With  Discussion) 188 

Automatic  Compensation  for  Cold-junction  Temperatures  of  Thermocouple 

Pyrometers.  By  F.  WUNSCH  (With  Discussion) 206 

Use  of  Modified  Rosenhain  Furnace  for  Thermal  Analysis.  By  H.  SCOTT  and 

J.  R.  FREEMAN,  JR 214 

A  Hot-wire  Anemometer  with  Thermocouple.  By  T.  S.  TAYLOR 221 

High-temperature  Thermometers.  By  R.  M.  WILHELM  (With  Discussion) 225 

Porcelain  for  Pyrometric  Purposes.  By  F.  H.  RIDDLE 240 

Pyrometer  Porcelains  and  Refractories.  By  R.  W.  NEWCOMB  (With  Discussion)  251 

Pyrometer  Protection  Tubes.  By  F.  A.  HARVEY  (With  Discussion) 255 

Protecting  Tubes  for  Thermocouples.  By  R.  B.  LINCOLN 258 

Pyrometer  Protection  Tubes.  By  OTIS  HUTCHINS 262 

Melting  Point  of  Refractory  Materials.  By  LEO  I.  DANA  (With  Discussion) .  .  .  267 
High-temperature  Scale  and  its  Application  in  Measurement  of  True,  Bright- 
ness and  Color  Temperature.  By  EDWARD  P.  HYDE 285 

Theory  and  Accuracy  in  Optical  Pyrometry  with  Particular  Reference  to  the 

Disappearing-filament  Type.  By  W.  E.  FORSYTHE  (With  Discussion) ....  291 
Optical  and  Radiation  Pyrometry.  By  PAUL  D.  FOOTE  and  C.  O.  FAIRCHILD 

(With  Discussion) . 324 

Industrial  Applications  of  Disappearing-filament  Optical  Pyrometer.  By  F.  E. 

BASH 352 

Emissive  Powers  and  Temperatures  of  Non-black  Bodies.  By  A.  G.  WORTHING  367 

Recording  Thermocouple  Pyrometers.  By  LEO  BEHR  (With  Discussion) 400 

V 


VI  CONTENTS 

Recording  Pyrometry.  By  C.  O.  FAIRCHILD  and  PAUL  D.  FOOTS  (With  Dis- 
cussion)    406 

High-temperature  Control.  By  C.  O.  FAIRCHILD  and  PAUL  D.  FOOTE  (With 

Discussion) 435 

Resistance  Thermometry.  By  F.  W.  ROBINSON 450 

Resistance  Thermometry  for  Industrial  Use.  By  CHARLES  P.  FREY  (With 

Discussion) • 458 

Tin,  an  Ideal  Pyrometric  Material.  By  E.  F.  NORTHRUP  (With  Discussion)...  464 
Thermocouple  Installation  in  Annealing  Kilns  for  Optical  Glass.  By  E.  D. 

WILLIAMSON  and  H.  S.  ROBERTS 466 

Annealing  of  Glass.  By  A.  Q.  TOOL  and  J.  VALASEK  (With  Discussion)  475 

Pyrometry  Applied  to  Bottle-glass  Manufacture.  By  R.  L.  FRINK 483 

Pyrometry  in  the  Manufacture  of  Optical  Glass.  By  ALBERT  J.  WALCOTT.  .  .  .  491 
Use  of  Optical  Pyrometers  for  Control  of  Optical-glass  Furnaces.  By  C.  N. 

FENNER  (With  Discussion) 495 

Pyrometry  as  Applied  to  the  Manufacture  of  Optical  Glass.  By  CARL  W. 

KEUFFEL 506 

Pyrometer  Shortcomings  in  Glass-house  Practice.  By  W.  M.  CLARK  and 

CHARLES  D.  SPENCER 509 

Pyrometry  in  the  Manufacture  of  Clay  Wares.  By  F.  K.  PENCE 513 

Application  of  Pyrometry  to  the  Ceramic  Industries.  By  C.  B.  THWING  (With 

Discussion) 516 

Pyrometry  in  Rotary  Portland  Cement  Kilns.  By  LEO  I.  DANA  and  C.  O. 

FAIRCHILD 522 

Application  of  Pyrometers  to  the  Ceramic  Industry.  By  JOHN  P.  GOHEEN.  .  535 
Pyrometry  in  Blast-furnace  Work.  By  P.  H.  ROYSTER  and  T.  L.  JOSEPH 

(With  Discussion) 544 

Pyrometry  and  Steel  Manufacture.  By  A.  H.  MILLER  (With  Discussion) 567 

Electric,  Open-hearth  and  Bessemer  Steel  Temperatures.  By  F.  E.  BASH 

(With  Discussion) 1 578 

Some  Thermal  Relations  in  the  Treatment  of  Steel.  By  C.  F.  BRUSH 590 

Pyrometry  in  the  Tool-manufacturing  Industry.  By  J.  V.  EMMONS 610 

Forging  Temperatures  and  Rate  of  Heating  and  Cooling  of  Large  Ingots.  By 

F.  E.  BASH  (With  Discussion) .  614 

Temperatures  of  Incandescent-lamp  Filaments.  By  BENJ.  E.  SHACKELFORD....  627 

Temperature  Measurements  of  Incandescent  Gas  Mantles.  By  H.  E.  IVES 632 

Application  of  Pyrometry  to  Problems  of  Lamp  Design  and  Performance.  By 

I.  H.  VAN  HORN  (With  Discussion) 638 

Temperature  of  a  Burning  Cigar.  By  T.  S.  SLIGH,  JR.  and  H.  R.  KRAYBILL 

(With  Discussion) 645 

Application  of  Pyrometry  to  the  Manufacture  of  Gas-mask  Carbon.  By  K. 

MARSH  (With  Discussion) 652 

Teaching  Pyrometry  in  our  Technical  Schools.  By  GEORGE  V.  WENDELL 669 

Teaching  Pyrometry  in  Technical  Schools.  By  C.  E.  MENDENHALL 678 

Teaching  Pyrometry.  By  O.  L.  KOWALKE 681 


PAPEKS 


Report  of  Pyrometer  Committee  of  National  Research  Council 

BY   GEORGE    K.   BURGESS,*   WASHINGTON,    D.    C. 
(Chicago  Meeting,  September,  1919) 

THE  Pyrometer  Committee  was  formed  Sept.  20,  1918,  at  the  sugges- 
tion of  Dr.  H.  M.  Howe,  Chairman  of  the  Engineering  Division  of  the 
Research  Council,  for  the  purpose  of  developing  a  pyrometric  method 
suitable  for  open-hearth  steel  practice  so  that  the  effects  of  temperature 
in  the  various  stages  of  the  processes  of  steel  making  might  be  correlated 
quantitatively  with  the  other  factors  influencing  the  production  of  sound 
steel.  As  finally  constituted  the  Committee  consists  of: 

Dr.  George  K.  Burgess,  Chief  Division  of  Metallurgy,  Bureau  of 
Standards,  Chairman. 

Dr.  Paul  D.  Foote,  Chief  of  Pyrometry  Section,  Bureau  of  Standards, 
Secretary. 

Dr.  H.  M.  Howe,  Chairman  Engineering  Division,  National  Research 
Council,  ex  officio. 

Prof.  G.  H.  Clevenger,  Chairman  Metallurgical  Section,  National 
Research  Council,  ex  officio. 

Mr.  F.  E.  Bash,  Leeds  &  Northrup  Co.,  Philadelphia,  Pa. 

Mr.  R.  P.  Brown,  The  Brown  Instrument  Co.,  Philadelphia,  Pa. 

Prof.  G.  H.  Brown,  Rutgers  College,  New  Brunswick,  N.  J. 

Mr.  R.  C.  Drinker,  metallurgical  engineer,  Boston,  Mass. 

Dr.  W.  E.  Forsythe,  Nela  Research  Laboratory,  Cleveland,  Ohio. 

Mr.  J.  T.  Hall,  Taylor-Wharton  Iron  &  Steel  Co.,  High  Bridge, 
N.  J. 

Mr.  J.  S.  McDowell,  Harbison- Walker  Refractories  Co.,  Pittsburgh, 
Pa; 

Mr.  Malcolm  McNaughton,  Joseph  Dixon  Crucible  Co.,  Jersey  City, 
N.  J. 

Mr.  A.  H.  Miller,  Midvale  Steel  Co.,  Philadelphia,  Pa. 

Mr.  F.  E.  Walduck  (since  deceased),  The  Norton  Co.,  Worcester, 
Mass. 

The  Committee  has  held  frequent  meetings  and  has  carried  out  an 
extensive  program  of  experimental  work  by  means  of  sub-committees; 
in  the  execution  of  which  the  Committee  has  been  aided  greatly  not  only 

*  Chairman  of  the  Committee. 
3 


REPORT    OF   PYROMETER    COMMITTEE 


by  the  manufacturing  companies  of  instruments,  refractories,  and  steel 
with  which  the  members  are  severally  associated,  but  also  by  help  and 
offers  of  facilities  from  several  other  companies. 

In  addition  to  the  experimental  work  carried  out  by  the  Committee, 
it  was  decided  to  hold  a  symposium  on  pyrometry,  and  the  Secretary, 
Dr.  Foote,  assisted  by  Dr.  Forsythe,  has  had  signal  success  in  collecting 
an  admirable  series  of  monographs  covering  both  the  theoretical  aspects 
of  the  subject  and  the  applications  of  pyrometry  to  many  industrial 
operations.  The  American  Institute  of  Mining  and  Metallurgical 
Engineers  has  accepted  the  invitation  to  publish  these  papers  and  provide 
a  forum  for  their  discussion. 

The  report  of  the  Committee  would  not  be  complete  without  this  list 
of  titles : 

Temperature,  by  J.  S.  AMES. 

Standard  Scale  of  Temperature,  by  C.  W.  WATDNEB,  E.  F.  MUELLER  and  PAUL  D. 

FOOTE. 

Metals  for  Pyrometer  Standardization,  by  C.  W.  WAIDNEB  and  GEO.  K.  BURGESS. 
Fundamental  Laws  of  Pyrometry,  by  C.  E.  MENDENHALL. 

Thermoelectric  Pyrometry,  by  PAUL  D.  FOOTE,  T.  R.  HARRISON  and  C.  O.  FAIRCHILD. 
Potentiometers  for  Thermoelement  Work,  by  WALTER  P.  WHITE. 
Self-checking  Galvanometer  Pyrometer,  by  H.  F.  PORTER. 

Some  Factors  Affecting  Usefulness  of  Base-metal  Thermocouples,  by  0.  L.  KOWALKE. 
Tables  and  Curves  for  Use  in  Measuring  Temperatures  with  Therm ocou pels, 

by  L.  H.  ADAMS. 

Reference  Standard  for  Base-metal  Thermocouples,  by  N.  E.  BONN. 
Alloys  Suitable  for  Thermocouples  and  Base-metal  Thermoelectric  Practice,  by  J.  M. 

LOHR. 

Recent  Improvements  in  Pyrometry,  by  R.  P.  BROWN. 

Automatic  Compensation  for  Cold-junction  Temperatures  of  Thermocouple  Pyro- 
meters, by  F.  WUNSCH. 

A  Hot-wire  Anemometer  with  Thermocouple,  by  T.  S.  TAYLOR. 

Porcelain  for  Pyrometric  Purposes,  by  F.  H.  RIDDLE. 

Pyrometer  Porcelains  and  Refractories,  by  R.  W.  NEWCOMB. 

Pyrometer  Protection  Tubes,  by  F.  A.   HARVEY. 

Protecting  Tubes  for  Thermocouples,  by  R.  B.  LINCOLN. 

Melting  Point  of  Refractory  Materials,  by  LEO  I.  DANA. 

High  Temperature  Scale  and  its  Application  in  Measurement  of  True,  Brightness 
and  Color  Temperature,  by  EDWARD  P.  HYDE. 

Theory  and  Accuracy  in  Optical  Pyrometry  with  Particular  Reference  to  the  Disap- 
pearing-filament  Type,  by  W.  E.  FORSYTHE. 

Optical  and  Radiation  Pyrometry,  by  PAUL  D.  FOOTE  and  C.  O.  FAIRCHILD. 

Industrial  Applications  of  Disappearing-filament  Optical  Pyrometer,  by  F.  E.  BASH. 

Use  of  Optical  Pyrometers  for  Control  of  Optical-glass  Furnaces,  by  C.  N.  FENNER. 

Emissive  Powers  and  Temperatures  of  Non-black  Bodies,  by  A.  G.  WORTHING. 

Recording  Thermocouple  Pyrometers,  by  LEO  BEHR. 

Recording  Pyrometry,  by  C.  O.  FAIRCHILD  and  PAUL  D.  FOOTE. 

High-temperature  Control,  by  C.  O.  FAIRCHILD  and  PAUL  D.  FOOTE. 

Resistance  Thermometry,  by  F.  W.  ROBINSON. 

Tin,  an  Ideal  Pyrometric  Material,  by  E.  F.  NORTHRUP. 


GEORGE    K.    BURGESS  5 

Resistance  Thermometry  for  Industrial  Use,  by  CHARLES  P.  FREY. 

Thermocouple  Installation  in  Annealing  Kilns  for  Optical  Glass,  by  E.  D.  WILLIAM- 
SON and  H.  S.  ROBERTS. 

Annealing  of  Glass,  by  A.  Q.  TOOL  and  J.  VALASEK. 

Pyrometry  Applied  to  Bottle-glass  Manufacture,  by  R.  L.  FRINK. 

Pyrometry  in  the  Manufacture  of  Optical  Glass,  by  ALBERT  J.  WALCOTT. 

Pyrometry  as  Applied  to  the  Manufacture  of  Optical  Glass,  by  CARL  W.  KETJFFEL. 

Pyrometer  Shortcomings  in  Glass-house  Practice,  by  W.  M.  CLARK  and  CHARLES  D. 
SPENCER. 

Some  Thermal 'Relations  in  the  Treatment  of  Steel,  by  C.  F.  BRUSH. 

Forging  Temperatures  and  Rate  of  Heating  and  Cooling  of  Large  Ingots,  by  F.  E. 
BASH. 

Pyrometry  and  Steel  Manufacture,  by  A.  H.   MILLER. 

Pyrometry  in  Blast-furnace  Work,  by  P.  H.  ROYSTER  and  T.  L.  JOSEPH. 

Electric,  Open-hearth  and  Bessemer  Steel  Temperatures,  by  F.  E.   BASH. 

Pyrometry  in  the  Tool-manufacturing  Industry,  by  J.  V.  EMMONS. 

Pyrometry  in  the  Manufacture  of  Clay  Wares,  by  F.  K.  PENCE. 

Application  of  Pyrometry  to  the  Manufacture  of  Gas-mask  Carbon,  by  K.  MARSH. 

Application  of  Pyrometry  to  the  Ceramic  Industries,  by  C.  B.  THWING. 

Pyrometry  in  Rotary  Portland  Cement  Kilns,  by  LEO  I.  DANA  and  C.  O.  FAIRCHILD. 

Pyrometry  in  the  Ceramic  Industry,  by  JOHN  P.  Go  KEEN. 

Temperatures  of  Incandescent-lamp  Filaments,  by  BENJ.  E.  SHACKELFORD. 

Temperature  Measurements  of  Incandescent  Gas  Mantles,  by  H.  E.  IVES. 

Application  of  Pyrometry  to  Problems  of  Lamp  Design  and  Performance,  by  I.  H. 
VAN  HORN. 

Use  of  Modified  Rosenhain  Furnace  for  Thermal  Analysis,  by  H.  SCOTT  and  J.  R 
FREEMAN,  JR. 

High-temperature  Thermometers,  by  R.  M.  WILHELM. 

Temperature  of  a  Burning  Cigar,  by  T.  S.  SLIGH  and  H.  R.  KRAYBILL. 

Teaching  Pyrometry  in  our  Technical  Schools,  by  GEORGE  V.  WENDELL. 

Teaching  Pyrometry  in  Technical  Schools,  by  C.  E.  MENDENHALL. 

Teaching  Pyrometry,  by  O.  L.  KOWALKE. 

Present  Status  of  Radiation  Constants,  by  W.  W.  COBLENTZ. 

Pyrometer  Protection  Tubes,  by  OTIS  HUTCHINS. 

Two  previous  symposia  on  pyrometry  may  be  noted,  that  held  by  the 
Iron  and  Steel  Institute  of  Great  Britain  in  1904,  and  the  recent  much 
more  pretentious  one,  giving  an  excellent  picture  of  the  present  industrial 
situation  of  pyrometry  in  England  (although  there  were  also  several 
American  contributions),  held  under  the  auspices  of  the  Faraday  Society 
on  Nov.  7, 1917. ! 

It  may  be  of  interest,  in  order  to  put  the  experimental  work  of  the 
Committee  in  its  proper  perspective,  to  mention  briefly  the  papers  relating 
to  open-hearth  furnace  practice  and  conclusions  reached  by  the  sympo- 
sium of  the  Faraday  Society.  These  papers  were:  1.  Determination  of 
the  Temperature  of  Liquid  Steel  Under  Industrial  Conditions,  by  Mr. 
Cosmo  Johns.  2.  Notes  on  Pyrometry  from  the  Standpoint  of  Ferrous 
Metallurgy,  by  Dr.  W.  H.  Hatfield.  3.  Applications  of  Optical  Pyro- 


1  Trans.  Faraday  Soc.  (1918)  13,  pt.  3. 


6  REPORT    OF   PYROMETER   COMMITTEE 

metry  in  Steel  Works  Practice,  by  Mr.  J.  Neill  Greenwood.  4.  Tempera- 
ture Determinations  of  Liquid  Steel,  by  Dr.  A.  McCance. 

In  the  main,  the  authors  of  these  papers  confirmed  the  results  and 
sustained  the  conclusions  of  the  investigations  published  by  the  Chairman 
of  this  Committee  in  May,  1917.2  No  new  methods  were  experimented 
with  by  these  authors  nor  new  principles  suggested,  although  there  were 
developed  several  important  details  of  technique,  particularly  by  Messrs. 
Johns  and  Greenwood. 

From  all  these  researches  it  appears  fair  to  conclude  that  the  problem 
of  measurement  of  the  temperature  of  metal  streams  or  running  clean 
surfaces  of  liquid  steel,  has  been  adequately  solved.  It  is  also  of  interest 
to  note  that  all  the  above  observers  agree  that  the  most  suitable  type  of 
pyrometer  for  this  purpose  is  the  optical,  or  to  quote  Mr.  Johns : 

"The  most  suitable  instrument  is  an  optical  pyrometer  using  mono- 
chromatic light  X  =  0.65ju,  which  it  is  suggested  should  be  adopted  as  a 
standard.  It  should  have  a  scale  that  can  be  read,  under  industrial  condi- 
tions, to  2°  C.  The  observer  should  be  able  to  read  to  ±5°  C." 

The  situation  as  regards  the  exploration  of  the  temperature  distri- 
bution within  the  open-hearth  furnace  and  metal  bath  is,  however,  far 
from  being  satisfactory,  and  the  Pyrometer  Committee  decided,  at  its 
first  meeting,  to  concentrate  its  endeavors  mainly  on  this,  the  most 
important  and  most  difficult  phase  of  the  problem;  for  it  is  in  the  fur- 
nace that  the  steel  is  made  and  the  reactions  are  all  functions  of  the 
temperature. 

Previous  investigations  had  shown  some  of  the  difficulties  to  be  over- 
come: mechanical,  metallurgical  and  thermal.  The  pyrometer  itself 
is  the  least  of  these.  The  greatest  is  a  refractory  that  has  the  requisite 
mechanical,  chemical  and  thermal  properties;  it  must  be  robust  enough, 
when  hot  and  cold,  to  withstand  abuse;  it  must  withstand  the  cor- 
rosive action  of  basic  and  acid  slags,  flames,  hot  gases  and  liquid  steel; 
be  non-porous  and  not  give  off  fumes,  smoke,  or  water  vapor  when  heated  ; 
it  must  not  crack  on  sudden  heating  or  cooling  and  must  be  able  to  with- 
stand a  temperature  of  nearly  1700°  C. ;  and  finally  it  must  be  capable  of 
being  manufactured  into  closed-end  tubes  of  convenient  size  with  a  thick- 
ness of  wall  not  too  great,  nor  of  too  low  thermal  conductivity,  to  allow  the 
interior  to  assume  rapidly  the  temperature  of  the  region  into  which  it 
is  thrust.  It  is  no  reflection  on  the  ability  of  the  members  of  the  sub- 
committee charged  with  this  problem  to  say  they  are  still  looking  for  a 
suitable  refractory.  When  such  is  found  the  question  of  the  pyrometer 
to  use  will  answer  itself. 

The  problem  of  determining  open-hearth  furnace  temperatures,  from 

1  George  K.  Burgess:  Temperature  Measurements  in  Bessemer  and  Open-hearth 
Practice,  Tech.  Paper  91,  U.  S.  Bureau  of  Standards.  Also  published  in  condensed 
form  in  Trans.  (1917)  66,  432. 


GEORGE   K.   BURGESS  7 

the  point  of  view  of  measurement,  appears  to  have  two  aspects  simi- 
lar to  other  cases  often  occurring  in  the  application  of  methods  of 
measurement : 

1.  A  primary  method  must  be  devised  which  will  give  temperatures 
directly;  such  a  method  may,  however,  not  be  practicable  for  other  than 
calibration  purposes. 

2.  A  secondary  method  may  be  used  in  practical  temperature  control 
of  the  operations;  such  a  secondary  method  may  be  quite  indirect  in  its 
operation  and  must  be  standardized  by  comparison  with  the  primary. 

It  would,  of  course,  be  desirable  if  the  primary  method  might  also 
be  used  as  the  practical  control  method  in  the  operation  of  the  open- 
hearth  furnace. 

An  illustration  of  a  primary  method  for  open-hearth  temperatures 
would  be  a  suitable  closed-end  tube  thrust  into  the  bath  to  the  desired 
depths,  the  temperature  of  the  inside  end  of  the  tube  being  measured 
by  any  suitable  pyrometer  as  optical  or  thermoelectric.  Practically, 
it  has  not  yet  been  found  possible  to  devise  such  a  satisfactory  outfit  of 
sufficient  permanence  to  remain  intact  in  the  bath  a  sufficient  time  to  make 
this  a  practical  control  method.  As  illustrations  of  possible  secondary 
methods  may  be  mentioned  that  of  removing  a  spoonful  of  metal  and 
estimating  the  temperature  of  the  bath  from  observations,  either  thermo- 
electric or  optical,  in  the  metal  spooned  out;  this  has  been  modified  by 
Mr.  Drinker  and  its  use  will  be  discussed  later.  Another  secondary  method 
would  be  to  insert  a  pyrometer  tube  in  the  furnace  lining  and  determine 
the  relation  of  the  temperature  of  bath  to  that  of  the  end  of  the  tube;  in 
this  case,  however,  due  to  time  lag  of  temperature,  it  would  be  difficult,  if 
not  impossible,  to  realize  a  practical  control. 

One  plan  suggested,  which  it  was  hoped  might  serve  as  both  primary 
and  secondary  method,  was  to  insert  vertically  through  the  roof  of  the 
open  hearth  a  closed-end  refractory  tube  which  could  be  lowered  into 
the  bath,  and  also  capable  of  being  raised  free  from  it  so  as  to  diminish 
deterioration  and  avoid  breakage  while  charging  the  furnace.  No  steel 
maker  appeared  to  want  to  take  the  responsibility  of  breaking  through 
the  roof  in  this  way,  and  our  subsequent  experience  with  refractory  tubes 
showed  this  idea  to  be  somewhat  premature.  Similar  objection  would 
apply  to  the  permanent  installation  of  a  tube  thrust  diagonally  into  the 
furnace  as  illustrated  by  Greenwood.3 

EXPERIMENTS  WITH  THE  DRINKER  METHOD 

The  method  suggested  by  Mr.  R.  C.  Drinker  for  determining  the 
temperature  of  the  metal  bath  in  the  open-hearth  furnace  is  a  modifica- 
tion of  the  "spoon"  method  described  in  Tech.  Paper  91.  This  method, 

3  Loc.  cit. 


8 


KEPOKT    OF    PYROMETER    COMMITTEE 


which  resembles  taking  a  fracture  test,  seemed  to  warrant  the  systematic 
study  which  was  given  it  by  a  sub-committee  consisting  of  Messrs.  Burgess, 
Bash,  R.  P.  Brown,  Drinker  and  Miller. 

In  the  form  used  by  the  Committee,  and  as  constructed  by  the  Brown 
Instrument  Co.,  the  Drinker  molten  metal  pyrometer  is  illustrated  in 
Fig.  1 ;  a  millivoltmeter  connected  to  F  with  copper  leads  of  any  conven- 
ient length  completes  the  outfit.  Theoretically,  the  operation  of  this 
apparatus  is  extremely  simple  and  consists  in  transferring  metal  from  the 
open-hearth  bath  in  a  spoon  and  filling  the  crucible  D,  containing  about 
2  lb.,  to  its  lip,  while  taking  readings  of  time  and  of  the  millivoltmeter 
until  a  maximum  is  reached,  from  which  data  the  temperature  of  the 
metal  in  the  furnace  may  be  estimated.  To  empty  the  metal  from  the 


FIG.  1. — DRINKER  MOLTEN  METAL  PYROMETER.  A,  METAL  SHIELD  FOR  THERMO- 
COUPLE; B,  PLATINUM  THERMOCOUPLE;  C,  THERMOCOUPLE  INSULATOR  OF  ALUNDUM; 

D,    CRUCIBLE   OF   STEEL;   E,    CRUCIBLE    SUPPORT;    F,    THERMOCOUPLE    CONNECTIONS. 

crucible,  it  is  lifted  from  the  support  E  and  treated  as  is  a  fracture  test 
mold ;  the  metal  shield  A  is  removed  with  the  metal  and  has  to  be  replaced ; 
the  crucible  is  then  chilled  in  water  and  reset  on  its  support,  and  the 
apparatus  is  ready  for  second  determination. 

In  practice,  there  are  many  factors  that  conspire  to  render  readings 
uncertain.  Some  of  the  variables  that  it  is  necessary  to  control  or  stand- 
ardize are: 

1.  A  standard-si  ted  spoon  must  be  adopted  of  exactly  the  same  di- 
mensions for  all  tests. 

2.  The  spoon  before  each  test  must  be  cooled  to  atmospheric  tempera- 
ture. 

3.  The  spoon  must  be  full  of  metal  in  each  instance. 


GEORGE    K.   BURGESS  9 

4.  The  man  who  skims  the  spoon  must  do  this  in  one  effort  and  with- 
out skimming  below  an  inch  or  two  of  the  surface. 

5.  The  time  interval  between  the  removal  of  the  metal  from  the  bath 
and  pouring  into  the  crucible  must  be  accurately  measured  and  repro- 
duced.    Pouring  must  occur  at  a  uniform  rate  into  the  crucible. 

6.  The  crucible  must  be  cooled  to  atmospheric  temperature   before 
each  test. 

In  practice  it  seems  almost  impossible  to  have  all  these  points  carried 
out  properly,  although  it  is  possible  that  if  the  instrument  should  be 
generally  adopted  and  the  men  become  familiar  with  its  use,  these  opera- 
tions might  be  performed  correctly,  but  it  is  difficult  to  get  the  necessary 
intelligent  and  adequate  cooperation  of  the  furnace  helpers.  Of  course  it 
is  also  evident  that  the  dimensions  and  relative  positions  of  the  crucible 
parts  and  thermocouple  accessories,  particularly  of  A,  B,  and  C,  must  be 
invariable,  otherwise  a  standard  practice  cannot  be  set  up  and  maintained. 

Tables  1, 2,  and  3  show  three  series  obtained  by  the  sub-committee  using 
the  Drinker  method,  and  it  will  be  noted  that,  although  every  effort  was 
made  to  develop  a  uniform  practice,  there  are  many  serious  discrepancies 
among  the  observations  which  were  also  checked  by  observations  taken 
with  an  optical  pyrometer  sighting  on  the  metal  stream  as  it  was  being 
poured  into  the  crucible.  It  is  evident  from  the  table  that  the  optical 
readings  are  much  more  satisfactory  than  the  others.  Another  variable 
of  unexpected  magnitude,  the  quantitative  effect  of  which  has  not  been 
worked  out,  is  the  variation  in  weight  of  the  metal  filling  the  crucible 
as  dependent  upon  the  state  of  the  bath;  this  is  undoubtedly  associated 
with  the  gas  content  of  the  steel. 

The  temperatures  given  by  the  Drinker  method  in  the  column  marked 
"maximum"  in  the  tables  cannot,  of  course,  be  true  temperatures  of  the 
metal  in  the  furnace;  it  would  be  necessary,  as  previously  stated,  to 
calibrate  any  given  apparatus  and  practice.  The  order  of  correction  to 
apply  to  such  readings  is  shown  by  a  comparison  of  this  column  with  that 
of  the  optical  pyrometer  readings  (which  are  corrected  for  emissivity  using 
e  =  0.40,  see  Table  4)  for  this  pyrometer  sighted  on  the  metal  stream, 
which  also  are  low. 

The  Committee  is  forced  to  the  conclusion  that  the  Drinker  method, 
as  above  described,  which  gave  promise  of  reliability  and  looked  particu- 
larly attractive  in  that  its  manipulation  was  almost  identical  with  an 
operation  familiar  to  the  furnace  man,  is  nevertheless  not  suited  for 
measurements  exact  to  10°  C.  or  20°  C.,  an  accuracy  that  is  required  and, 
moreover,  can  be  obtained  by  use  of  the  optical  pyrometer. 


10 


REPORT    OF    PYROMETER    COMMITTEE 


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REPORT    OF   PYROMETER    COMMITTEE 


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13 


Dr.  Howe,-  at  the  first  meeting  of  the  Committee,  called  attention 
to  the  fact  that  in  the  fireclay  insulator,  with  its  graphite  tip  and  re- 
enforcing  iron  core,  of  the  ladle  stopper  used  for  years  in  steel  teeming, 
we  have  the  possibilities  of  a  protective  sheath  for  a  pyrometer  in  the 
open  hearth.  With  this  idea  as  a  basis,  an  extended  series  of  experiments 
has  been  carried  out,  mainly  by  a  sub-committee  consisting  of  Messrs. 
Bash  and  Miller  assisted  part  of  the  time  by  Messrs.  Burgess  and 
Forsythe. 

The  reports  of  this  sub-committee  follow,  and  for  convenience  in 
interpreting  the  results  obtained  with  the  optical  pyrometer,  there  is 
here  included  the  table  of  corrections  for  emissivity  as  given  in  Tech. 
Paper  91,  to  which  has  been  added  a  corresponding  table  in  Fahrenheit 
degrees.  All  the  optical-pyrometer  readings,  unless  otherwise  indicated, 
have  been  corrected  for  emissivity.  It  will  be  recalled  that  for  liquid 
steel  e  =  0.40  and  the  value  of  e  for  liquid  slag  is  about  0.65,  both  for  a 
pyrometer  using  light  or  wave  length  X  =  0.65/i. 

TABLE  4. — Corrections  to  Add  to  Temperature  Readings  for  Emissivity 
Pyrometer  using  red  light,  wave-length  X  =  0.65^,  at  observed  temperatures. 


Emissivity 

900° 
C. 

1000° 
C. 

1100° 
C. 

1200° 
C. 

1300° 
C. 

1400° 
C. 

1500° 
C. 

1600° 
C. 

1700° 
C. 

1800° 
C. 

2000° 
C. 

0.30 

80 

94 

110 

127 

146 

166 

188 

211 

235 

262 

318 

0.40 

59 

70 

82 

95 

108 

123 

139 

156 

174 

193 

234 

0.50 

44 

53 

62 

71 

81 

92 

104 

116 

129 

143 

173 

0.60 

32 

38 

45 

51 

59 

67 

75 

84 

93 

103 

124 

0.65 
0.70 

27 
22 

32 
26 

37 
31 

43 
36 

49 
41 

56 
46 

63 
52 

70 
58 

78 
64 

86 
71 

104 
86 

0.80 

14 

16 

19 

22 

25 

28 

32 

36 

40 

44 

53 

0.90 

7 

8 

9 

10 

12 

14 

15 

17 

19 

21 

25 

Emissivity 

1600° 
F. 

1800° 
F. 

2000° 
F. 

2200° 
F. 

2400° 
F. 

2600° 
F. 

2800° 
F.» 

3000° 
F. 

3200° 
F. 

3400° 
F. 

3600° 
F. 

0.30 

139 

166 

196 

230 

268 

310 

355 

402 

452 

506 

564 

0.40 

101 

122 

147 

173 

200 

229 

261 

297 

335 

374 

414 

0.50 

76 

92 

110 

130 

151 

173 

195 

220 

247 

276 

306 

0.60 

56 

67 

79 

93 

108 

124 

141 

159 

178 

198 

220 

0.70 

38 

47 

56 

65 

75 

85 

97 

110 

123 

137 

153 

0.80 

24 

29 

34 

40 

47 

54 

61 

68 

76 

85 

94 

0.90 

11 

13 

16 

19 

22 

25 

29 

32 

36 

40 

43 

FIRST  REPORT  OF  SUB-COMMITTEE  ON  OPEN-HEARTH  PYROMETFR 


After  talking  the  matter  over  with  steel  metallurgists  and  open-hearth 
men,  it  was  decided  to  be  impracticable  to  try  to  put  the  pyrometer  tube 


14 


REPORT    OF   PYROMETER    COMMITTEE 


through  the  roof  of  the  open-hearth  furnace  for  the  reasons  that  it  would 
tend  to  weaken  the  roof,  the  temperature  above  the  roof  would  be  too 
high  for  men  to  work  and  observations  to  be  made  and  the  risk  of  the 

roof  falling  in  at  any  time  would  make  the 
work  very  dangerous.  Also,  it  would  mean 
some  sort  of  mirror  in  the  pyrometer  tube  and 
this  could  be  done  away  with  if  the  tube  were 
put  in  from  the  side. 

It  was  decided  not  to  use  wrought-iron 
pipe,  as  it  could  not  be  obtained  in  the  size 
wanted  and,  if  it  got  hot  enough  through  the 
clay  sleeves,  the  tube  would  sag  whether  made 
of  wrought  iron  or  steel.  For  this  reason, 
drawn-steel  pipes  2^  in.  (5.7  cm.)  outside 
diameter  were  used,  each  being  about  11  ft. 
(3.4  m.)  long.  The  graphite  tip  was  made 
as  per  specifications  as  shown  in  Figs.  2  and  3. 
While  the  pyrometer  tube  and  tip  were 
originally  designed  for  5^-in.  (14  cm.)  clay 
sleeves,  it  was  decided  to  try  4^-in.  (11  cm.) 
sleeves  as  well  in  a  preliminary  test  as  they 
would  make  a  considerably  lighter  tube, 
which  is  very  desirable. 

For  a  preliminary  test,  two  pyrometer 

FIG.  2. — REFRACTORY  TUBE  tubes  were  made  up,  one  with  4^-in.  sleeves 
AKING  STEEL  TEMPERA'  and  one  with  5^-in.  sleeves.     To  fasten  the 
graphite  tip,  which  had  a  bayonet  joint,  to 

the  11-ft.  drawn-steel  pipe,  an  iron  sleeve  was  made  which  would  fit  over 
the  pipe  and  which  had  two  lugs  on  it  to  fit  the  graphite  tip.  This  sleeve 
was  welded  to  the  pipe  and  the  tip  fitted  in  place.  The  clay  sleeves 


SECTION  ON  A-B 


FIG.  3. — ARRANGEMENT  OF  TUBE  FOR  TAKING  STEEL  TEMPERATURES.  1.  WROUGHT- 
IRON  TUBE  2%-2%  IN.  OUTSIDE  DIAMETER;  2.  CERAMIC  SLEEVE;  3.  WROUGHT- 
IRON  RING  TO  SUPPORT  SLEEVE;  4.  CLAY  OR  CEMENT;  5.  WROUGHT-IRON  LUGS  1  IN. 

LONG;  6.  GRAPHITE  TUBE;  7.  WELD  TO  w.i.  PIPE. 

were  then  slipped  on,  the  joint  between  the  tip  and  the  bottom  sleeve 
being  filled  with  asbestos  rope  and  clay. 

In  order  to  handle  the  tube  at  the  furnace,  an  iron  sleeve  with  a  ring 


GEORGE   K.   BURGESS  15 

attached  was  slipped  over  the  outside  of  the  tube  to  a  point  about  8  ft. 
from  the  tip.  A  crane  hook  could  be  attached  to  this  ring  and  the  tube 
manipulated.  For  counterbalancing  the  tube,  another  ring  was  at- 
tached at  the  open  end  of  the  tube  from  which  heavy  chain  links  could 
be  hung. 

After  the  two  tubes  were  made  up,  they  were  thoroughly  dried,  and, 
just  before  using,  the  tips  were  heated  in  a  blacksmith's  forge  fire.  It 
was  decided  to  try  the  tube  with  the4^-in.  sleeves  first,  so  it  'was  counter- 
balanced and  put  directly  into  a  75-T.  acid  open-hearth  furnace.  The 
tube  was  put  through  the  door  about  7  ft.  and  the  opening  covered  with 
corrugated  iron  to  protect  the  observers.  Considerable  smoke  was 
forming  in  the  tube,  so  a  j^-in.  iron  pipe  connected  to  an  air  hose  was 
pushed  down  the  pyrometer  tube  and  the  smoke  blown  out;  it  reformed 
quite  rapidly,  however,  so  that  to  make  an  observation  with  the  optical 
pyrometer,  the  air  pipe  was  quickly  withdrawn,  and  a  quick  reading 
made.  This  procedure  was  repeated  a  number  of  times,  but  the  tem- 
perature read  is  questionable. 

After  the  tube  had  been  in  the  furnace  14  min.,  it  was  withdrawn  and 
found  to  be  in  good  condition  and  straight.  It  was  laid  on  the  floor  and 
covered  with  sand  to  keep  it  from  cooling  too  rapidly. 

The  tube  with  the  5^-in.  sleeves  was  not  tried,  as  the  end  joint  was 
not  in  good  shape  and  we  did  not  consider  it  necessary  to  try  it  after  the 
result  with  the  smaller  sleeves,  which  are  not  so  bulky  and  therefore  easier 
to  handle  and  more  satisfactory. 

After  about  2  hr.,  the  tube  which  had  already  had  one  trial,  which 
we  will  designate  as  No.  1,  was  dug  out  of  the  sand  for  another  test. 
This  time  a  Mi-in.  iron  pipe  was  put  inside  the  pyrometer  tube  and  left 
in  to  be  used  in  blowing  out  any  smoke  that  might  form.  To  do  this,  an 
air  hose  was  held  at  intervals  to  the  outside  end  of  the  pipe  and  the  smoke 
cleared  from  the  tube.  This  time  the  tube  was  left  in  the  furnace  15 
min.  and  a  reading  could  be  easily  made  as  the  smoke  was  kept  clear. 
The  tip  appeared  to  come  up  to  temperature  in  from  6  to  8  min.  Di- 
rectly after  the  tube  was  withdrawn  from  the  furnace,  the  four  or  five 
clay  sleeves  on  the  hot  end  split  lengthwise  and  dropped  off,  although 
the  pipe  remained  straight.  The  reason  for  the  splitting  is  probably  as 
follows :  The  clay  sleeves  fitted  very  snugly  on  the  steel  pipe  and  when  the 
tube  was  put  in  the  furnace,  the  sleeves  heated  up  and  expanded  but  the 
heat  did  not  penetrate  to  the  steel  tube.  When  the  tube  was  covered 
with  sand  and  allowed  to  lie  covered  for  a  few  hours,  the  heat  was 
equalized  between  the  clay  sleeves  and  the  steel  pipe.  On  putting  the 
tube  into  the  furnace  the  second  time,  the  steel  tube  became  much  hotter 
than  at  first  and  expanded  enough  to  split  the  sleeves.  The  remedy 
would  be  to  have  a  slightly  larger  internal  diameter  for  the  clay 
sleeves. 


16  REPORT    OF    PYROMETER    COMMITTEE 

The  graphite  tip  of  tube  No.  1  was  removed  and  found  to  be  in 
good  condition  except  for  a  few  small  cracks  near  the  top. 

It  was  thought  possible  that  the  source  of  the  smoke  in  the  tube  was 
due  to  the  asbestos  rope  in  the  joint  next  to  the  tip  so  two  more  tubes  were 
prepared  which  we  will  designate  as  No.  3  and  No.  4.  No.  3  tube  was 
made  up  in  the  same  manner  as  No.  1  except  that  the  joints  were  made 
without  any  asbestos  and  sodium  silicate  mixed  with  clay  was  used.  No. 
4  was  the  same  as  No.  3  but  had  clay  only  at  the  joints. 

No.  3  and  No.  4  tubes  were  made  up  and  dried  in  the  usual  way  and 
then  subjected  to  a  temperature  of  1000°  F.  (538°  C.)  for  1  hr.  to  drive  off 
any  remaining  volatile  matter.  Before  being  put  into  the  furnace,  the 
tubes  were  rested  on  a  stand  in  front  of  the  open-hearth  door  and  the 
tip  pushed  against  the  open  peephole.  This  brought  it  to  a  red  heat 
in  a  few  minutes.  A  ^-in.  (3.2  mm.)  iron  pipe  was  prepared  to  be  used 
in  blowing  out  smoke  from  the  pyrometer  tubes.  This,  being  smaller 
than  the  34~m-  pipe,  did  not  obstruct  the  view  down  the  tube  so  much. 
No.  3  tube  was  put  into  the  furnace  and  left  in  19  min.,  a  number  of 
readings  being  made  during  that  time.  The  tube  sagged  slightly  and 
smoke  collected  as  badly  as  in  No.  1  and  had  to  be  blown  out. 

No.  4  tube  was  put  into  the  furnace  right  after  No.  3  was  withdrawn 
and  left  in  14  min.,  without  sagging.  Smoke  also  formed  in  this  tube, 
but  by  blowing  it  out  at  intervals,  readings  could  be  made  with  ease. 
Slight  explosions  would  occur  in  the  tube  when  the  air  was  blown  in. 
This  seems  to  point  to  a  partial  oxidation  of  the  graphite  tip  due  to  the 
air  in  the  tube.  An  inert  or  reducing  gas  such  as  nitrogen  or  hydrogen 
would  probably  do  away  with  the  smoke.  It  is  probable  that  the  smoke 
all  comes  from  the  graphite  tip.  After  2  hr.,  No.  4  tube  was  put  into  the 
furnace  again  for  12  min.,  and  at  the  end  of  this  time,  the  clay  sleeves 
in  the  furnace  started  cracking  off. 

On  taking  the  No.  3  tube  out  of  the  sand  to  put  it  into  the  furnace,  it 
was  found  that  a  couple  of  the  clay  sleeves  were  cracked  so  that  the  tube 
could  not  be  used.  On  using  a  tube  for  the  second  time,  there  is  not  so 
much  smoke  formed,  the  reason  probably  being  that  the  graphite  on  the 
inside  surface  of  the  tip  is  oxidized  at  first,  leaving  a  layer  of  clay  which 
protects  the  graphite  from  further  oxidation.  If  this  is  the  case,  it 
might  be  well  to  line  the  tip  with  clay  in  the  first  place  or  to  burn  it  out 
by  subjecting  it  to  a  high  temperature. 

A  tabulation  of  the  observations  made  in  the  tubes  and  in  the  furnaces 
and  on  the  taps  of  different  heats  is  here  shown. 


GEORGE    K.    BURGESS 


17 


TEMPERATURES  OF  OPEN-HEARTH  STEEL  WITH  PYROMETER  TUBE  AND 

L.  &  N.  OPTICAL  PYROMETER 
Heat  No.  12/4104— Midvale  Steel  Co.,  11/21/18. 


TIME 

10:14 


10:28 
10:30 

11:11:00 
11:12:20 

13:45 
14:15 
14:50 

15:20 
16:10 
16:45 
17:25 

11:17:18 

No.  7  Furnace. 

12:00 

12:10 

12:20 


TEMP., 
DEGREES  F. 


2740? 
2843? 
2843? 
2842? 


2768? 

2835 

2821 

2805 

2828 

2821 

2805 

2805 

2805 

2813 

2813 

2805 

2805 

2797 

2797 

2805 

2797 

2797 

2807 

2858 

2858 


2658 
2658 
2658 


TAP 


12:25 

Heat  No.  8/5225,  11/22/18. 

11:50 

2606 
2645 
2645 
2678 
2937 
2678 
2678 


11:56 

12:02 
12:04 
12:05 
12:08 
12:09 
12:15 


REMARKS 

No.  1  tube  into  furnace. 

Readings  in  tube,  smoky. 
J  Snap  readings  taken  after  smoke 
\  blown  out. 

Tube  out  of  furnace. 
Finals  added. 

Steel  stream,  smoky. 


Stream  observed  by  sighting  di- 
rectly on  it  with  optical  pyrometei 


2917 


First  slag. 

Slag. 

Slag  finish. 

Charge  melted. 

No.  1  tube  in. 

Reading  in  pyrometer  tube. 

End  of  tube  approximately  8  in. 

under  slag  surface. 

Tube  out,  sleeves  cracked. 

No.  3  tube  in. 
Reading  in  tube. 
Reading  in  tube. 
Reading  in  tube. 
Reading  in  tube. 
Slag  surface,  flame  on. 
Tube  reading. 
Tube  reading. 
Tube  out,  slightly  bent. 
Steel  in  spoon  (e  =  0.40). 


18  REPORT    OF   PYROMETER    COMMITTEE 

TEMP., 
TIME  DEGREES  F.  REMARKS 

2924  Slag  surface  in  furnace. 

'12:20  No.  4  tube  in. 

12:27  2534  Reading  in  tube,  smoky. 

12:30  2672  Reading  in  tube,  smoky. 

12:32  2672  Reading  in  tube,  smoky. 

12:33  2672  Tube  out. 

12:34  2918  Slag  surface. 

2:18  2902  Steel  in  spoon  (e  =  0.40). 

2:19  2942  Slag  surface. 

2910  Steel  in  spoon  (e  =  0.40). 

2:37  No.  4  tube  in. 

2:42  2740  Tube  reading. 

2:44  2740  Tube  reading. 

2 :45  Sleeves  cracked  off  in  furnace. 

TAP 

3:28  2902  Steel  stream  (e  =  0.40). 

2858 
3:30:15  2843 

2872 

2834 
3:31:30  2843 

2851 

2843 

2843 
3:33:15  2833  Slag  stream  (e  =  0.65) 

2900  Slag  stream  finish. 

To  attempt  to  overcome  the  smoke  difficulties  and  to  ascertain  if  the 
inside  of  the  graphite  tip  actually  comes  to  the  temperature  of  the  steel 
bath,  four  more  tubes  were  made  up  as  follows : 

No.  5. — Tip  baked  in  a  crucible  furnace  until  outside  surface  was 
glazed.  Inside  glazed  with  oxyhydrogen  torch. 

No.  6. — Tip  glazed  inside  with  oxyhydrogen  torch. 

No.  7. — Tip  machined  to  %Q  m-  wa^  from  %  in.  Burnt  in  crucible 
furnace  and  glazed  inside  with  oxyhydrogen  torch. 

No.  8. — Tip  burnt  in  crucible  furnace.  Inside  untouched.  Nos.  5 
and  7  tubes  were  made  up  with  sodium  silicate  and  clay  and  6  and  8  with 
plain  clay. 

On  Nov.  27,  1918,  the  tubes  were  tested  with  the  following  results: 

Tube  No.  5  was  prepared  and  put  into  an  acid  open  hearth  after  pre- 
heating the  tip.  Fumes  appeared  and  were  blown  out  and  one  reading 
was  made  before  the  tip  came  up  to  temperature.  After  10  min.  the  tip 
broke  and  the  tube  was  withdrawn. 

Tube  No.  7  with  the  thin  tip  was  then  put  into  the  open  hearth  and 
a  clear  reading  was  made  after  the  tube  had  been  in  4  min.  After  5  min. 
the  tip  broke  and  the  tube  had  to  be  withdrawn. 

It  was  thought  possible  that  the  reason  for  the  failure  of  the  first  two 
tips  was  partly  a  rapid  oxidation  of  the  graphite,  due  to  the  stream  of 


GEORGE    K.   BURGESS 


19 


air  from  the  3^-in.  iron  pipe,  so  a  tank  of  hydrogen  was  secured  and  a  hose 
attached. 

Tube  No.  6  was  laid  before  the  open-hearth  door  with  the  tip  tilted 
up  and  the  hydrogen  run  in  through  the  small  iron  pipe.  The  open  end 
was  then  plugged  with  asbestos  wool  and  a  small  amount  of  hydrogen 
kept  flowing  into  the  tube  while  it  was  being  inserted  into  the  steel 
bath.  Heavy  smoke  and  steam  were  observed  coming  out  of  the  tube 
and  no  readings  could  be  made.  As  it  was  not  considered  safe  to  try  to 
blow  the  fumes  and  gas  out  with  air,  the  tube  was  taken  out  of  the  furnace. 

Tube  No.  8,  the  tip  of  which  had  been  baked  in  a  crucible  furnace  but 
which  had  not  been  glazed  inside  with  the  oxyhydrogen  torch,  was  finally 
warmed  up  and  put  into  the  furnace  and  a  temperature  observation  on 
the  surface  of  the  slag  made  directly  afterward.  When  the  tube  had 
been  in  6  min.,  a  reading  was  made,  after  blowing  out,  with  air,  the  smoke 
which  had  formed.  This  temperature  remained  constant  for  a  number  of 
readings  afterward  showing  that  the  tip  was  up  to  temperature  after 
6  min.  There  was  comparatively  little  smoke  in  this  tip  and  it  was 
easy  to  keep  the  tube  clear  by  blowing  air  in  at  intervals. 

Readings  made  by  two  observers  checked  to  17°  F.  (9°  C.).  After 
the  tube  had  been  in  the  furnace  15  min.,  it  was  withdrawn  and  found 
to  be  in  good  condition.  Directly  after  the  removal  of  the  pyrometer 

TEMPERATURE   OF  OPEN-HEARTH  STEEL  WITH  PYROMETER  TUBE 
AND  L.  &  N.  OPTICAL  PYROMETER 


Heat  No.  7/5186,  Nov.  27,  1918. 


TIME 

12:19 
12:24 

12:25 
12:29 
12:44 
12:48 

2:08 

2:18 
2:23 

2:29 
2:31 
2:32 
2:33 
2:35 
2:37 
2:39 
2:40 


TEMP., 
DEGREES  F. 


2606 


2830 


2747 
2893 


2942 
2697 
2697 
2697 
2697 
2714 
2703 
2948 
2677 


REMARKS 

Tube  No.  5  in. 

In  tube,  air  blowing  during  read- 
ing. 

Under  flames  on  slag  surface. 

Tip  broke. 

Tube  No.  7  in. 

In  tube,  no  smoke. 

On  slag  surface,  no  flame. 

Tube  No.  6  in.,  flowing  hydrogen, 
smoke. 

Tube  out. 

Tube  No.  8  in. 

On  slag  surface. 

In  tube,  smoke  blown  out. 

In  tube. 

In  tube. 

In  tube. 

In  tube. 

In  tube. 

On  slag  surface. 

Stream  from  spoon  (e-0.40). 


20  REPORT    OF   PYROMETER    COMMITTEE 

tube,  a  spoon  of  steel  was  taken  from  the  bath  for  a  sample  and  a  reading 
made  on  the  stream.  The  temperature  read  was  corrected  for  emissivity 
of  0.40  and  found  to  be  somewhat  lower  than  that  read  in  the  tube,  as 
would  be  expected.  The  temperature  observations  made  during  this 
test  are  shown  on  page  19. 

From  the  experience  with  the  graphite  tips  and  the  trouble  with  the 
smoke,  the  sub-committee  decided  that  it  would  be  desirable  to  try  clay 
tips  on  the  pyrometer  tubes,  as  the  clay  appears  to  stand  sudden  immer- 
sion in  the  steel  and  the  corrosive  effect  of  the  slag.  A  design  was  made 
showing  the  thin  portion  of  the  tip  an  inch  longer  and  the  Joseph  Dixon 
Co.  agreed  to  make  up  a  number  of  tips  for  another  trial. 

Conclusions 

Open-hearth  steel  temperatures  can  be  taken  with  a  tube  similar  to 
the  one  used  in  our  experiments,  though  it  is  a  difficult  matter  to  handle 
the  tube  and  protect  the  operators. 

With  a  graphite  tip  some  trouble  is  experienced  with  smoke  which  at 
times  makes  the  readings  questionable.  This  would  possibly  be  elimi- 
nated by  a  clay  tip  if  that  should  prove  satisfactory  in  other  respects. 

SE*COND  REPORT  OF  SUB-COMMITTEE  ON  OPEN-HEARTH  PYROMETER 

Temperature  Measurements  on  Open-hearth  Steel 

Temperature  data  were  taken  of  two  steel  heats  of  the  following  com- 
position, Midvale — Feb.  12,  1919: 

Heat  No.  8—5317  Hear  No.  11—5204 


c  

0.490 

0  038 

C     . 

.    .   0  60 

Ni 

...      3  50 

Mn  

0.650 

Si.. 

0  230 

Mn 

0  26 

Cr 

.     2  25 

P  

0.038 

Ni 

...   3  000     Si  

0  23 

Data 
Furnace  No.  8,  Heat  No.  5317. 

Three  tubes,  with  clay  tips  made  by  Joseph  Dixon  Co.,  were  put  in  the 
furnace;  the  tips  cracked  off  in  each  case,  the  break  occurring  in  two  cases 
at  the  junction  of  the  thin  and  thick  sections  of  the  tip. 

Temperature  Observations 

TEMP.,  » 

DEGREES  F.  REMARKS 

2931  Slag  surface  1:35  P.  M. 

2893  Stream  from  small  spoon  in  furnace. .  About  3  ft.  in  from 

door.  Door  open  about  18  in.  Stream  appeared  much 
darker  than  background  of  flame.  No  correction  for 
emissivity — 2:05  P.  M. 


GEORGE    K.    BURGESS 


21 


TEMP., 
DEGREE  F. 

3015 


2885 
2893 

2714 
2753 

2866 
2830 

3092 
2805 


REMARKS 

Slag  surface  2:09  P.  M. 
Door  opened  at  2:13  to  warm  up  tube. 
Slag  surface  2:15  P.  M. 
Stream  from  small  spoon  in  furnace — Reading  made  on 

second  dip.  No  correction. 
Slag  surface — about  3 :00  p.  M. 
Stream  of  slag  from  large  spoon  in  furnace  uncorrected — 

4th  dip. 

Above  corrected  for  e  =  0.65. 
Stream  of  steel  from  small  spoon  in  furnace — uncorrected. 

Good  reading. 

Above  corrected  for  e  =  0.40. 
Stream  from  small  spoon  into  mold  corrected  fore  =  0.40 — 

Approximately  12  sec.  after  spoon  left  furnace. 


A  number  of  small  spoons  of  metal  were  taken  from  the  furnace  and 
poured  onto  the  floor  at  different  intervals  of  time  between  lifting  the 
spoon  clear  of  the  bath  and  pouring  it.  A  cold  spoon  was  taken  in  each 
case.  The  first  set  was  made  about  3 :30  P.  M.  and  the  second  set  about 
4 :00  P.  M. 


3. 


4. 


FIRST  SET 
DEGREES  F. 

2805       16  sec.  time;  e  =  0.40. 

6  sec.  from  furnace  to 
brick  where  it  was 
skimmed. 

10  sec.  on  brick. 

7^  sec.  to  read. 
2768       23>i  sec.  total  e  =  0.40. 

6  sec.  to  brick. 

15  sec.  on  brick. 

6  sec.  to  read. 
2713       27  sec.  total;  e  =  0.40. 

5  sec.  to  brick. 

7%  sec.  to  read. 
2797       12%  sec.  total. 


SECOND  SET 
DEGREES  F. 

1.  5  sec.  to  brick. 


2. 


3. 


4. 


2790 


2730 


2705 


2821 


sec.  to  read. 

sec.  total. 
5  sec.  to  brick. 
10  sec.  on  brick. 
21  sec.  total. 
5  sec.  to  brick. 
15  sec.  on  brick. 
5  sec.  to  read. 
25  sec.  total. 
5  sec.  to  brick. 
5  sec.  on  brick. 
3  sec.  to  read. 
13  sec.  total. 


Two  tests  were  made  by  spooning  out  a  test  spoon,  clearing  it  of  slag 
and  then  making  readings  at  intervals  on  the  surface  of  the  steel  until  it 
froze  over.  Time  was  counted  from  lifting  the  spoon  from  the  bath. 


TEMP., 
DEGREES  F. 

TIME, 
SECONDS 

TEMP., 
DEGREES  F. 

TIME, 
SECONDS 

TEMP., 
DEGREES  F. 

TIME, 
SECONDS 

2790 

15K 

2813 

13 

2672 

42 

2768 
2705 
2689 

'  26 
33 

44 

2768 
2737 
2721 

20K 
29K 
35 

2638 

51 

2629 

55 

.  ^  ' 

22 


REPORT    OF    PYROMETER    COMMITTEE 


Immediately  after  these  two  tests,  a  reading  on  the  stream  from  a  small 
spoon  in  the  furnace  and  slag  readings  were  made. 


TEMP., 
DEGREES  F. 

2796 
2913 
3053 
2783 
2900 
3040 
2772 
2818 


2761 
2937 
2813 
2843 
2931 
2835 
2824 
2942 

2772 
2887 
3025 
2778 
2894 
3033 
2887 

TEMP., 
DEGREES  F. 

2805 
2782 
2768 
2761 
2775 
2782 
2790 


REMARKS 

Stream  from  small  spoon;  no  correction. 
Stream  from  small  spoon;  slag  correction  (e  =  0.65). 
Stream  from  small  spoon;  steel  correction  (e  =  0.40). 
Stream  from  small  spoon;  no  correction. 
Stream  from  small  spoon;  slag  correction  (e  =  0.65). 
Stream  from  small  spoon;  steel  correction  (e  =  0.40). 
Slag  surface;  cooled  by  open  door. 
Slag  surface;  8  min.  after  door  closed. 

Furnace  No.  11,  Melt  No.  5204.     Chrome-nickel  Steel. 

Stream  from  spoon  into  mold  (e  =  0.40). 

Stream  from  small  spoon  in  furnace,  no  correction. 

Slag  surface  in  furnace;  cooled. 

Slag  surface  5  min.  after  previous  reading. 

Stream  from  small  spoon  in  furnace,  no  correction. 

Stream  from  small  spoon  into  mold  (e  =  -0.40). 

Slag  stream  from  large  spoon  in  furnace;  uncorrected. 

Above  corrected  for  slag  (e  =  0.65).     Approximately    1^   nr- 

the  following  readings  were  taken  5  min.  before  the  tap. 
Stream  from  small  spoon  in  furnace;  uncorrected. 
Above  corrected  for  e  =  0.65. 
Above  corrected  for  e  =  0.40. 

Stream  from  small  spoon  in  furnace.     Second  dip,  uncorrected. 
Above  corrected  for  e  =  0.65. 
Above  corrected  for  e  =  0.40. 
Slag  surface,  door  shut. 

TAP 


later 


TIME  FROM  BEGINNING  TEMP., 

OF  TAP  DEGREES  F. 

30  sec. — Steel  stream.  2775 

39  sec.  2782 

57  sec.  2768 

1  min.  15  sec.  2790 

1  min.  30  sec.  2742 

1  min.  54  sec.  2638 

2  min.    5  sec. 


TIME  FROM  BEGINNING 
OF  TAP 

2  min.  25  sec. 

2  min.  47  sec. 

3  min.    7  sec. 
3  min.  27  sec. 
Slag  stream. 
First  ingot. 


From  the  data,  it  appears  that  the  readings  on  the  stream  from  a 
spoon  in  the  furnace  should  have  some  emissivity  correction,  although 
smaller  than  for  a  steel  stream  in  the  open.  This  will  have  to  be  checked 
up  by  further  tests.  Until  temperature  measurements  can  be  made  with 
black-body  conditions,  nothing  definite  can  be  said  or  proved  as  to 
actual  temperatures. 

The  whole  matter  is  up  to  the  ceramic  men  of  the  Committee  to 
develop  a  material  suitable  for  a  tip.  Clay  is  satisfactory  from  a  com- 
position standpoint  and,  if  made  so  that  it  will  stand  unequal  rapid  heat- 


GEORGE    K.   BURGESS  23 

ing,  could  be  used  for  tips  that  would  not  smoke  and  would  take  the 
furnace  temperature. 

THIRD  REPORT  OF  SUB-COMMITTEE  ON  OPEN-HEARTH  PYROMETER,  ELEC- 
TRIC-FURNACE AND  MISCELLANEOUS  TEMPERATURES 

In  view  of  the  difficulties  in  making  experiments  in  open-hearth 
furnaces,  the  sub-committee  decided  to  make  further  tests  in  electric 
furnaces. 

A  number  of  steel  tubes  2^  in.  (5.7  cm.)  outside  diameter  and  about 
5  ft.  (1.5  m.)  long  were  prepared  with  small  lugs  on  the  end  as  shown  in 
Fig.  4.  A  number  of  end  sleeves  with  grooves  in  them  to  fit  the  lugs 
were  prepared  by  Hiram  Swank's  Sons  and  tubes  for  tips  were  secured, 
made'  from  different  materials. 

The  first  test  on  an  electric  furnace  was  made  at  the  Taylor  Wharton 
Iron  &  Steel  Co.  at  High  Bridge,  N.  J.  Pyrometer  tubes  having  tips  of 
carborundum  and  clay  were  prepared  and  after  preheating  were  each  put 


xClay  Sleeves  Tube  madded  in 

/  Acheson  Graphite 

/    ~  /  ,  it 


FIG.  4. — ACHESON  GRAPHITE  MOUNTING^ 

into  the  steel  in  the  furnace,  which  was  a  3-ton  basic  Heroult.  Each 
tube  broke  before  coming  to  temperature  and  both  were  very  badly 
corroded  by  the  slag. 

Tapping  and  teeming  temperatures  were  taken  on  a  number  of  heats 
and,  just  before  one  tap,  four  spoons  full  of  steel  were  dipped  out;  the 
spoon  in  each  case  was  set  on  the  floor  and  readings  made  on  the  steel 
surface  until  it  crusted  over.  The  values  obtained  were  plotted  against 
time  and  curves  drawn  as  shown  in  Fig.  5. 

Optical  pyrometer  readings  were  also  made  on  the  flame  from  a 
Bessemer  converter  and  the  apparent  temperatures  obtained  are  given  in 
the  accompanying  tabulation  of  data  made  at  High  Bridge. 

Fig.  5  shows  four  cooling  curves  for  four  spoons  of  steel  which  were 
dipped  out  of  the  electric  furnace  within  a  few  minutes  of  each  other,  just 
before  the  tap.  Since  there  were  only  a  few  points  taken  on  each  spoon 
through  which  a  number  of  different  shapes  of  curves  might  have  been 
drawn,  the  nature  of  cooling  of  a  spoonful  of  steel  was  considered. 


24 


REPORT    OF   PYROMETER    COMMITTEE 


TEMPERATURE  DATA  AT  TAYLOR  WHARTON  IRON  &  STEEL  Co 

APR.  22,  1919 
Tapping  3-ton  Basic  Electric  Furnace  Carbon  Steel  for  Castings 


TIME 
4: 18  P.M. 


TIME 

TEMP., 
DEGREES  F. 

4:23 

2797 

4:24 

2790 

4:26 

2872 

4:26.5 

2656 

4:27 

2782 

4:29 

2775 

4:30 

2761 

4:31 

2704 

4:31.5 

2730 

1st  shank 
2d  shank 
3d  shank 


TEMP.,  DEGREES  F.  * 

2932 

2910 

2895 

TEEMING 

RKS                 TIME 
c               4:31.7 

TEMP., 
DEGREES  F. 

2720 

4:32 

2704 

4:32.5 

2688    2< 

3d  shank  4:33 

2697     3< 

k              4:33.5 

2704     41 

k                4:34 

2656    5t 

4:35 

2713     B 

4:36 

2680    7- 

REMARKS 
Tap  steel. 
Tap  steel. 
Tap  steel. 

REMARKS 

2d  mold. 

2d  mold. 

2d  mold  small  stream. 
3d  mold. 
4th  mold. 

5th  mold  small  stream. 
Riser  of  2d  mold. 
7th  mold,  last.] 


4th  shank 
5th  shank 
1st  mold 
2d  mold 
2d  mold 

PULLING  SLAG 
TIME  TEMP.,  DEGREES  F.       '  REMARKS 

9:58  A.  M.  2638  Stream  from  spoon  into  sample  mold. 

10:14  2712  On  slag  (e  =  0.65). 

2902  On  metal  (oxidized)  (e  =  0.40). 

2917  On  metal  (oxidized)  (e  =  0.40). 

2828  On  metal  dark  streak,  some  smoke. 

10:16  2887  On  metal  oxidized. 

2895  On  metal  oxidized. 


TIME  PROM  LIFTING 
FROM  SLAG — 11:24  A.M. 

4  sec. 
12  sec. 
19.5  sec. 
27. 5  sec. 
37  sec. 
46  sec. 


TIME  FROM 

17  sec. 

2960 

28  sec. 

2920     Oi 

46  sec. 

2982 

64  sec. 

2952 

2  min.  30  sec. 

St 

3  min.  45  sec. 

Fi 

4  min.  45  sec. 

Fi: 

5  min.  22  sec. 

2887     Is 

7  min.  30  sec. 

2865     2d 

8  min.  20  sec. 

2821     PC 

9  min.  13  sec. 

2835     3d 

9  min.  53  sec. 

2797     PC 

10  min.  9  sec. 

2797     PC 

10  min.  46  sec. 

2828     4t 

11  min.  5  sec. 

2782     PC 

1  1  min.  43  sec. 

2775     PC 

READINGS  ON  SPOON 
TEMP., 

DEGREES  F.  REMARKS 

To  floor. 

2968  Spoon  surface. 

2895  Spoon  surface. 

2813  Spoon  surface. 

2790  Spoon  surface. 

Spoon  surface  oxidized. 

TAPPING  CARBON  STEEL  FOR  CASTINGS 
Started  to  tap  11:26  A.  M. 

On  slag. 


Started  to  skim. 

Finished  skimming. 

Finished  weighing. 

1st  shank. 

2d  shank. 

Pouring  2d  shank  in  small  mold. 

3d  shank. 

Pouring  3d  shank  into  mold. 

Pouring  3d  shank  into  mold. 

4th  shank. 

Pouring  4th  shank  into  mold. 

Pouring  4th  shank  into  mold. 


*  All  readings  on  steel  streams  corrected  for  an  emissivity  of  0.40  and  slag  for  0.65. 


GEORGE    K.   BURGESS 


25 


TIME  TEMP.,  DEGREES  F.  REMARKS 

12  min.  10  sec.  2835  5th  shank. 

12  min.  35  sec.  2761  Pouring  5th  shank. 

13  min.  59  sec.  2775  Pouring  from  ladle  into  1st  mold. 

14  min.  41  sec.  2730  Pouring  from  ladle  into  2d  mold. 

15  min.  15  sec.  2688  3d  mold,  small  stream. 

16  min.    3  sec.  2782  4th  mold,  large  stream. 
16  min.  21  sec.  2768  4th  mold. 

16  min.  41  sec.  2761  4th  mold. 

16  min.  52  sec.  2753  4th  mold. 

17 -min.  12  sec.  2761  4th  mold  (oxidized  stream). 

18  min.  19  sec.  2753  5th  mold. 

20  min.    3  sec.  2797  6th  mold  (oxidized  stream). 

21  min.    2  sec.  2745     '        7th  mold. 


TIME  AFTER 
START  OF  BLOW 

2  min. 

3  min. 

3  min. 

4  min. 

5  min. 

6  min. 

7  min. 

8  min. 

9  min. 
11  min. 


50  sec. 
10  sec. 
37  sec. 
10  sec. 
12  sec. 
43  sec. 
18  sec. 
0  sec. 
22  sec. 
53  sec. 


3-TON  ACID  BESSEMER  FURNACE 
2:07  P.  M.     2433.     Charging  on  stream 

TEMP.,  DEGREES  F.  REMARKS 

2574  Flame  of  Bessemer. 

2632  Flame  of  Bessemer. 

2722  Flame  of  Bessemer. 

2824  Flame  of  Bessemer. 

2860  Flame  of  Bessemer. 

2747  Flame  of  Bessemer. 

2752  Flame  of  Bessemer. 

2527  Started  to  pour  ferromanganese  into  ladle. 

3060  Poured  Bessemer  charge  into  ferromanganese. 
Finished  pouring. 


SPOONS  FROM  ELECTRIC  FURNACE  (MANGANESE  STEEL  MELTED  SCRAP) 

Test  started  2:42  p.  M. 

IST  SPOON 

TIME  TEMP.,  DEGREES  F.  REMARKS 

0  sec.  Just  lifted  spoon  from  metal. 

10  sec.  2790  Steel  surface. 

18  sec.  2637  Steel  surface. 

23  sec.  2603  Steel  surface. 

33  sec.  2456  no  cor.  Steel  surface  oxide. 


0  sec. 
19  sec. 
24  sec. 
34  sec. 
45  sec. 
50  sec. 
52  sec. 
68  sec. 


2D  SPOON 

Just  in  furnace. 

Just  lifted  from  metal. 

On  floor. 

2775  Steel  surface. 

2745  Steel  surface. 

2672  Steel  surface. 

2603  Steel  surface. 

2471  Steel  surface  oxide. 


26 


REPORT    OF    PYROMETER    COMMITTEE 


TIME 
0  sec. 

15  sec. 

19  sec. 

25  sec. 

31  sec. 

34  sec. 

46  sec. 


0  sec. 
19  sec. 
22  sec. 
29  sec. 
36  sec. 
45  sec. 
53  sec. 


3o  SPOON 

TEMP.,  DEGREES  F.  REMARKS 

Just  dipped  in  metal. 

Just  lifted  out. 

On  floor. 

2775  Steel  surface. 

2680  Steel  surface. 

2629  Steel  surface. 

2566  Steel  surface  dirty. 

4TH  SPOON 

In  metal. 

Out. 

On  floor. 

2821  Spoon  surface. 

2721  Spoon  surface. 

2745  Spoon  surface. 

2556  Spoon  surface. 

2796  Flame  of  2d  Bessemer. 

2843  Flame  of  2d  Bessemer. 

2758  Flame  of  2d  Bessemer. 

3100  Pouring  into  ladle  (bright  streak),     (e  =  0.40) 

2895  On  dark  streak. 

TAPPING  MANGANESE  STEEL 


TIME  FROM 
START  OP  TAP 

15  sec. 

34  sec. 

52  sec. 

1  min.  14  sec. 
1  min.  38  sec. 


TEMP.,  DEGREES  F. 

3053 
3025 
3025 
3037 
3025 


REMARKS 
Steel  stream. 
Steel  stream. 
Steel  stream. 
Steel  stream. 
Steel  stream. 


There  are  three  factors  which  enter  into  the  cooling  of  a  hot  body — 
radiation,  conduction  and  convection.  Of  these  the  effect  of  radiation 
is  much  greater  than  conduction  or  convection  at  the  temperature  of 
molten  steel,  so  that  the  shape  of  the  cooling  curve  must  be  largely 
influenced  by  the  fourth-power  radiation  law  and  approach  that  of  an 
exponential  curve. 

In  the  operation  of  dipping  out  a  spoonful  of  steel  and  bringing  it  out 
into  the  room  to  cool,  there  are  a  number  of  conditions  which  may  affect 
the  rate  of  cooling  of  the  steel.  The  spoon  is  first  dipped  into  the  slag 
and  turned  over  a  number  of  times  to  get  a  good  slag  coating  on  it.  This 
operation  may  take  from  10  to  30  sec.  depending  on  conditions  and  the 
skill  of  the  operator.  The  spoon  in  this  operation  is  heated  up  consider- 
ably and  may  become  so  hot  that  the  handle  will  bend  on  lifting  out  the 
spoonful  of  metal.  When  the  spoon  is  sufficiently  "slagged,"  it  is  dipped 
into  the  steel  and  a  spoonful  lifted  out.  The  steel  in  the  spoon  immedi 
ately  begins  to  cool,  as  there  is  at  least  1000°  F.  (555°  C.)  difference  be- 
tween the  spoon  and-  steel  temperatures,  but  since  the  temperatures  of 
the  furnace  gases  are  about  that  of  the  steel  and  a  third  of  the  total 


GEORGE    K.    BURGESS 


27 


area  bounding  the  steel  in  the  spoon  is  exposed  to  them,  the  cooling  of  the 
molten  steel  is  probably  not  very  rapid  while  in  the  furnace.  As  soon 
as  the  spoon  passes  the  furnace  door,  however,  there  is  an  abrupt  change 
of  cooling  and  the  rate  must  be  largely  exponential  in  character. 

Curves  1  and  3,  the  cooling  curves  for  two  spoons  of  steel  taken  out 
of  the  furnace  under  identical  conditions,  are  of  the  nature  of  exponential 
curves.  The  part  of  the  curves  from  the  point  where  the  spoon  passed 
through  the  door  to  the  first  reading  is  guessed  at,  as  is  the  cooling  curve 
in  the  furnace.  The  steel  temperature  is  taken  as  the  average  of  the 
readings  made  on  the  tapping  stream. 

Curve  4  is  clearly  defined;  curve  2  is  rather  poor  but  it  was  drawn 
to  be  as  nearly  like  the  others  as  possible  and  still  conform  to  the  points. 


3100 


3000 


2900 


fa   2800 


2700 


2600 


2500 


2400 


36 


44 


48 


12  16  20  24  28  32 

Seconds  after  Raising  Spoon  from  Sins 

FIG.  5. — TEMPERATURE  MEASUREMENTS  ON  SURFACE  OF  MANGANESE  STEEL  SPOONED 

FROM  ELECTRIC  FURNACE  . 

An  attempt  will  be  made  to  get  points  nearer  to  zero  time  in  the  near 
future. 

After  the  failure  of  the  clay  and  carborundum  tubes  in  the  basic 
furnace,  a  test  was  planned  in  the  acid  furnace  at  Lebanon,  Pa.,  but  since 
they  were  not  in  operation  at  the  time,  two  tubes  were  prepared  with 
Acheson  graphite  tips  and  a  test  was  made  at  the  Disston  Saw  Works 
in  Philadelphia,  in  a  3-ton  basic  Heroult  furnace. 

The  first  tube  was  put  in  without  preheating,  and  the  end  sleeve 
cracked  almost  immediately,  due  to  too  sudden  heating.  The  other 
tube  was  preheated  and  pushed  into  the  steel.  It  did  not  smoke  and  came 
to  temperature  in  70  sec.,  at  which  time  a  very  satisfactory  reading  was 
obtained.  About  an  hour  later  the  same  tube  was  put  into  the  furnace 
again  and  another  reading  taken.  This  time,  however,  part  of  the  end 


28 


REPORT    OF    PYROMETER    COMMITTEE 


clay  sleeve  broke  off  so  that  the  graphite  tube  was  not  held  in  line  but 
was  atj  an  angle  to  the  axis  of  the  steel  tube.  For  this  reason  the  optical 
reading  could  not  be  made  on  the  end.  The  sight  was  taken  on  the  side 
of  the  tube  and  a  reading  was  made  after  it  had  come  up  to  temperature 
and  remained  constant  for  a  minute.  This  reading  was  probably  very 
near  the  true  temperature  but  it  is  not  put  on  the  curve  as  it  is  somewhat 
questionable. 

After  each  time  that  the  tube  was  put  in  the  furnace,  it  was  withdrawn 
and  readings  made  on  the  graphite  tip  and  the  time  noted.  These  curves 
are  shown  in  Fig.  6.  It  will  be  noted  that  a  smooth  curve  can  be  readily 
drawn  through  the  points.  The  furnace  was  tapped  about  15  min.  after 


2800 


2700: 


2600 


^2500 


2400 


22300 

£ 
E 
v 
H2200 


2100 


2000 


1900 


\ 


LEGEND 

Optical  Pyrometer  Reading  In  Tube 
Cooling  in  Air  No.  1 
"         "     "    No.  2 
Averaga  Tapping  Temp.  Degrees  F. 


>.  2 


0 


72 


3  1C  24  32  40  48  56  6 

Time  after- Pulling  through  Door,  Seconds 

FIG.  6. — COOLING  CURVES  FOR  GRAPHITE  TUBES  AFTER  IMMERSION  IN  STEEL. 

the  last  tube  was  put  in,  and  in  the  intervening  time,  the  steel  was  heated 
somewhat.  The  tapping  temperature  is  25°  F.  (13.8°  C.)  above  the 
extrapolated  curve  for  tube  No.  2. 

The  data  taken  on  this  test  are  as  follows: 

TEST  AT  DISSTON  SAW  WORKS  (JUNE  16,  1919) 
3-Ton  Basic  Heroult  Furnace 

First  tube  in,  the  end  sleeve  broke  due  to  sudden  heating.  Second 
tube  came  to  temperature  after  70  sec.;  reading,  2728°  F.  (1497°  C.). 
The  tube  was  pulled  from  the  furnace  and  readings  made  on  its  end  while 
it  cooled.  Time  taken  from  passing  through  door. 


GEORGE    K.    BURGESS  29 


TIME 

TEMP., 
DEGREES  F. 

REMARKS 

TIME 

TEMP., 
DEGREES  F. 

REMARKS 

0 

Out  door. 

42  sec. 

2130 

On  end  of  tube. 

14  sec. 

2380 

On  side  of  tube. 

51  sec. 

2032 

On  end  of  tube. 

25  sec. 

2298 

On  end  of  tube. 

59  sec. 

1964 

On  end  of  tube. 

34  sec. 

2185 

On  end  of  tube. 

The  same  tube  was  put  in  later.  The  end  sleeve  cracked  but  the  tube 
was  held  partly  under  the  steel  and  was  at  an  angle  to  the  sight  tube  so 
the  readings  were  made  on  a  point  about  9  in.  from  the  end.  The  tem- 
peratures gradually  increased  to  a  steady  value  after  2  min.  30  sec. 
Reading,  2672°  F.  (1467°  C.) 

The  graphite  tube  was  pulled  out  in  a  shovel  and  laid  on  the  floor 
where  readings  were  made  on  it  at  different  times  as  follows : 

TlME  DECREES  F.        REMARKS  TIME         _  DJ*B™^  F.      REMARKS 

0  Out  door.  32  2110        On  tube. 

13  2355         On  tube.  40  2021         On  tube. 

24  2193         On  tube.  48  1964        On  tube. 

A  number  of  readings  were  attempted  on  the  spoon  surfaces  but  the 
metal  chilled  over  almost  immediately  after  the  spoon  was  set  on  the 
floor. 

Tap  started  16  min.  after  the  tube  was  put  in,  the  steel  being  heated 
up  between  times  for  a  period  of  about  7  min.  Tapping  and  teeming 
temperatures  were  as  follows : 

TAP 


TIME 

0 

1  min. 

TEMP., 
DEGREES  F. 

2688 
2713 
2697 
2697 

REMARKS 

On  steel  stream. 
On  steel  stream. 
On  steel  stream. 
On  steel  stream. 
TEEM 

TIME 

TEMP., 
DEGREES 

,,    REMARKS 
r  . 

TIME         DEGREE'S  F.    ^MARKS 

2  min. 
3  min. 
3  min. 
8  min. 

10  sec. 
8  sec. 
58  sec. 
20  sec. 

2656 
2647 
2637 
2595 

1st  ingot. 
2d  ingot. 
3d  ingot. 
8th  ingot. 

12 
13 
17 

min. 
min. 
min. 

30  sec. 
30  sec. 
00  sec. 

2576 
2566 
2556 

12th  ingot 
13th  ingot. 
16th  ingot  last. 

Carbon  was 

about  1. 

35  per  cent. 

Ingots  weighed  470  Ib 

,  each. 

Two  more  tubes  were  prepared  with  the  same  graphite  tips  which  had 
been  previously  used.  One  tube  was  put  into  the  furnace  and  a  very  good 
reading  was  made.  The  tube  did  not  smoke  in  the  least  and  was  very 
satisfactory.  After  the  reading  was  taken,  the  end  clay  sleeve  broke  off 
but  the  graphite  tip  was  pulled  out  quickly  and  readings  made  on  it  as 
in  the  previous  test.  The  points  are  plotted  in  Fig.  7. 

The  second  tube  had  not  been  thoroughly  dried  out  and  had  steam  in 
it,  so  that  although  a  reading  was  made  while  the  steam  was  blown  out, 


30 


REPORT    OF    PYROMETER    COMMITTEE 


it  is  somewhat  questionable.  A  cooling  curve  was  also  taken  on  this 
graphite  tip  and  is  shown  as  curve  No.  2  on  Fig.  7.  It  will  be  noted  how 
closely  the  extrapolated  curves  come  to  the  readings  made  in  the  tube. 
In  no  case  is  the  difference  greater  than  20°  F.  (11°  C.).  It  was  not  pos- 
sible to  lift  the  tube  above  the  bath  and  take  a  reading  on  the  outside 
of  the  graphite  tip  in  the  furnace  because  the  smoke  was  too  heavy. 

The  clay  end  sleeves  that  were  used  in  these  tests  were  overheated  in 
making  and  were  not  as  strong  as  the  regular  stopper  rod  sleeves.  For 
this  reason  most  of  them  cracked  after  being  in  the  furnace  a  short  time. 
The  temperature  data  on  this  test  are  given  here. 


TIME 


TEMP., 
DEGREES  F. 


REMARKS 


9:14  A.  M. 

2753 

Reading  in  1st  tube. 

0  sec. 

Tube  No.  1  through  door.     Arc  off. 

9.4 

2574 

On  graphite  tip  in  air. 

18.8 

2411 

27.4 

2254 

35.0 

2157 

45.6 

2032 

54.0 

1943 

9:28  A.  M. 

2703 

Reading  on  slag  in  furnace.     Arc  on. 

9:29  A.  M. 

Made  new  slag,  spar  and  lime. 

10:10  A.  M. 

2836(?) 

Reading  in  tube.     Air  blowing  out  steam. 

0  sec. 

Tube  No.  2  through  door. 

8.4 

2567 

Reading  on  graphite  tube  in  air. 

16.6 

2478 

25.0 

2347 

33.0 

2203(?) 

40.0 

2082 

47.0 

2053 

54.8 

1964 

10:  25  A.  M. 

2807 

Slag  in  furnace  —  door  open,  corrected  for  e  = 

0.65. 

10:25:30  A. 

M. 

Start  to  tap. 

7  sec. 

2775 

Tap. 

22 

2768 

31 

2768 

44 

2775 

60 

2768 

72 

2768 

83 

2768 

108 

2761 

10:27:35  A. 

1C. 

Finish  tap. 

Held  5  min. 

in  ladle. 

TIME  FROM 
START  OF 
TEEM 

TEMP., 
DEGREES  F. 

TIME  FROM          TEMP., 
REMARKS           START  OF      DEGREES  F.             REMARKS 
TEEM 

30  sec. 

2656 

1st  ingot.          8:00            2602            10th 

ingot. 

1:30 

2647 

2d  ingot.         10:45            2595            12th 

ingot. 

3:00 

2629 

4th  ingot.        12:45            2576            14th 

ingot. 

4:40 

2620 

6th  ingot.        13:05            2566            16th 

ingot 

6:15 

2611 

8th  ingot.                                                  last. 

GEORGE    K.   BURGESS 


31 


2900 


2800 


2700 


2600 


£2500 


2400 


2300 


2200 


2100 


2000 


1900 


\ 


LEGEND 

e  Cooling  in  Air  No.  1 
x  -  ••  H  No.  2 
«>  Optical  Reading  in  Tube,  in  Steel 


A  Xa 


Nos.  1  &  2 


No.  1 


).  2 


1G 


24  32  40 

Time  in  Seconds 


48 


Fio.  7. — COOLING  CURVES  FOB  GRAPHITE  TUBES   AFTER  IMMERSION  IN   STEEL  IN 

HEROULT  FURNACE. 

CONCLUSIONS  OF  SUB-COMMITTEE 

Acheson  graphite  is  a  very  satisfactory  material  for  a  tube  to. use  in 
steel.  It  does  not  smoke,  only  oxidizes  slowly,  and  has  plenty  of  strength. 
One  tube  with  3^-in.  wall  which  was  immersed  five  times  and  allowed  to 
cool  in  air  appeared  to  have  lost  only  a  small  amount  in  diameter.  An- 
other advantage  is  that  the  slag  does  not  stick  to  it,  and  optical  readings 
may  be  made  on  it  as  soon  as  it  is  lifted  from  the  bath. 

Steel  temperatures  may  be  taken  by  allowing  a  tube  of  Acheson  graph- 
ite to  come  to  the  temperature  of  the  bath  and  then  either  sighting  on 
the  tube  when  it  is  raised  above  the  bath  or  pulling  it  out  into  the  air 
and  taking  the  cooling  curve  and  extrapolating  to  zero4ime.  Tempera- 
tures, may  also  be  taken  by  sighting  down  the  tube.  In  an  open  hearth 
it  would  probably  be  easier  to  use  the  graphite  as  a  target. 

PROJECTED  EXPERIMENTS  AND  SUMMARY 

The  use  of  Acheson  graphite,  either  in  block  or  tube  form,  as  a  target 
after  immersion  to  the  desired  depth  and  location  in  the  metal  bath  and 
sighted  upon  by  the  optical  pyrometer,  as  shown  by  the  experiments  of 
the  sub-committee,  gives  promise  of  being  a  serviceable  method  of  tern- 


32  REPORT    OF    PYROMETER    COMMITTEE 

perature  control.  This  graphite  has  the  advantage  of  remaining  clear 
of  slag  and,  in  addition,  the  optical  pyrometer  requires  no  correction 
when  sighted  on  graphite  whether  within  the  furnace  or  not.  More 
observations,  however,  should  be  taken  by  this  method  to  more  com- 
pletely determine  its  limitations. 

Another  method  suggested,  which  has  been  used  by  Dr.  Northrup 
with  small  laboratory  furnaces  and  which  the  Committee  expects  to  try 
out,  is  to  thrust  a  quartz-glass  rod  or  thick-walled  closed-end  tube  into 
the  bath  to  the  depth  at  which  the  temperature  is  wanted,  and  sight  in 
the  outer  end  of  the  rod  with  an  optical  pyrometer.  The  light  being 
totally  reflected  along  the  quartz — which  may  even  bend  considerably 
without  loss  of  light — temperatures  not  requiring  any  appreciable  correc- 
tion should  be  given  by  this  method.  The  quartz  will  probably  not  last 
very  long,  especially  in  basic  practice,  but  it  will  probably  be  found 
unnecessary  to  leave  the  quartz  immersed  more  than  a  fraction  of  a 
minute  in  order  to  take  a  satisfactory  observation.  If  held  too  long  in 
the  bath,  of  course  the  quartz  will  melt,  but  on  account  of  its  transparency 
it  should  be  possible  to  obtain  a  reliable  observation  before  this  happens. 

The  work  of  the  Committee  has  shown  the  very  great  practical  dif- 
ficulties which  obtain  with  available  refractories  when  attempt  is  made 
to  sight  down  a  tube  thus  protected.  The  best  material  thus  far  found 
for  this  purpose  is  an  Acheson  graphite  tube  supported  and  extended  by 
a  steel  tube  which  is  in  turn  protected  by  a  fireclay  sleeve.  All  other 
refractories  tried  are  unsuited  for  the  purpose. 

When  care  is  exercised,  it  has  been  shown  by  the  Committee  that  the 
method  of  spooning  out  metal  and  taking  simultaneous  observations  of 
time  and  temperature  will  give  reliable  results. 

The  modifications  suggested  by  Mr.  Drinker,  in  the  hands  of  the  Com- 
mittee, do  not  give  dependable  results. 

APPENDIX 

REPORT  OF  SUB-COMMITTEE 
Test  on  Crucible  Steel  at  Midvale  Steel  Co.,  Sept.  17>  1919 

F.  E.  BASH,  Philadelphia,  Pa. — In  order  to  make  a  definite  check  on 
the  emissivity  of  crucible  steel  under  works  conditions,  a  test  was  planned 
and  executed  as  follows:  Five  crucibles  were  put  in  a  crucible  furnace 
with  the  normal  charge  and  were  treated  in  the  same  manner  as  a  number 
of  others  that  were  charged  at  the  same  time;  the  only  difference  was 
that  the  five  crucibles  had  lids  with  2-in.  holes  in  the  center.  It  was 
thought  that  after  the  crucibles  were  removed  from  the  furnace  a  read- 
ing could  be  made  through  the  hole  in  the  lid  with  the  optical  pyrometer 
and  that  such  a  reading  would  give  the  true  temperature  of  the  steel  as 


GEORGE    K.    BURGESS 


33 


black-body  conditions  should  prevail  inside  the  crucible.  If  after 
obtaining  the  true  temperature,  the  crucible  lid  were  removed  and  the 
steel  poured  into  the  ladle,  the  apparent  temperature  could  be  taken 
by  sighting  on  the  steel  stream  and  the  emissivity  obtained  from  the 
two  temperature  values. 

Such  a  test  was  carried  out  with  five  crucibles.  About  a  third  of  the 
crucibles  charged  into  the  furnace  with  the  five  experimental  ones  were 
first  drawn  from  the  furnace  and  poured  into  the  ladle.  The  experimental 
crucibles  were  then  drawn  and  readings  of  the  true  temperature  were  made 
through  the  hole  in  the  lid  from  a  raised  stand  erected  for  the  purpose; 
the  crucibles  were  then  poured  into  a  small  ladle.  While  pouring  them, 
a  reading  was  made  on  the  stream.  It  was  not  an  easy  matter  to  get 
this  reading  as  the  crucible  was  poured  in  about  8  to  10  sec. ,  and  often 
only  a  glimpse  of  the  steel  stream  could  be  obtained  as  it  was  covered 
with  slag  during  most  of  the  time  of  pouring.  However,  by  setting  the 

Temperature  Measurements  on  Crucible  Steel 


Time  from  Furnace, 
Seconds 

Temperature,  Degrees  F. 

Remarks 

Apparent 

True 

2886 

Temperature  in  furnace  before  drawing 

crucibles 

12 

2824 

First  experimental  crucible 

33 

2542  (?) 

(2761) 

First  experimental  crucible,  steel 

15 

2783 

Second  experimental  crucible 

30 

2560 

(2783) 

Second  experimental  crucible,  steel 

17 

2818 

Third  experimental  crucible 

30 

2708 

(2820) 

Third  experimental  crucible,  slag 

30 

2824        Fourth  experimental  crucible 

45 

2703 

(2814) 

Fourth  experimental  crucible,  slag 

24 

2802 

Fifth  experimental  crucible 

40 

2632 

(2738) 

Fifth  experimental  crucible,  fumes 

Average 

time 

2722 

(2834) 

Reading  on  slag  stream  from  pots 

from 

furnace 

2708 

(2820) 

Reading  on  slag  stream  from  pots 

to 

pour 

2581 

(2805) 

Reading  on  steel  stream  from  pots 

20  sec. 

2567 

(2790) 

Reading  on  steel  stream  from  pots 

2581 

(2805) 

Reading  on  steel  stream  from  pots 

2581 

(2805) 

Reading  on  steel  stream  from  pots 

2574 

(2797) 

2588 

(2813) 

2594 

(2821) 

2588 

(2813) 

2581 

(2805) 

2574 

(2797) 

34  REPORT    OF    PYROMETER    COMMITTEE 

lamp  at  a  temperature  very  nearly  that  of  the  apparent  temperature  of 
the  steel  stream,  some  readings  were  obtained.  In  the  case  of  three  of 
the  crucibles  the  reading  had  to  be  made  on  the  slag  stream  as  the  steel 
was  not  visible. 

As  a  further  check  on  the  apparent  temperature  of  the  steel,  readings 
were  made  on  the  streams  from  all  the  remaining  crucibles  in  that  heat; 
these  temperatures  are  tabulated  on  page  33.  The  time  recorded  in  the 
first  column  is  the  time  that  elapsed  when  the  crucible  passed  through 
the  door  and  the  reading  was  made. 

TEEMING  READINGS  CORRECTED  FOR  AN  EMISSIVITY  OF  0.40 

TRUE  TEMPERATURE,  DEGREES  P.  TRUE  TEMPERATURE,  DEGREES  F. 

Third  ingot 2680  Ninth  ingot 2647 

Fourth  ingot 2665  Tenth  ingot 2647 

Fifth  ingot 2665  Eleventh  ingot 2647 

Sixth  ingot 2665  Twelfth  ingot 2629 

Seventh  ingot 2656  Thirteenth  ingot 2629 

Eighth  ingot 2656  Fourteenth  ingot 2629 

NOTE. — All  temperature  values  enclosed  in  parenthesis,  in  the  columns  for  true 
temperatures,  are  corrected  for  an  emissivity  of  0.40  in  the  case  of  steel  and  0.65 
for  slag. 

The  readings  in  the  table  for  the  five  experimental  crucibles  show 
that  the  corrections  for  emissivity  of  0.40  for  steel  and  0.65  for  slag, 
when  applied  to  the  apparent  temperatures,  give  practically  the  same 
temperature  values  as  the  readings  made  in  the  crucible  under  black-body 
conditions,  the  greatest  difference  being  10°  F.  The  correctness  of  these 
emissivity  values  is  further  verified  by  taking  the  mean  of  the  true  tem- 
peratures obtained  by  sighting  into  the  five  experimental  crucibles  and 
comparing  them  with  the  mean  of  the  corrected  temperature  values  ob- 
tained by  sighting  on  the  steel  streams  from  the  crucibles.  The  values 
are  as  follows : 

DEGREES  F 

Mean  of  true  temperatures  for  five  crucibles 2810 

Mean  of  corrected  readings  on  steel  streams 2805 

Difference 5 

The  conclusions  drawn  from  the  above  data  are  that  under  industrial 
conditions,  the  values  for  the  emissivity  of  steel  and  slag  are  0.40  and 
0.65,  respectively. 

DISCUSSION 

H.  SCOTT,  Washington,  D.  C.  (written  discussion*). — It  appears  to 
me,  from  observations  taken  in  a  number  of  plants  under  Dr.  Burgess' 
direction,  that  the  simple  and  natural  expedient  of  sighting  on  the  bath 
will  give  the  steelmaker  the  desired  information.  The  measurements 

*  Received  Sept.  24,  1919. 


DISCUSSION 


35 


show  remarkable  consistency  among  themselves.  On  the  other  hand, 
it  may  be  noted  from  other  work,  as  Dr.  Burgess'  "Temperature  Measure- 
ments in  Bessemer  and  Open-hearth  Practice"4  and  the  report  under 
discussion,  that  readings  taken  by  this  method  (sighting  on  the  bath) 
are  not  uniform  enough  to  be  used  as  a  direct  control  of  open-hearth 
heats  for  the  variations  in  tapping  temperatures  are  less  than  the  observed 
bath  temperatures. 

To  support  this  opinion  I  have  prepared  the  accompanying  figure, 
which  shows  the  temperature  rise  in  open-hearth  furnaces  after  the  charge 


OPEN-HEARTH  STEEL  FURNACE. 
TEMPERATURES 


1500 


50  100  . 150 

Time  from  First  Observation 


200  mins. 


has  melted  down.  Each  value  represents  the  average  of  several  un- 
corrected  readings  taken  through  the  opening  in  the  furnace  door  with 
the  pyrometer  used  by  Dr.  Burgess.  It  may  be  noted  that  a  smooth 
curve  can  be  drawn  representing  the  temperature  rise  with  time,  that 
only  an  exceptional  value  lies  more  than  15°  C.  off  the  curves,  and  that 
the  extrapolated  values  for  the  tapping  temperatures  all  lie  between 
1600°  and  1620°  C.  (2912°  and  2948°  F.). 

4U.  S.  Bureau  of  Standards  Tech.  Paper  91;  also  Trant.  (1917)  56,  432. 


36  REPORT   OF   PYROMETER    COMMITTEE 

The  discrepancies  generally  observed  in  readings  taken  on  the  bath 
may  be  associated  with  the  two  disconcerting  conditions  that  always  exist 
in  open-hearth  furnaces;  namely,  flames  shooting  across  the  line  of 
vision  and  the  boiling  of  the  bath.  As  both  of  these  features  are  inter- 
mittent, it  is  possible,  with  the  development  of  some  skill,  to  obtain 
rational  observations  on  the  background  of  the  slag  surface.  Thus, 
for  example,  by  decreasing  the  brightness  of  the  pyrometer  lamp  from  that 
of  the  flames,  a  point  is  reached  at  which  the  filament  no  longer  flashes 
bright  when  the  flame  momentarily  ceases  and  vice  versa.  These 
readings  represent  temperatures  just  below  or  above,  as  the  case  may  be, 
that  of  the  bath.  Care  must  be  taken,  however,  that  the  reading  is  not 
on  the  front  surface  of  bubbles  as  these  reflect  the  dark  hole  in  the  door 
and  probably  represent  non-black-body  conditions.  I  therefore  think 
that  the  question  can  be  properly  brought  up  as  to  whether  direct  observa- 
tions on  the  bath  in  the  furnace  do  or  do  not  give  an  acceptable  criterion 
of  open-hearth  furnace  temperatures. 

A.  L.  FEILD,  Niagara  Falls,  N.  Y. — Have  any  attempts  been  made  to 
correlate  the  temperatures  in  the  steel  bath  with  the  reactions  that  go  on 
in  the  smelting  process?  Has  the  temperature  been  related  to  the  ques- 
tion of  the  life  of  the  refractories,  especially  in  the  roof?  It  seems  to  me 
that  these  two  points  are  of  practical  importance  to  steel  manufacture. 
A  third  question  is  the  relation  of  the  temperature  of  the  steel  to  the 
properties  of  the  cooled  ingot.  So  far,  it  seems  to  me,  the  Pyrometer 
Committee  has  been  devising  methods  and  means  of  measuring  tempera- 
ture, which  should  be  regarded  only  as  preliminary  to  the  really  important 
.  questions  I  have  mentioned. 

G.  K.  BURGESS. — Mr.  Feild  is  perfectly  correct  in  stating  that  the 
object  of  the  Committee's  work  was  the  question  of  determining  tempera- 
tures. The  fundamental  idea,  of  course,  is  that  if  you  can  get  a  reliable 
method  of  temperature  observation  and  control,  it  is  obvious  that  the 
thing  to  do  is  to  tie  it  up  to  the  factors  in  steel  making  that  may  be  of 
interest  and  related  to  temperature,  such  as  the  reaction  in  the  furnace, 
life  of  the  roof,  and  ingot  properties,  etc. 


TEMPERATURE  37 


Temperature 

BY  JOSEPH   S.    AMES,*  BALTIMORE,    MD. 
(Chicago  Meeting,  September,  1919) 

THERE  are  two  distinct  questions  associated  with  the  concept  of  tem- 
perature: one  is  practical,  the  other  is  theoretical.  Our  fundamental 
ideas  of  temperature  come  from  our  senses;  we  know  what  we  mean  by 
the  words  "hot"  and  "cold,"  or  by  saying  one  body  is  "hotter  than 
another."  But  for  scientific  purposes  words  require  definition.  We  are 
guided  in  this  matter,  as  in  all  other  scientific  questions,  by  our  knowl- 
edge of  facts  of  observation. 

When  two  bodies  at  different  temperatures  are  intimately  associated, 
e.g.,  a  hot  stone  is  dropped  into  a  pail  of  water,  our  experience  is  that 
ultimately  they  come  to  the  same  temperature  as  far  as  our  senses  can 
tell;  the  hot  body  becomes  colder  and  the  cold  body  hotter.  In  the  case 
of  a  block  of  ice  immersed  in  water,  the  ice  melts,  forming  cold  water, 
which  then  mixes  with  the  other  water,  the  final  result  being  water  colder 
than  the  original  water.- 

From  our  knowledge  of  the  nature  of  heat  phenomena,  we  learn  that 
in  this  process  one  body  loses  heat  and  the  other,  gains  heat,  the  condi- 
tion of  equilibrium  being  one  in  which  each  body  gains  as  much  heat 
as  it  loses.  It  is  distinctly  assumed  in  this  statement  that  only  two 
bodies  are  concerned  in  the  transfer  of  heat,  all  other  bodies  being  ren- 
dered in  some  way  impervious  to  heat.  The  body  that  in  the  process 
loses  heat  is  said  to  have  the  "higher  temperature"  while  the  body  that 
gains  the  heat  is  said  to  have  the  "lower  temperature;"  and,  when  ther- 
mal equilibrium  is  reached,  the  two  bodies  are  said  to  have  the  "same 
temperature."  Consequently,  the  temperature  of  a  body  may  be  de- 
fined, as  it  was  by  Maxwell,  as  "its  thermal  state  considered  with  refer- 
ence to  its  power  of  communicating  heat  to  other  bodies." 

Experience  also  proves  that  if  we  have  three  bodies  A,  B,  and  C; 
if  A  is  at  the  same  temperature  as  B  and  as  C,  then  B  will  be  at  the  same 
temperature  as  C,  This  fact  is  the  basis  of  all  methods  of  thermometry. 
For  instance,  if  A  and  B  are  two  bodies  in  thermal  equilibrium,  and  if  C 
is  an  ordinary  mercury  thermometer  in  equilibrium  with  A,  then  if  C  is 
placed  in  B,  it  will  indicate  the  same  "reading,"  showing  that  B  is  in 
equilibrium  with  it.  This  emphasizes  an  essential  feature  of  the  use  of  a 

.  .      *  Professor  of  Physics,  Johns  Hopkins  University. 


38  TEMPERATURE 

thermometer:  it  must  be  in  such  a  relation  with  the  body  whose  tempera- 
ture is  desired  that  these  two  bodies  are  in  complete  thermal  equilibrium. 
Experiments  have  shown  that  we  have  at  our  disposal  in  laboratories 
many  methods  by  which  definite  thermal  states  can  be  secured.  For 
instance,  if  a  rod  of  copper  is  placed  in  a  mixture  of  ice  and  water,  it 
always  assumes  the  same  length,  regardless  of  when  or  where  the  experi- 
ment is  performed;  a  piece  of  platinum  wire  will,  in  such  a  bath,  always 
have  the  same  electrical  resistance,  etc.  We  therefore  believe  that  we 
are  dealing  with  a  definite  thermal  condition.  The  same  is  true  of  the 
steam  rising  from  boiling  water  if  the  pressure  of  the  air  on  the  surface 
of  the  water  is  unchanged,  and  of  countless  other  so-called  "changes  of 
state. " 

THERMOMETRY 

The  practical  question  of  thermometry  is  to  devise  a  method  by  which 
numbers  may  be  given  the  temperature  of  any  state  of  thermal  equilib- 
rium ;  and,  obviously,  the  method  should  be  such  as  to  assign  always  a 
greater  number  to  the  higher  temperature.  The  theoretical  question 
is  to  learn  what  physical  property  of  the  molecules  of  a  body  it  is  that 
determines  its  temperature.  Great  difficulties  arise  instantly.  Assume 
that  we  have  adopted  a  thermometer  and  a  thermometric  scale  (as  will 
be  explained  later)  and  that  it  is  possible  to  insert  the  thermometer  into 
a  flame  in  such  a  manner  that  it  and  the  flame  come  to  equilibrium;  then, 
if  there  are  two  types  of  flames  or  even  the  same  type  of  flame  under  two 
conditions,  we  can  obtain  numbers  by  the  thermometer  "readings." 
Are  they  temperatures?  Supposing  one  reading  is  1500°  and  the  other 
1000°;  does  this  mean  that,  when  the  two  flames  are  placed  in  thermal 
communication,  the  former  will  lose  heat  and  the  latter  gain  heat?  In 
such  a  case  as  this  there  are  at  least  two  causes  of  uncertainty:  (1)  Is 
the  state  of  a  flame  such  as  to  justify  one  in  using  the  word  "tempera- 
ture" in  connection  with  it?  (2)  Is  the  condition  of  the  thermometer 
when  it  ceases  to  change  such  that  it  is  in  thermal  equilibrium  with  the 
flame?  This  same  uncertainty  arises  when  one  considers  inserting  a 
thermometer  in  an  arc-light,  in  an  electric  spark,  in  a  vacuum  tube  carry- 
ing an  electric  discharge,  and  in  numerous  other  cases. 

Therefore  in  the  preliminary  discussion  of  temperature  and  ther- 
mometry we  shall  exclude  all  such  cases  and  shall  assume:  (1)  That  the 
bodies  to  which  temperature  numbers  are  to  be  assigned  are  in  thermal 
equilibrium  free  of  all  electrical  or  chemical  changes,  and  (2)  that  the 
presence  of  the  thermometer  does  not  give  rise  to  any  such  changes. 
Thermal  equilibrium  between  the  body  and  the  thermometer  is  brought 
about  by  processes  of  heat-conduction.  The  process  of  radiation  is 
involved  in  those  methods  of  thermometry  in  which  the  thermometer  is 
not  inserted  in  the  body.  It  is  convenient,  therefore,  to  divide  the  sub- 


JOSEPH    S.    AMES  39 

ject  of  thermometry  into  two  divisions:  one  involving  the  insertion  of  the 
thermometer  in  the  body  and  therefore  heat-conduction;  the  other,  the 
use  of  a  thermometer  at  a  distance  and  therefore  radiation. 

CONDUCTION  METHODS 

In  order  to  assign  a  number  to  a  thermal  state,  it  is  impossible  J;o  make 
use  of  our  temperature-sense,  but  an  obvious  method  is  to  make  use  of 
some  physical  property  of  a  definite  piece  of  matter,  which  property 
changes  in  amount  as  heat  leaves  this  body  or  is  added  to  it  and  which 
can  be  measured;  e.g.,  the  length  of  a  selected  copper  rod;  the  volume  of  a 
definite  quantity  of  mercury  held  in  some  solid;  the  pressure  of  a  definite 
volume  of  a  definite  mass  of  nitrogen;  the  electrical  resistance  of  a  definite 
platinum  wire;  etc.  Such  an  instrument  is  called  a  thermometer.  Of 
course  it  would  cease  to  be  useful  if  (1)  its  thermometric  property  ceased 
to  change  as  heat  was  abstracted  from  it,  or  (2)  it  underwent  such  changes 
owing  to  use  that,  when  replaced  in  the  same  thermal  state  after  such  use, 
it  gave  a  different  reading.  (In  some  cases  this  second  cause  of  trouble 
may  be  obviated). 

Having  selected  a  definite  physical  property  of  a  definite  body,  for 
instance,  the  length  of  a  copper  rod,  this  length  may  be  measured  at 
two  definite  thermal  states  to  which  arbitrary  numbers  are  assigned. 
Let  these  numbers  be  t\  and  t2,  where  t2  >  ti}  and  let  the  measured  quanti- 

ties be  ai  and  a2;  the  ratio  -~        —  is  chosen  as  the  measure  of  a  "degree." 

za  —  h 

Then,  for  the  number  to  be  given  the  temperature  of  any  thermal  state, 
let  a  be  the  measured  quantity  in  that  state,  t  the  temperature  of  that 
state  may  be  defined  as 


It  is  obvious  that  this  is  only  one  of  a  number  of  ways  in  which  a 
method  can  be  devised  for  assigning  a  number  to  the  temperature;  but 
it  certainly  is  the  simplest.  This  definition  of  t  leads  at  once  to  the 
proportion 

t   —  ti  _  a  —  «it 

tz  —  t\      0,2  —  a\ 

and  a  reason  for  adopting  it,  apart  from  its  simplicity,  is  that,  when 
various  "thermometers"  are  used,  experiments  made,  taking  advantage 
of  a  large  number  of  definite  thermal  states,  prove  that  the  values  of 
t  obtained  do  not  differ  widely.  Of  course  they  differ,  but  not  as  much 
as  they  would  if  any  less  simple  definition  were  adopted. 

In  this  connection  another  definition  is  useful;  this  is  the  "mean 


40  TEMPERATURE 

coefficient  of  change  of  a  between  t\  and  tz  with  reference  to  ti."     This 

is  obviously  A  and  may  be  written  a. 

a\  (iz  —  h) 

"Absolute  zero"  on  such  a  scale  is,  by  definition,  the  value  t  assumes 
when,  in  the  formula,  a  is  put  equal  to  zero.  (This  is  quite  regardless  of 
whether  or  not  it  is  possible  to  reduce  a  to  zero  by  any  physical  means.) 
Calling  this  calculated  value  td,  we  have 

/     -  /  ai        -  t      i  1 

to  —  iii  —  -•  -  —  '1 

dz  —  PI  a 

tz  —  t\ 

(?  The  "absolute  temperature"  on  such  a  scale  is  defined  to  be  t  —  t0; 
and,  calling  this  ta,  we  have 

.    _  1   ,   a   -  di 


a        0,2  —  d\ 

tz  —  t\ 

On  the  centigrade  system,  which  is  now  universally  adopted,  the  two 
standard  thermal  states  selected  are  those  of  "equilibrium  of  ice  and 
water"  and  "equilibrium  of  water  and  steam,"  both  under  a  pressure  of 
76  cm.  of  mercury;  "  and  to  this  interval  of  temperature  the  number  100  is 
given.  Hence,  on  this  centigrade  system, 

•   dz  —  o>i 
100  a! 


a  dz  —  d\ 

If  the  ordinary  scale,  not  the  absolute-one,  is  used,  it  is  necessary  to 
assign  also  a  number  to  either  of  the  standard  thermal  states;  and  in  all 
scientific  measurements  it  is  customary  to  give  the  number  zero  to  the 
temperature  of  "melting  ice."  Thus 

ir\f\     a      —    aO 

t  =  100 


—  do 
OOao 


Certain  obvious  facts  should  be  noted  : 

1.  If  different  properties  of  different  bodies  are  selected  for  thermo- 
metric  purposes,  that  is,  if  different  thermometers  are  used,  different 
values  of  t  will  be  obtained  for  the  same  thermal  state. 

2.  The  value  of  absolute  zero  will  be  different  for  different  thermometers. 

3.  The  value  of  absolute  zero  does  not  have  any  physical  importance. 

4.  To  say  that  a  body  expands  uniformly  with  change  in  temperature 
has  no  meaning  unless  the  particular  thermometer  used  to  give  the  numer- 
ical values  of  the  temperature  is  specified. 

Any  one  thermometer  may  be  calibrated  in  terms  of  another  and  the 
demands  of  scientific  statement  require  that  all  temperatures  quoted  in 


JOSEPH   S.    AMES  41 

memoirs  and  reports  should  refer  to  the  same  instrument.  In  order  to 
secure  this,  a  definite  temperature  scale  has  been  adopted  known  as  the 
"  Hydrogen  Scale,"  whose  specification  is  well  known. 

Certain  obvious  advantages  would  follow  if  a  method  of  thermometry 
could  be  devised  that  would  be  independent  of  the  nature  of  the  thermo- 
metric substance.  Lord  Kelvin  was  the  first  to  show  that  there  was  an 
indefinite  number  of  such  methods,  if  one  could  make  use  of  Carnot's 
principle.  If  in  a  Carnot  cycle  Q2  is  the  heat  absorbed  by  the  "  working 
substance"  at  one  temperature  and  Qi  that  given  out  at.  the  other, 
Carnot's  principle  is  that  the  ratio  Qz/Qi  is  independent  of  the  nature  of 
Ihe  working  substance,  being  a  function  only  of  the  temperatures  of  the 
two  baths  (or  reservoirs).  There  is  no  practical  way  by.  which  this 
simple  fact  may  be  used  directly  for  thermometric  purposes,  owing  to 
the  impossibility  of  securing,  in  practice,  a  Carnot  cycle;  but  Kelvin 
showed  how  indirect  methods  could  be  used.  He  finally  adopted  the  fol- 
lowing definition  of  a  thermometric  scale;  calling  the  temperatures  of 
the  two  baths  T2  and  TV 

"ft  =  Qi 

Either  a  definite  number  is  assigned  one  definite  thermal  state  or 
a  definite  number  is  assigned  a  definite  thermal  interval,  e.g.,  100  to  the 
interval  between  "ice"  and  "steam." 

He  then,  in  collaboration  with  Joule,  devised  a  method  by  which  his 
scale  could  be  compared  experimentally  with  that  given  by  any  constant- 
pressure  gas  thermometer.  This  method  depends  on  the  expansion  of 
the  gas  through  a  porous  plug,  due  to  a  small  drop  in  pressure,  and  the 
measurement  of  the  ratio  of  the  drop  in  temperature  to  the  drop  in  pres- 
sure. Calling  this  ratio  /i,  the  purpose  of  the  experiment  was  to  deter- 
mine n  as  a  function  of  temperature  and  pressure.  Kelvin  showed  from 
thermodynamic  reasoning  that,  if  /*  is  known  as  &f(t,p),  it  is  possible  to 
obtain  exact  numerical  relations  between  his  scale  and  the  gas  scale. 
He  also  showed,  by  approximate  methods,  that,  if  he  adopted  the  centi- 
grade system,  T  =  273  +  t  approximately  over  a  limited  range;  so 
that  in  the  "correction  terms"  in  the  formulas  t  may  be  replaced  by 
T  —  273.  The  best  modern  experiments  on  the  porous-plug  expansion, 
those  of  L.  G.  Hoxton  for  air  as  the  gas,  give  the  value  of  ju  over  the  range 
of  temperature  from  melting  ice  to  boiling  water.  Using  this  value, 
and  writing 

1  =  IT*  dT 

the  value  of  the  Kelvin  temperature  for  melting  ice  becomes 

i 

HP  T  r1    ( 7         —  T  \ 

1  -*  0-*  100         \jp  \-L  100        •*  O/  i 

T     -  J  -L  Po  A     i 

-tO—         T~  7XX —  —         ~T  *7 

a  lUOa  a 


42  TEMPERATURE 

where  Ti00  =  T0  +  100,  p0  =  density  of  air  at  melting  ice,  CP  is 
the  mean  value  of  the  specific  heat  of  air  at  constant  pressure,  and  a  is 
the  mean  coefficient  of  expansion  of  air  at  constant  pressure.  t\  is  a 
"correction  term;"  and  in  it  T0  may  be  placed  equal  to  273,  and  in  7, 
T  may  be  replaced  by  t  +  273.  This  gives  T0  =  273.36.  For  any  tem- 
perature, the  theory  gives 

TT  — 

T  -  To  =  t  +  r,at  -       JCP(7  -  70)  =  t  +  e 
Po 

where  e  is  the  correction  term. 
Two  facts  should  be  noted : 

1.  Kelvin's  theory  proves  that  there  is  a  definite  minimum  tempera- 
ture, which  may  properly  be  called  "absolute  zero;"  it  is  the  state  for 
which  T  becomes  equal  to  zero. 

2.  Any  temperature,  if  given  in  the  gas  scale,  may  be  expressed  on 
the  Kelvin  scale  provided  it  is  in  the  range  of  temperature  for  which  /* 
is  known. 

RADIATION   METHODS 

The  thermometric  methods  just  described  are  evidently  not  applicable 
to  extremely  hot  states,  e.g.,  the  carbon  poles  of  an  arc  light,  furnaces,  etc. 
Instead,  therefore,  of  attempting  to  secure  thermal  equilibrium  between 
a  thermometer  and  the  hot  body,  application  may  be  made  of  the  facts 
that  all  bodies  are  radiating  energy,  that  the  amount  radiated  varies 
with  the  temperature,  and  that  this  energy  may  be  measured.  It  is 
easily  observed  that  the  surfaces  of  different  bodies,  when  at  the  same 
temperature,  emit  different  amounts  of  radiation.  But  Kirchhoff  proved, 
from  theoretical  considerations,  that  if  a  hollow  is  made  in  a  solid  body 
maintained  at  a  constant  temperature,  the  radiation  out  through  a  small 
opening  in  the  wall  is  independent  of  the  nature  of  the  solid  and  is  a  func- 
tion of  its  temperature  alone.  (Of  course,  "freak"  cases  are  excluded.) 
Such  radiation  is  called  "black-body  radiation;"  and  it  is,  as  Kirchhoff 
proved,  greater  than  is  emitted  from  the  surface  of  any  body  at  the  same 
temperature,  provided  this  emission  is  due  solely  to  the  thermal  state  of 
the  body,  that  is,  is  not  due  to  what  has  been  called  "lumeniscence." 
If  it  were  possible  to  obtain  a  body  that  would  absorb  all  radiation  in- 
cident upon  it,  it  would  be  called  a  black  body  and  its  surface  would 
emit  the  same  radiation  as  the  enclosure  just  described,  at  the  same 
temperature. 

The  radiation  from  such  an  enclosure  is,  then,  a  measure  of  its  tem- 
perature, and  it  is  possible  to  define,  in  any  way  deemed  desirable,  a  new 
scale  of  temperature  t  in  terms  of  the  radiation  E  emitted  per  unit  of  time 
through  an  opening  of  a  definite  cross-section,  e.g.,  of  unit  area. 
Simple  observations  prove  that  E  increases  when  the  temperature  is 
increased;  so  possible  scales  would  be 


JOSEPH    S.    AMES  43 

t  =  AE,  where  A  is  an  arbitrary  constant 

t  =  AE2  +  BE,  where  both  A  and  B  are  constants,  etc. 

Naturally  one  would  select  such  a  definition  as  would  make  t  on  this 
new  scale  have,  if  possible,  the  same  value  as  t  has  on  some  standard 
scale,  e.g.,  a  gas  thermometer  using  the  centigrade  system,  for  those 
thermal  states  that  are  not  so  hot  as  to  preclude  the  use  of  such  a  conduc- 
tion instrument.  Experiments  have  shown  that  if  the  definition  adopted 
is 

E  =  A(t  +  273)4 

where  A  is  a  definite  constant,  this  condition  is  satisfied.  This  new 
scale  may  be  called  "radiation  temperature."  Obviously,  it  may  be  used 
for  temperatures  far  beyond  the  range  covered  by  a  gas  thermometer. 
Further,  if  the  radiation  from  the  surface  of  any  body  is  measured, 
this  same  formula  in  the  form  AS*  —  E  may  be  applied;  and  the  numerical 
value  of  S  may  be  deduced.  It  is  called  the  "black-body  temperature" 
of  the  body,  but  this  number  is  obviously  less  than  273  +  its  true  tem- 
perature. This  last  may  be  obtained  in  certain  cases,  however,  by  an 
indirect  method.  If  the  body  is  one  whose  properties  are  conditioned  by 
its  temperature,  we  have  the  law : 

Reflection  coefficient  +  transmission  coefficient  +  emissivity  =  1 
Let  us  assume  that  these  first  two  may  be  measured.  The  emissivity 
is,  by  definition,  the  ratio  of  the  emission  of  the  body  to  that  of  a  black 
body  at  the  same  temperature.  Hence,  calling  the  reflection  coefficient 
R,  the  transmission  coefficient  T,  the  black-body  temperature  of  the 
body  S, 

S* 

1       _    Z?    T7 

(t  +  273)4 

and  therefore  t  may  be  deduced. 

Other  radiation  methods  have  been  evolved.  If  that  part  of  the 
radiation  from  a  black  body  that  is  due  to  waves  having  wave-lengths  in 
the  range  from  X  to  X  +  AX  is  called  E^\,  experiments  prove  that  for 
radiation  in  the  visible  spectrum,  through  a  range  of  temperatures  where 
a  gas  thermometer  may  be  used. 


E^  =   d\~5e      M<  +  273) 

where  t  is  gas-thermometer  temperature  and  Ci  and  c2  are  definite  con- 
stants. Therefore  tjiis  formula  may  be  used  to  define  a  new  temperature 
scale,  specially  for  very  hot  bodies;  and  the  values  of  t  obtained  agree 
with  both  those  given  by  the  radiation  method  previously  described. 

Both  the  radiation  formulas  for  black  bodies: 

Total  radiation 

E  =  A(t  +  273) 4  (Stefan's  Law) 

and  partial  radiation 


44  TEMPERATURE 

£x  =  Cl\-5e  ~  \(t  +  273)  (Wien's  Law) 

have  a  thermodynamic  foundation.  It  may  be  proved  from  thermo- 
dynamic  reasoning,  with  certain  assumptions,  that  the  total  and  partial 
radiations  from  a  black  body  obey  the  laws 

E  =  AT*  and  #x  =  ClX-5e~  M? 

where  T7  is  Kelvin's  absolute  temperature.  Therefore  radiation  methods 
will  give  us  -a  knowledge  of  Kelvin's  temperatures  for  very  hot  bodies; 
for,  as  stated  before,  in  order  to  define  the  Kelvin  scale,  it  is  sufficient  to 
assign  an  arbitrary  number  to  some  one  thermal  state,  e.g.,  373.36  to 
that  of  "boiling  water;"  then  E  may  be  measured  for  a  black  body  having 
that  temperature  and  A  may  be  deduced;  or,  using  two  known  values  of 
T  for  definite  thermal  states,  ci  and  c2  may  be  deduced;  consequently  the 
constants  being  known,  T  will  be  given  by  measurements  of  E  or  E^. 

The  results  of  experiments  on  porous-plug  expansion  are  to  show  that 
over  the  range  of  temperatures  between  melting  ice  and  boiling  water 
T  =  t  +  273,  approximately,  where  i  is  gas  temperature  on  the  centi- 
grade system,  and  radiation  experiments  prove  that  the  same  is  true 
approximately  for  higher  temperatures,  as  far  as  the  gas  thermometer  may 
be  used,  that  is,  the  constants  in  the  two  radiation  formulas  are  found  to 
be  the  same  when  obtained  by  either  process. 

THEORY  OF  TEMPERATURE 

In  the  discussion  of  both  conduction  and  radiation  methods,  care, 
has  been  taken  to  exclude  from  consideration  bodies  in  which  chemical 
or  electrical  actions  were  going  on  and  bodies  that  were  not  in  what  has 
been  called  statistical  equilibrium.  The  question  now  arises  as  to  whether 
or  not  it  is  allowable  to  speak  of  the  temperature  of  such  bodies.  Light 
is  thrown  upon  this  from  the  dynamical  study  of  the  properties  of  bodies, 
considering  them  formed  of  molecules.  It  has  been  proved  that  for  a 
gas  in  a  state  of  equilibrium,  that  is,  when  it  is  not  flowing  or  in  a  turbu- 
lent condition,  there  is  an  intimate  connection  between  its  mean  kinetic 
energy  per  molecule,  and  its  temperature.  If  m  is  the  mass  of  each  of 
its  molecules,  and  if  u,  v,  w  are  the  components  of  the  velocity  of  the  center 
of  mass  of  any  one  molecule,  we  may  write  for  the  mean  kinetic  energy 
of  translation  of  the  molecules  ^(mu2  +  mv2  +  raw2) 

It  has  been  proved  that  mu2  =  mv2  =  mwz  =  RT  where  R  is  a  definite 
constant  and  T  is  Kelvin's  temperature.  (If  we  adopt  the  centigrade 
system  and  use  other  deductions  from  the  kinetic  theory  as  applied  to 
actual  gases,  it  is  found  that  R  =  9.3  X  10~17  approximately.)  Further, 
if  the  molecules  have  kinetic  energy  of  rotation,  each  of  the  "  degrees  of 
freedom"  has  an  amount  of  mean  kinetic  energy  equal  to  %RT.  So 
far  as  the  mean  kinetic  energy  of  translation  is  concerned,  this  equals, 


JOSEPH    S.    AMES  45 

therefore,  %RT;  and  the  same  is  true  of  solids,  liquids,  and  the  "free 
electrons"  in  solids. 

It  is  clear,  then,  that  for  a  body  not  in  a  state  of  statistical  equilibrium 
it  is  not  allowable  to  use  the  word  temperature.  This  means  that  the 
word  should  not  be  used  with  reference  to  a  single  molecule  or  to  such 
phenomena  as  occur  in  most  flames,  or  in  an  electric  spark  or  discharge. 
It  is  true  that  bodies  placed  in  flames,  sparks,  etc.,  may  assume  definite 
temperatures,  but  this  does  not  affect  the  statement  just  made. 

From  the  standpoint  of  experiment,  the  fundamental  question  is  to 
determine  when  a  body  is  in  a  state  of  statistical  equilibrium  and,  there- 
fore, has  its  properties  conditioned  by  temperature.  Unfortunately 
this  cannot  be  done  with  certainty  in  all  cases;  general  principles  are  the 
only  guides. 

There  are,  however,  certain  cases  in  which  the  body  is  not  in  thermal 
equilibrium,  and  where  the  word  "temperature"  may  be  used.  Consider 
a  vessel  of  water  placed  on  a  stove,  we  say  that  its  "temperature  rises," 
thus  attaching  a  meaning  to  the  word  at  any  instant.  Again,  consider 
a  current  of  air  moving  with  a  uniform  velocity,  if  a  thermometer  were 
to  move  with  the  gas  at  the  same  velocity,  it  would  register  the  true 
temperature  of  the  gas;  but  this  temperature  is  not  that  which  would  be 
indicated  by  a  thermometer  at  rest  in  the  moving  gas,  nor  is  it  the  tem- 
perature of  the  solid  walls  of  the  tube  through  which  the  gas  is  flowing. 
We  may  often  think  of  certain  limits  of  temperature  between  which  a 
certain  body  must  lie  when  it  is  not  in  a  condition  of  equilibrium;  thus, 
one  limit  would  be  that  which  corresponds  to  its  mean  molecular  kinetic 
energy  of  translation.  But,  in  general,  it  is  safest  to  limit  the  use  of  the 
word  temperature  to  bodies  in  the  state  of  equilibrium  and  even  then  to 
those  bodies  for  which  there  is  reason  for  believing  that  their  state  is 
conditioned  by  their  mean  molecular  kinetic  energy. 


46  STANDARD  SCALE  OF  TEMPERATURE 


Standard  Scale  of  Temperature 

BY   C.    W.    WAIDNER,    E.    F.    MUELLER,    AND    PAUL   D.    FOOTE,    WASHINGTON,    D.    C. 
(Chicago  Meeting,  September,  1919) 

THE  standard  scale  of  temperature  that  it  is  attempted  to  realize  in 
practice  is  the  centigrade  thermodynamic  scale,  as  defined  by  Kelvin 
about  the  middle  of  the  last  century.  This  scale  would  be  exactly 
realized  with  a  perfect1  gas  in  an  ideal  gas  thermometer,  and  is  closely 
realized  with  the  more  permanent  gases.  From  the  departure  of  certain 
properties  of  these  gases  from  those  of  a  perfect  gas,  it  is  possible  to  deduce 
the  amount  of  the  departure  of  the  temperature  scales,  defined  by  their 
use,  from  the  thermodynamic  scale. 

In  nearly  all  the  precise  experimental  work  done  during  the  latter 
part  of  the  last  century,  it  was  necessary  for  the  experimenter  to  estab- 
lish his  own  gas-thermometer  scale,  and  the  labor  involved  in  this  was 
often  greater  than  that  involved  in  the  prime  object  of  the  experimental 
work  undertaken.  The  experimental  difficulties  involved  in  gas  ther- 
mometry  led  to  the  establishment  of  temperature  scales  differing  from  one 
another  by  amounts  considerably  greater  than  could  be  accounted  for 
by  the  differences  in  the  properties  of  the  gases  used.  Results  were, 
therefore,  expressed  in  terms  of  somewhat  different  temperature  scales, 
making  correlation  of  such  results  difficult  and  uncertain. 

INTERNATIONAL  HYDROGEN  SCALE 

The  first  standard  temperature  scale  to  receive  fairly  general  scientific 
recognition  was  the  so-called  international  hydrogen  scale  defined  by 
the  following  resolution  of  the  International  Committee  on  Weights  and 
Measures,  adopted  Oct.  15,  1887.  "The  International  Committee  on 
Weights  and  Measures  adopts  as  the  standard  thermometric  scale  for  the 
international  service  of  weights  and  measures,  the  centigrade  scale  of  the 
hydrogen  thermometer,  having  as  fixed  points  the  temperature  of  melting 
ice  (0°)  and  of  the  vapor  of  distilled  water  boiling  (100°)  at  standard 
atmospheric  pressure,  the  hydrogen  being  taken  at  an  initial  manometric 

pressure  of  1  meter  of  mercury,  that  is  to  say,   7^,ft   =  1.3158  times  the 

i  OU 

standard  atmospheric  pressure." 


1  Buckingham:  U,  S.  Bureau  of  Standards  Bull.  6  (1910)  409.     (Bureau  of  Stand- 
ards Sci.  Paper  136.) 


C.    W.    WAIDNER,    E.    F.    MUELLER,    AND    PAUL   D.    FOOTE  4V 

In  practice,  this  scale  in  the  interval  —  25°  to  +  100°  has  been 
realized  and  made  available  by  means  of  primary  standard  mercurial 
thermometers  made  of  French  hard  glass  (verre  dur).  Eight  thermome- 
ters, four  in  the  range  below  0°  and  four  in  the  range  0°  to  100°,  were 
compared  by  Chappuis2  with  the  hydrogen  gas  thermometer.  In  this 
way  Chappuis  determined  the  difference  between  the  mean  verre  dur  scale 
defined  by  the  eight  standard  thermometers  and  the  scale  defined  by  the 
hydrogen  thermometer. 

The  scale  is  distributed  to  the  scientific  world  by  the  International 
Bureau  of  Weights  and  Measures  through  verre  dur  thermometers,  the 
several  corrections  (calibration,  internal  and  external  pressure  coeffi- 
cients, and  fundamental  interval3)  for  which  have  been  determined  by 
that  Bureau.  The  assumption  is  made  that  the  verre  dur  scale  defined  by 
these  standardized  thermometers  differs  from  the  hydrogen  scale  by  the 
same  amount  as  does  the  mean  scale  of  the  thermometers  originally 
standardized  by  Chappuis.  The  thermometers  distributed  by  the  Inter- 
national Bureau  are  but  rarely  compared  with  the  primary  standards 
of  that  Bureau  to  determine  to  what  extent  the  scale  defined  by  the  newer 
thermometers  differs  from  that  of  the  original  standards.  It  has  been 
found  that  the  scales  defined  by  two  different  thermometers  of  verre 
dur  may  differ  from  each  other  by  over  0.01°. 

PROPOSED  INTERNATIONAL  SCALE  OF  TEMPERATURES  IN  THE  INTERVAL 

-  40°  TO  1100°  C. 

When  the  several  national  laboratories4  undertook  the  standardiza- 
tion of  temperature-measuring  instruments,  it  became  necessary  to 
extend  the  limited  temperature  scale  distributed  by  the  International 
Bureau  to  higher  temperatures  on  the  one  hand  and  to  lower  temperatures 
on  the  other.  The  method  used  by  all  of  the  laboratories  in  extending 
the  standard  temperature  scale  consisted  in  the  adoption  of  certain  fixed 
points  defined  by  the  freezing  (melting)  or  boiling  points  of  certain  pure 
substances,  these  fixed  points  having  been  determined  on  the  scale  of 
some  gas  thermometer  by  a  number  of  different  observers.  According 
to  the  interpretation  of  the  available  data,  small  differences  in  the  tem- 
perature scales  adopted  by  the  national  laboratories  thus  resulted.  With 
a  view  to  reaching  international  uniformity  in  the  temperature  scales 
employed,  correspondence  was  carried  on  between  the  several  laborato- 

2  Trav.  et  Mem.,  Bur.  Int.  (1888)  6. 

3  For  discussion  of  these  corrections  see  Waidner  and  Dickinson,  U.  S.  Bureau  of 
Standards    Bull.   3    (1907)  667  (Bureau  of   Standards  Sri.  Paper  69)  or  Bureau  of 
Standards  Circular  8,  2d  Edition,  20. 

4  The  Physikalisch-Technische   Reichsanstalt  at  Charlottenburg,  Germany;  the 
National  Physical  Laboratory  at  Teddington,  England,  and  the  National  Bureau  of 
Standards  at  Washington. 


48  STANDARD  SCALE  OF  TEMPERATURE 

ries  during  the  years  1912  to  1914.  When  these  negotiations  were  inter- 
rupted by  the  war,  agreement  had  been  reached  on  the  essential  principles 
to  be  followed  and  on  most  of  the  necessary  details.  The  plan  practi- 
cally agreed  upon,  but  which  failed  to  reach  the  stage  of  ratification, 
was  similar  to  that  followed  in  defining  the  electric  units.  It  was  agreed 
to  define  as  the  fundamental  scale,  the  centigrade  thermodynamic  scale 
and,  in  addition,  to  establish  a  practical  or  working  scale  that  should 
realize  this  fundamental  scale  as  closely  as  was  possible  in  the  existing 
state  of  knowledge.  The  practical,  or  working,  scale  that  had  been  out- 
lined in  the  correspondence  has  served  as  the  working  scale  of  the  several 
laboratories  since  1914,  although  there  are  a  few  minor  differences 
due  to  the  fact  that  final  agreement  was  not  reached.  In  December, 
1915,  the  Reichsanstalt,  through  its  President,  Warburg,5  announced 
that  after  Apr.  1,  1916,  it  would  adopt  a  new  temperature  scale.  In 
the  interval  —  40°  to  1100°,  this  scale  is  practically  identical  with  that 
outlined  in  the  correspondence  between  the  laboratories. 

Temperature  Scale  in  the  Interval  —  40°  to  450°  C.  —  In  the  interval 
-  40°  to  450°  G.,  the  temperature  scale  now  used  is  that  defined  by  the 
platinum-resistance  thermometer,  calibrated  at  the  temperature  of  melt- 
ing ice  (0°),  the  temperature  of  saturated  steam  (100°),  and  of  sulfur  vapor 
(444.6°)  all  under  standard  atmospheric  pressure.6 

The  relation  between  the  resistance  R,  the  platinum  temperature  and 
the  temperature  t  is  expressed  by  the  Callendar  formulas 

•n/  -     2LH  ^° 

pt  —  ^~    —  =- 

jtlioo  —  **« 


The  purity  of  the  platinum  is  specified  by  the  mean  temperature  co- 
efficient of  resistance  between  0°  and  100°,  which  coefficient  should  be  not 
less  than  0.00388  and  by  the  constant  5  in  the  above  formulas,  which 
should  be  not  greater  than  1.52. 

*Ann.  Phys.  (4)  (1915)  48,1034. 

6  The  standard  atmospheric  pressure  is  defined  as  equivalent  to  the  pressure 
exerted  by  a  column  of  mercury  760  mm.  in  height,  the  mercury  being  at  0°  C.  and  sub- 
ject to  a  gravitational  force  corresponding  to  g  =  980.665  cm.  per  sec. 

Previous  to  1914  the  only  precise  formulas  for  expressing  the  variation  with  pres- 
sure of  the  boiling  point  of  sulfur  were  these  of  Holborn  and  Henning  and  of  Marker 
and  Sexton.  These  are  respectively: 

t  •=  t7to  +  0.0910  (p  -  760)  -  0.000043  (p  -  760)  2 
t  =  <76o  +  0.0904  (p  -  760)  -  0.000052  (p  -  760)  2 

In  the  correspondence  between  the  national  laboratories,  it  had  been  agreed  that 
the  equation  t  =  tlto  +  0.0908  (p  —  760)  -  0.000047  (p  -  760)  2  best  represents  the 
relation.  Later  experimental  work  at  the  Bureau  of  Standards  (published  in  Jnl. 
Am.  Chem.  Soc.  41,  759,  1919)  led  to  the  equation 

t  =  <76o  +0.0910  (p  -  760)  -  0.000049  (p  -  760)2 


C.    W.    WAIDNER,    E.    F.    MUELLER,    AND   PAUL   D.    FOOTE  49 

The  sulfur  boiling  point  is  to  be  determined  in  the  conventional  form 
of  sulfur-boiling  apparatus  originally  used  by  Callendar  and  Griffiths. 
Full  details  concerning  the  experimental  methods  to  be  used  and  the 
precautions  to  be  observed  have  been  published  by  several  investigators, 
the  most  recent  being  Meissner7  and  Mueller  and  H.  A.  Burgess.8  Day 
and  Sosman9  have  summarized  the  more  important  determinations  of 
the  temperature  of  the  sulfur  boiling  point.  To  their  table  should  be 
added  the  determination  made  by  Eumorfopoulous10  in  1914.  Their 
table  shows  that  determinations,  previous  to  1908,  indicate  a  value  of 
about  444.9°  C.  on  the  thermodynamic  scale,  while  the  later  determina- 
tions indicate  a  value  somewhat  higher  than  444.5°.  It  appears  that  if 
any  weight  whatever  is  to  be  given  to  any  determinations  other  than  those 
of  Holborn  and  Henning  and  Day  and  Sosman,  the  value  444.6°  should 
be  chosen  in  preference  to  444.5°. 

However,  since  final  ratification  of  the  agreement  on  the  sulfur 
boiling  point  had  not  been  reached,  the  several  laboratories  are  not 
using  identical  values  for  this  constant.  The  National  Physical  Labora- 
tory has  apparently11  not  changed  from  the  old  value  of  444.53°,  as  de- 
termined by  Callendar  and  E.  H.  Griffiths,  on  the  scale  of  the  constant- 
pressure  air  thermometer.  This  temperature  is  now  interpreted  by 
that  laboratory  as  being  on  the  thermodynamic  scale.  The  announce- 
ment of  the  Reichsanstalt,  already  referred  to,  shows  that  that  institu- 
tion is  using  444.55°  as  the  temperature  of  the  sulfur  boiling  point,  while 
the  Bureau  of  Standards,  for  reasons  given  above,  is  using  444. 6Q.  A 
change  of  0.1°  in  the  temperature  chosen  for  the  sulfur  boiling  point 
corresponds  to  a  change  of  about  0.4  per  cent,  in  d,  which  would  affect 
the  temperature  scale  by  less  than  0.002°  at  50°  C. 

It  is  worthy  of  note  that  the  several  national  laboratories  have,  in 
effect,  abandoned  the  international  hydrogen  scale  either  as  an  ultimate 
standard  or  as  a  working  standard.  It  is,  therefore,  a  matter  of  great 
interest  to  determine  whether,  in  actual  practice,  the  scale  used  by  these 
laboratories  differs  measurably  from  the  hydrogen  scale  of  the  Inter- 
national Bureau.  The  results  of  numerous  unpublished  intercompari- 
sons  of  the  scale  defined  by  platijnum  resistance  thermometers  and  the 
hydrogen  scale,  derived  from  the  mean  of  a  large  number  of  verre  dur 
thermometers,12  show  that  the  scale  defined  by  the  platinum  resistance 


7  Ann.  Phys.  [4]  (1912)  39,  1230. 

8  Jnl.  Am.  Chem.  Soc.  (1919)  41,  745. 

9  Am.  Jnl.  Sci.  (1912)  33,  530;  Ann.  Phys.  (1912)  38,  865. 

10  Proc.  Royal  Soc.  London    (1914)  A  90,  189. 

11  See  Ezer  Griffiths:  "Methods  of  Measuring  Temperature,"  8.     Charles  Griffin 
&  Co.,  1918. 

12  Waidner  and  Dickinson:  Op.  cit. 

4 


50  STANDARD  SCALE  OF  TEMPERATURE 

thermometer  is  in  agreement  with  the  hydrogen  scale  of  the  International 
Bureau  to  well  within  the  limits  of  accuracy  with  which  the  latter  scale 
has  been  distributed  to  the  scientific  world  by  means  of  standardized 
primary  standard  verre  dur  thermometers.  The  advantage  of  using  the 
platinum  resistance  thermometer  to  define  the  standard  scale  lies  in  the 
fact  that  platinum  of  the  required  purity  is  readily  available,  that  its 
purity  is  easily  verified,  and  that  the  standard  scale  in  the  interval  0° 
to  100°  can  be  reproduced  in  any  part  of  the  world  to  an  accuracy  of 
0.002°  or  0.003°. 

It  may  be  noted  that  the  adoption  of  the  platinum  resistance  ther- 
mometer to  define  the  standard  scale  of  temperature  embodies  the  essen- 
tial features  of  the  practical  thermometric  standard  proposed  by  Callendar 
to  the  British  Association  in  1899.13 

A  number  of  fixed  points  within  the  above  range,  useful  for  the  cali- 
bration of  temperature  measuring  instruments  other  than  standard  resist- 
ance thermometers  had  been  practically  agreed  upon,  as  shown  in  Table  1. 
The  freezing  points  of  cadmium  and  zinc  are  convenient  lower  fixed  points 
for  the  calibration  of  high  temperature  thermocouples. 

TABLE  I.— Fixed  Points  in  the  Interval  -  40°  to  450°  C. 

DEGREES 

Freezing  point  of  mercury —  38.88 

Freezing  point  of  tin 231 . 84° 

Freezing  point  of  cadmium 32Q. 9 

Freezing  point  of  zinc 419.4° 

Boiling  point  of  naphthalene 217.96  +  0.058  (p  -  760) 

Boiling  point  of  benzophenone 305.9    +  0.063  (p  -  760)° 

•  Recent  determinations  with  the  resistance  thermometer  calibrated  at  0°,  100°, 
and  the  boiling  point  of  sulfur  (444.6°),  of  the  freezing  points  of  very  pure  specimens 
of  tin  and  zinc  lead  to  the  values  231.88°  and  419.44°,  respectively.  It  is  apparently 
difficult  to  produce  benzophenone  of  sufficient  purity  to  serve  for  reproducing  the  stand- 
ard temperature  and  so  far  as  known  the  temperature  given  is  obtained  only  with 
Kahlbaum's  purest  material.  See  U.  .S.  Bureau  of  Standards  Bull.  7  (1910)  5. 
(Bureau  of  Standards  Sri.  Paper  143.) 

Temperature  Scale  in  the  Interval  450°  to  1100°  C. — In  the  interval 
450°  to  1100°  C.,  the  practical  scale  is  defined  by  the  fixed  (freezing) 
points  given  in  Table  2,  which  are  practically  identical  with  the  values 
of  Day  and  Sosman14  when  reduced  to  the  thermodynamic  scale,  the 
resulting  values  being,  in  general,  rounded  off  in  the  direction  of  higher 
temperatures  in  view  of  the  fact  that  the  previously  accepted  values  for 
these  fixed  points  were  higher. 

13  Report  British  Assn.  Dover  (1899)  242. 

14  Am.  JnL  Sti.  (1910)  29,  93;  (1912)  33,  517.     Also  L.  H.  Adams:  Jnl.  Am. 
Chem.  Soc.  (1914)  36,  65. 


C.    W.    WAIDNER,    E.    F.    MUELLER,    AND    PAUL   D.    FOOTE  51 

TABLE  2. — Standard  Freezing  Points0 

DEGREES 

Antimony 630 

Silver 960. 5 

Gold 1063 

Copper,  free  from  oxide -. 1083 

0  The  freezing  point  given  for  antimony  is  apparently  attained  only,  with  the 
pure  metal  supplied  by  Kahlbaum.  See  U.  S.  Bureau  of  Standards  Bull.  6  (1910) 
164.  (Bureau  of  Standards  Sci.  Paper  124.)  Final  agreement  was  not  reached  as  to 
whether  960.5°  or  961°  is  the  better  value  to  be  taken  as  the  freezing  point  of  silver. 
The  Reichsanstalt  announcement  gives  960.5°. 

The  instrument  commonly  employed  for  interpolation  between  these 
fixed  points  is  the  platinum,  90-per  cent,  platinum,  10-per  cent,  rhodium 
thermocouple  calibrated  at  three  temperatures;  the  relation  between 
temperature  t  and  electromotive  force  e  being  expressed  by  the  formula 
e  =  a  +  bt  +  dz.  The  points  usually  chosen  for  the  standardization 
of  these  couples  are  the  freezing  points  of  zinc  (or  cadmium),  antimony, 
and  copper.  Other  convenient  secondary  calibration  points  in  the  in- 
terval are  aluminum  (freezing  point)  658.7°  for  a  sample  99.66  per  cent, 
pure,  and  sodium  chloride  (freezing  point)  801°  C.  The  freezing  point 
of  the  latter  substance  is  less  sharply  defined  and  is  not  quite  as  satis- 
factory as  a  fixed  point  as  are  the  freezing  points  of  the  metals. 

It  is  a  rather  remarkable  fact  that  the  scale  defined  by  the  platinum 
resistance  thermometer  calibrated  in  ice,  steam,  and  sulfur  vapor  (444.6°) 
is  in  agreement  up  to  1100°  with  the  scale  defined  by  the  thermocouple 
calibrated  at  three  points  as  above  described.15 

Temperature  Scale  Above  1100°  C. — In  the  correspondence  between  the 
national  laboratories,  no  attempt  had  been  made  to  agree  on  a  uniform 
scale  above  1100°  C.  The  only  gas-thermometer  data  that  could  receive 
consideration  for  the  purpose  of  establishing  a  standard  temperature 
scale  above  1100°  is  that  of  Day  and  Sosman.  Whatever  temperature  is 
considered  as  the  upper  limit  of  precise  gas  thermometry,  it  is  evident 
that  the  scale  above  this  limit  must  be  based  on  physical  laws  having 
sound  theoretical  or  experimental  support.  The  method  generally 
followed  was  to  extend  the  thermometric  scale  from  the  region  of  known 
temperatures,  as  determined  by  the  gas  thermometer,  by  the  aid  of  the 
radiation  laws.  The  two  laws  most  satisfactory  for  this  purpose  are 
represented  by  the  Planck  and  the  Stefan-Boltzmann  equations.  Of 
these  two  laws  the  latter  has  a  stronger  theoretical  basis,  but  has  been  less 
frequently  applied  on  account  of  the  greater  difficulties  of  experimental 
manipulation  involved.  The  former,  for  the  wave-lengths  of  the  visible 
spectrum,  reduces  to  the  well-known  Wien  formula  from  which  it  follows 
that 

15  Waidner  and  Burgess:  U.  S.  Bureau  of  Standards  Bull.  6  (1910)  182.  (Bureau 
of  Standards  Sci.  Paper  124). 


52  STANDARD  SCALE  OF  TEMPERATURE 


l        1\ 

~  ej 


c2logi<,e 

=       x 

where  Ji  and  J2  are  the  intensities  of  black-body  radiation  of  wave-length 
X  at  absolute  temperatures,  6\  and  02,  respectively,  and  c2  is  the  funda- 
mental constant  of  the  Planck  equation. 

It  will  be  seen  from  this  equation  that  any  high  temperature  02 
(absolute)  may  be  determined  from  a  measurement  of  the  ratio  of  the  in- 
tensity of  black-body  radiation  for  a  given  wave-length  X  to  the  intensity 
of  black-body  radiation  for  the  same  wave-length  at  some  accu- 
rately determined  lower  temperature  0i  (absolute).  The  temperature 
scale  will  depend  on  the  value  chosen  for  c2,  the  constant  of  the  Planck 
equation.  In  all  of  the  earlier  work,  the  value  taken  for  this  constant 
was  14,500  or  14,600  micron  degrees.  In  1906,  Holborn  and  Valentiner16 
published  the  results  of  an  investigation  that  led  them  to  assign  the 
value  14,200  to  this  constant.  The  large  difference  between  this  value 
and  the  ones  previously  accepted  led  to  further  extensive  researches  at 
the  Reichsanstalt  and  at  the  Bureau  o£  Standards.  A  summary  of  this 
later  work  may  be  found  in  a  paper  by  Coblentz.17  As  a  result  of  this 
more  recent  work,  the  Bureau  has  been  led  to  adopt  14,350  for  the  value 
of  the  constant  c2  of  the  Planck  equation.  The  Reichsanstalt  is  using 
the  value  14,300  for  c2. 

The  most  important  fixed  points  in  the  range  above  1100°  C.  are  the 
melting  points  of  palladium  and  platinum.  Some  of  the  more  important 
experimental  work  that  has  been  done  in  the  determination  of  these 
fixed  points  is  discussed  below. 

Melting  Point  of  Palladium.  —  Day  and  Sosman  extended  their  gas- 
thermometer  measurements  to  the  melting  point  of  palladium.  Using 
a  crucible  method,  the  e.m.f.  of  standard  rare-metal  thermocouples  was 
obtained  at  the  melting  point  of  palladium  and  the  couples  were  standard- 
ized by  direct  comparison  with  the  gas  thermometer.  The  couples, 
accordingly,  served  only  as  a  means  of  transfer  from  the  crucible  of 
palladium  to  the  furnace  containing  the  thermometer  bulb.  The  result 
of  the  single  experiment  attempted  at  this  extreme  temperature  for  the 
gas  thermometer  gave  1549.2  +  2.0°  C.  for  the  melting  point  of  palladium. 
To  this  value  may  be  added  a  rather  uncertain  correction  of  about  0.8° 
to  take  account  of  the  deviation  of  the  temperature  scale  defined  by  the 
constant-volume  nitrogen  gas  thermometer,  at  the  pressures  employed, 
from  the  ideal  thermodynamic  scale.  As  a  final  corrected  value  we  thus 
obtain  1550°  ±  2°  C.  A  chemical  analysis  of  the  palladium  showed  that 
it  was  of  extremely  high  purity.  This  value  of  1549°  C.  was  used  quite 
generally  for  several  years.  Since  1916,  however,  the  Reichsanstalt18 

"  Ann.  Phys.  [4]  (1906)  22,  1. 

17  U.  S.  Bureau  of  Standards  Bull.  13  (1917)  459.  (Bureau  of  Standards  Sci. 
Paper  284.)  1S  Loc.  cit. 


C.    W.    WAIDNER,    E.    F.    MUELLER,    AND    PAUL   D.    FOOTE  53 

has  adopted  the  value  1557°  C.  for  the  melting  point  of  palladium,  and 
1764°  C.  instead  of  1755°  C.  for  the  melting  point  of  platinum.  More 
recently  the  laboratories  of  the  General  Electric  Co.19  have  agreed  to  use 
the  value  1555°  C.  for  the  melting  point  of  palladium. 

The  evidence  that  the  determination  of  Day  and  Sosman  is  low 
is  based  on  the  data  obtained  by  extrapolation  of  the  optical  temperature 
scale  using  Wien's  (or  Planck's)  law.  The  melting  point  of  gold  is  gen- 
erally agreed  upon  as  1063°  C.  Hence  if  we  measure  for  a  given  wave- 
length the  ratio  of  the  brightness  of  a  black  body  at  the  temperature  of 
melting  palladium  to  that  of  a  black  body  at  the  temperature,  of  melting 
gold,  knowing  c2,  the  melting  point  of  gold,  and  the  value  of  this  ratio 
R,  the  temperature  of  the  melting  palladium  may  be  computed  by  the 
formula  already  given  : 


•          (D 

where 

OAU  =  absolute  temperature  of  melting  gold; 

6pd   =  absolute  temperature  of  melting  palladium  ; 

Jpd  =  intensity  of  radiation  for  wave-length  X  from  a  black 

body  at  a  temperature  Op*; 

JAu  =  intensity  of  radiation  for  wave-length  X  from  a  black 
body  at  a  temperature  6Au- 

If  in  this  formula  we  substitute  the  values  c2  =  14,350  and  6Au  =  1336, 
we  obtain: 

.  A  =  0.0007485  -***  '       '      (2) 

Nernst  and  V.  Wartenberg20  measured  the  value  of  the  ratio  R  for 
X  =  0.5896/i  by  means  of  a  Wanner  pyrometer,  which  is  essentially 
the  Konig-Martens  spectrophotometer.  The  gold  or  palladium  in  the 
form  of  wire  was  gradually  heated  in  an  iridium-tube  furnace.  The 
melting  point  was  indicated  by  the  breaking  of  an  electric  circuit  of 
which  the  wire  formed  a  part.  The  value  thus  found  was  R  =  131. 
Substituting  this  value  in  formula  2  we  obtain  6Pd  =  1551°  C.  on  the  basis 
of  6AU  =  1063°  C.  and  c2  =  14,350. 

Holborn  and  Valentiner21  using  the  wire  method,  an  iridium  furnace, 
and  a  Lummer-Brodhun  spectrophotometer  obtained  the  following 
values  of  R  from  which  are  recomputed,  in  column  3,  on  the  basis  of  c2  = 
14,350,  the  corresponding  values  of  the  melting  point  of  palladium.  The 
mean  1573°  C.  is  confirmed  by  their  work  with  the  gas  thermometer, 

19  Hyde:  Gen.  Elec.  Rev.  (Oct.,  1917).  80  Verh.  Phys.  Ges.  (1906)  8,  48-58. 

21  Loc.  tit. 


54  STANDARD  SCALE  OF  TEMPERATURE 

which  gave  a  final  value  of  1575°  C.  It  is  impossible  to  account  for  these 
high  determinations  since  the  experimental  work,  at  least  from  the  optical 
side,  appears  most  carefully  done. 

x  R  PC. 

0.656  93.1  1575 

92.6  1574 

90.3  1570 

0.590  153.5  1574 

0.546  229.0  1573 


Mean 1573 

Hoffmann  and  Meissner22  have  performed  a  series  of  experiments, 
only  the  summary  of  which,  however,  has  been  published.  Using  a 
spectrophotometer  at  various  wave-lengths  and  using  black  bodies 
immersed  in  baths  of  freezing  gold  and  of  palladium  as  well  as  employing 
the  usual  wire  method,  the  following  values  of  R  reduced  to  a  common 
wave-length  were  obtained.  In  the  third  column  is  given  the  corre- 
sponding melting  point  of  palladium  computed  for  c2  =  14,350.  On 
the  basis  of  R  =  81.5  for  X  =  0.6563/*  and  c2  =  14,300,  the  above  referred 
to  temperature  scale  of  Reiehsanstalt  was  adopted. 

x  R  >  c. 

0.6563*1    .  80.5  1552 

81.4  1554 

81.6  155.4 

The  work  of  Hyde,  Cady,  and  Forsythe,23  which  forms  the  basis  for 
the  adoption  by  the  General  Electric  Co.  of  the  value  1555°  C.  for  palla- 
dium, has  never  been  published  in  detail.  They  state  that  for  the  effect- 
ive wave-length  of  a  piece  of  red  glass  "slightly  thicker  than  the  standard 
sample  here  investigated,"  for  which  this  wave-length  was  determined 
"as  a  result  of  ten  determinations  extending  over  a  year  and  a  half,  the 
ratio  76.9  was  obtained."  The  wave-length24  employed  was  X  = 
0.6661/i  (private  letter  to  the  authors).  Hence  on  the  basis  of  gold  = 
1063°  C.  and  c2  =  14,350,  this  ratio  gives  1555°  C.  for  the  melting  point 
of  palladium. 

Mendenha.ll,26  using  the  wire  method  and  a  spectral  pyrometer  and 
assuming  the  melting  points  of  gold  and  palladium  to  be  1063°  C.  and 
1549°  C.  respectively,  obtained  c2  =  14,413  from  measurements  of  the 


"Zeitf.  Inst.  (1912)  32,  201;  (1913)  33,  95,  157. 

23  Astrophys.  Jnl  (1915)  42,  300. 

24  For  methods  of  computing  and  for  definition  of  effective  wave  length  see  Hyde, 
Cady  and  Forsythe,  loc.  cit.  and  Footer  U.  S.  Bureau  of  Standards  Bull.  12  (1915) 
483-501. 

**Phys.  Rev.  [2]  (1917)  10,  522. 


C.    W.    WAIDNER,    E.    F.    MUELLER,    AND    PAUL   D.    FOOTE 


55 


ratio  R.  The  actual  values  of  R  are  not  given  but  the  data  permit  the 
calculation  of  R  for  any  arbitrary  wave-length.  Thus,  for  X  =  0.65/t 
the  foregoing  data  require  that  R  =  83.7.  Using  this  value  of  R  and 
c2  =  14,350,  we  find  from  formula  2  that  BPd  =  1825°  absolute  or  1552°  C- 

In  another  series  of  experiments  in  which  the  ratio  of  brightnesses  of 
a  black  body  at  a  temperature  61  =?  1604°  absolute,  and  a  black  body  at  a 
temperature  02  =  2734°  absolute  was  measured,  the  value  c2  =  14,394 
was  obtained.  Since  assigning  values  to  6Au,  6Pd  =  1822°  absolute  and 
c2  fixes  the  ratio  JPa/  JAU  for  any  color,  this  ratio  may  be  computed  for 
c2  =  14,394.  Using  the  ratio  thus  obtained  6Pd  may  be  recomputed  for 
c2  =  14,350.  This  gives  1551°  C.  for  the  melting  point  of  palladium. 
The  temperature  6\  was  defined  in  terms  of  the  Day  and  Sosman  scale 
based  on  the  melting  point  of  palladium  1549°  C.,  and  62  was  measured 
by  an  application  of  the  Stefan-Boltzmann  law.  This  method  of  com- 
putation accordingly  assumes  that  an  error  in  the  temperature  scale  at 
1549°  C.  necessitates  some  error  at  1604°  absolute,  an  assumption  which 
is  not  absolutely  justified.  Hence  the  value  of  1551°  C.  is  omitted  from 
Table  3. 

Many  other  determinations  o'f  the  melting, point  of  palladium  have 
been  made  by  various  investigators  but  most  of  these  have  taken  no 
account  or  improper  account  of  the  wave-length  of  the  glass  employed  in 
the  eyepiece  of  the  pyrometer  and  hence  their  data,  either  with  a  sector 
disk  or  absorption  glass,  are  unreliable.  We  have  recomputed  the  data 
of  Waidner  and  Burgess,26  in  which  a  sector  disk  was  used.  The  proper 

TABLE  3. — Melting  Point  of  Palladium. 
ca  =  14,350;  Melting  Point  of  Gold  =  1063°  C. 


Investigator 

Method 

Melting 
Point, 
Degrees  C. 

Day  and  Sosman  

Gas  thermometer  corrected  to  thermo- 

1550 

Nernst  &  Wartenberg  

dynamic  scale. 
Ratio    of    brightness    Pd/Au,    Wanner 

Hoffman  and  Meissner  

pyrometer. 
Ratio    of   brightness    Pd/Au,    spectro- 

1551 

Hyde,  Cady  and  Forsythe  .... 
Mendenhall  

photometer. 

Ratio  of  brightness   Pd/Au,   H.  &  K. 
pyrometer. 
Ratio    of    brightness    Pd/Au,    spectral 

1552 
1554 
1554 

1555 

Waidner  and  Burgess  

pyrometer. 
•H.  &  K.  pyrometer  and  sector  disk. 

1552 
1549 

Mean 

1552 

26  U.  S.  Bureau  of  Standards  Bull.  3  (1907)  163-208,  Table  XI. 


56 


STANDARD  SCALE  OF  TEMPERATURE 


effective  wave-length  is  X  =  0.6663/u.  Hence  on  reducing  their  values  to 
c2  =  14,350,  the  copper  melting  point  of  1083°  C.  instead  of  1084°  C. 
we  obtain  1549°  C.  for  the  melting  point  of  palladium. 

Table  3  summarizes  the  determinations  of  the  melting  point  of  palla- 
dium as  discussed,  omitting  the  work  of  Holborn  and  Valentiner,  which 
gives  a  value  20°  higher  than  that  of  all  other  observers.  Hence  the 
unweighted  mean  of  all  the  best  published  values  of  the  melting  point  of 
palladium,  properly  corrected  for  c2  =  14,350  and  the  melting  point  of 
gold  of  1063°  C.,  gives  1552°  C.  The  value  that  has  been  used  by  the 
Bureau  of  Standards  for  the  past  4  years  is  1550°  C.  The  evidence  here 
presented  indicates  that  this  temperature  is  possibly  low  by  an  uncertain 
amount,  probably  by  not  more  than  5°  C.  In  view  of  the  uncertainties 
involved,  the  Bureau  has  not  deemed  it  advisable  to  change  the  scale  in 
present  use. 

In  only  one  of  the  investigations  discussed,  that  of  Day  and  Sosman, 
was  the  purity  of  the  palladium  determined.  Hoffman  and  Meissner, 
however,  stated  that  the  palladium  used  by  them  melted  within  1°  C.  of 
that  of  a  sample  used  by  Day  and  Sosman.  The  natural  tendency  in  the 
wire  method  is  toward  too  high  values,  although  with  care  Hoffman  and 
Meissner  obtained  consistent  results  by  both  the  wire  and  the  crucible 
methods.  In  the  Lummer-Kurlbaum  black  body,  it  is  possible  to  obtain 
too  high  values  on  account  of  stray  reflection  from  the  hotter  side  walls. 
For  this  reason  further  experimental  work  with  crucible  melts  of  a  type 
described  by  Hoffman  and  Meissner  are  highly  desirable.  If  the  meas- 
urements are  made  with  a  spectrophotometer,  attention  must  be  given  to 
the  slit  width  and  to  the  shift  of  wave-length  with  temperature  of  the 
source  when  the  slit  width  is  large.  If  the  measurements  are  made  with  a 
pyrometer,  the  effective  wave-length  of  the  ocular  screen  must  be  care- 
fully considered.  That  these  factors  are  of  importance  is  evident  when 
one  notes  that  the  ratio  of  brightness  Pd/Au  increases  from  about  74 
at  X  =  0.67/i  to  about  600  at  X  =  0.45  M- 

Melting  Point  of  Platinum. — Experimental  determinations  of  the 
melting  point  of  platinum  are  much  less  satisfactory  than  those  of  palla- 
dium. Table  4  summarizes  the  work  of  investigators  who  have  made 

TABLE  4. — Melting  Point  of  Platinum 


Investigator 

• 

Palladium, 
Degrees  C. 

Platinum, 
Degrees  C. 

Platinum- 
palladium, 
Degrees  C. 

Nernst  and  Wartenberg  

1551 

1761 

210 

Holborn  and  Valentiner  

1573 

1777 

204 

Waidner  and  Burgess  

1549 

1753 

204 

Mean  

206 

C.  W.  WAIDNER,  E.  F.  MUELLER,  AND  PAUL  D.  FOOTE        57 

determinations  with  both  metals.  The  data  have  been  recomputed  for 
gold  =  1063°  C.,  c2  =  14,350,  and  the  proper  effective  wave-lengths  have 
been  employed.  If  the  mean  difference  of  206°  C.  is  added  to  the  mean 
value  of  the  determination  of  the  melting  point  of  palladium,  1552°  C., 
we  obtain  1758°  C.  for  the  melting  point  of  platinum.  The  Bureau  has 
used  the  value  1755°  C.  for  the  past  several  years,  hoping  to  obtain  more 
satisfactory  data  before  adopting  a  new  value. 

Melting  Point  of  Tungsten. — Consideration  of  the  pioneer  work  on 
tungsten  by  Waidner  and  Burgess,  V.  Wartenberg,  and  Pirani  may  be 
omitted  because  of  the  fact  that  at  this  early  date  pure  tungsten  was  not 
procurable,  and  because  of  the  empiric  methods  employed,  especially  in 
the  determination  of  emissivity  of  the  metal,  and  finally  because  of  the 
unsatisfactory  computation  of  effective  wave-lengths  and  transmission 
coefficients  of  the  absorbing  glasses.  Worthing27  has  summarized  what 
are  believed  to  be  the  most  reliable  measurements,  reduced  to  c2  =  14,350, 
gold  =  1063°  C.,  and  X  =  0.665M  as  follows: 

APPARENT 
TEMPERATURE 

INVESTIGATORS  X  =  0.665/1 

DEGREES 

AbSQLUTE 

Mendenhall  and  Forsythe 3174 

Langmuir 3187 

Worthing 3174 

Luckey 3169 


Mean 3176 

On  the  basis  of  Worthing's  determination  of  the  emissivity  of  tungsten 
at  this  wave-length,  3176°  absolute  is  equivalent  to  a  true  temperature  of 
3674°  absolute  or  in  round  numbers  3400°  C. 

Reproduction  of  the  High  Temperature  Scale. — The  standard  tem- 
perature scale  as  used  and  distributed  by  the  Bureau  of  Standards  is 
fixed  in  the  interval  —40°  to  450°  C.  by  means  of  platinum  resistance 
thermometers  calibrated  in  ice,  steam,  and  sulfur  vapor  (444.6°  C.)  as 
previously  described.  The  standard  temperature  scale  in  the  interval 
450°  to  1100°  C.  is  defined  by  the  fixed  points  cited  in  Table  2  and  inter- 
polation between  these  points  is  based  on  the  temperature  scale  defined  by 
the  rare-metal  thermocouple  (Pt,  Pt  90  —  Rh  10)  calibrated  at  two  of 
these  fixed  temperatures,  usually  antimony  and  copper,  and  at  the  zinc 
point,  Table  1. 

The  temperature  scale  above  1100°  C.  is  based  upon  the  extrapola- 
tion of  Wien's  (or  Planck's)  law  using  as  the  fiducial  point  the  melting 
point  of  gold  =  1063°  C.  and  c2  =  14,350. 

27  Worthing:  Phys.  Rev.  [2]  (1917)  10,  392. 


58  STANDARD  SCALE  OF  TEMPERATURE 

DISCUSSION 

CHARLES  E.  GUILLAUME,*  Sevres,  France  (written  discussion f). — 
Referring  to  p.  47,  experiments  made  at  the  end  of  the  year  1884  showed 
a  remarkable  agreement  between  the  indications  of  various  verre  dur 
thermometers  of  which  all  the  corrections  had  been  determined  with  the 
greatest  possible  precision.  The  maximum  deviations  not  attributable 
to  the  comparisons  themselves  were  then  about  0.002  to  0.003  degree. 
Much  later,  greater  differences  began  to  show.  They  were  caused  by 
the  fact  that  the  glassmaker  had  modified  the  composition  by  adding  to 
it  a  small  quantity  of  lead.  Efforts  are  now  being  made  in  France  to 
obtain  a  regular  supply  of  glass  made  according  to  the  old  formula. 

The  determination  of  the  boiling  point  of  sulfur  made  by  Chappuis,28 
444.6°,  accords  very  well  with  the  value  noted  on  p.  49. 

The  International  Committee  of  Weights  and  Measures  and  the 
General  Conference  have  already  considered  the  future  abandonment 
of  the  hydrogen  scale  in  favor  of  the  thermodynamic  scale.  The  hydro- 
gen scale  appears  to  be,  in  their  estimation,  a  transitory  one,  happily  so 
closely  approaching  the  final  scale  that  no  correction  should  be  necessary 
for  all  measurements  of  temperature  made  during  the  last  30  years. 

It  must  not  be  forgotten  that  the  decision  reached  by  the  Inter- 
national Committee  in  1887,  and  endorsed  by  the  Conference  of  1889, 
applied  only  to  the  international  service  of  weights  and  measures ;  that  is 
to  say,  the  domain  of  temperatures  where  the  hydrogen  scale  or  the  ther- 
modynamic scale  presents  divergences  which  are,  up  to  date,  within  the 
limit  of  measurable  quantities.  But  it  is  not  at  all  contrary  to  this  deci- 
sion to  adopt  other  representations  in  the  region  outside  of  the  one  ex- 
pressly indicated.  At  very  low  temperatures,  the  best  representation 
that  one  can  at  the  present  time  propose  is  undoubtedly  that  furnished 
by  the  helium  thermometer;  and  at  high  temperatures,  hydrogen  has 
such  a  tendency  to  diffuse  that  it  is  necessary  to  replace  it  by  another 
gas.  But  then  the  errors  in  reference  to  the  thermodynamic  scale  are 
considerable  enough  to  necessitate  a  correction.  All  our  efforts  ought  to 
tend  now  to  determining  the  corrections  in  such  a  way  as  to  extend  the 
realization  of  the  thermometric  scale  far  into  the  region  where  the  nitro- 
gen thermometer,  for  example,  overlaps  the  radiation  pyrometer.  As 
the  authors  say,  the  latter  is  the  only  thing  that  one  can  use  for  high  tem- 
peratures, whichever  law  is  applied.  It  may  be  noted  that,  the  funda- 
mental points  of  a  nitrogen  thermometer  under  low  initial  pressure  having 
been  chosen,  for  example,  at  the  boiling  point  of  water  and  at  the  boiling 
point  of  sulfur,  the  linear  extrapolation  toward  high  temperatures  will 

*  Bureau  International  des  Poids  et  Mesures. 
t  Received  Oct.  11,  1919.     Translated  from  the  French. 
28  Bureau  International,  Travaux  et  Mtmoires,  16. 


DISCUSSION  59 

give  a  scale  very  close  to  the  thermodynamic  scale.  It  would  seem  pos- 
sible to  make  a  direct  experimental  determination  of  corrections  by  the 
comparison  of  two  nitrogen  thermometers  having  very  different  initial 
pressures. 

On  p.  56,  the  authors  mention  the  old  measure  of  Holborn  and 
Valentiner  of  the  melting  temperature  of  platinum.  However,  this  old 
value  seems  to  have  been  abandoned  by  the  Reichsanstalt,  see  p.  52. 
On  the  other  hand,  Harker  indicated  a  much  lower  temperature  for  the 
melting  of  platinum;  but  the  process  that  he  employed  in  his  determina- 
nation  leads  one  to  think  that  he  used  a  metal  containing  an  appreciable 
amount  of  carbon  in  solution,  either  in  the  form  of  carbon  or  in  the  form 
of  platinum  carbide. 

LEASON  H.  ADAMS,*  Washington,  D.  C.  (written  discussionf). — It 
would  be  difficult  to  point  to  anything  more  vitally  important  to  the 
industries  and  to  scientific  research  than  a  temperature  scale  that  is 
trustworthy  and  reproducible.  This  paper  is  a  clear  and  illuminating 
exposition  of  the  present  state  of  our  knowledge  of  the  scale  of  tempera- 
ture and  it  is  pleasing  to  note  that,  by  means  of  the  standard  scale  which 
the  authors  present,  temperatures  may  now  be  defined  with  such  satis- 
factory precision.  Thus  at  room  temperatures  the  possible  uncertainty 
in  the  absolute  magnitude  of  a  given  temperature  need  not  be  greater  than 
a  few  thousandths  of  a  degree;  at  400°  C.,  the  maximum  error  is  not 
more  than  a  few  hundredths  of  a  degree;  and  at  1100°,  a  few  tenths. 

Above  1100°,  the  determination  of  the  temperature  scale  hinges 
largely  upon  the  melting  point  of  palladium,  which  is  taken  as  1550°, 
although  the  average  as  obtained  by  several  independent  investigators 
is  somewhat  higher.  As  pointed  out  by  the  authors,  in  only  one  of  the 
investigations,  that  of  Day  and  Sosman,  was  the  purity  of  the  palladium 
determined.  This  circumstance  brings  GO  mind  certain  observations  I 
made  some  time  ago  on  the  difference  in  melting  point  of  three  samples  • 
of  palladium  wire,  one  of  which  was  drawn  from  a  piece  of  the  metal  used 
by  Day  and  Sosman.  The  melting  points  were  determined  with  a  plati- 
num-platinrhodium  thermocouple  using  the  wire  method.  One  sample 
melted  2°  and  another  12°  higher  than  the  Day  and  Sosman  palladium, 
which  according  to  the  analysis  was  very  pure,  and  the  thermoelectric 
properties  of  the  three  kinds  gave  a  qualitative  support  to  the  conclusion 
that  as  a  rule  the  purest  palladium  has  the  lowest  melting  point.  This 
being  the  case,  it  is  not  at  all  improbable  that  some  of  the  higher  values 
that  have  been  obtained  for  the  melting  point  of  palladium  are  influenced 
by  lack  of  purity  of  the  material,  and  this  supposition  lends  support  ti 
the  lower  value,  1550°,  which  the  Bureau  of  Standards  has  very  wisely 
chosen. 

*  Physical  Chemist,  Geophysical  Laboratory.  f  Received  Sept.  25,  1919. 


60  STANDARD  SCALE  OF  TEMPERATURE 

The  scale  of  temperature,  as  given,  terminates  at  the  lower  end  at 
—  40°.  It  is  to  be  hoped  that  the  Bureau  of  Standards  will,  as  soon  as 
feasible,  extend  the  standard  scale  down  to  liquid-air  temperatures  or 
below. 

E.  P.  HYDE,  Nela  Park,  Cleveland,  O. — I  want  to  emphasize  the  impor- 
tance of  determining  the  palladium  point.  I  do  not  know  how  pure  the 
palladium  is  for  these  melting-point  determinations.  I  know  it  is  quite 
constant  and  the  values  obtained  from  time  to  time  using  different  sam- 
ples of  this  wire  are  approximately  the  same.  In  calibrating  optical 
pyrometers  in  practical  use,  the  palladium  point  is  the  one  of  greatest 
value.  The  General  Electric  Co.  has  arrived  at  the  same  conclusion. 
It  would  be  better  to  establish  our  scale  on  the  melting  point  of  gold,  using 
for  the  constant  c2,  the  value  14,350  micron  degrees.  We  find  it  necessary 
to  use  palladium  in  calibrating  pyrometers,  and  I  trust  that  before  long 
the  Bureau  of  Standards  will  determine  whether  or  not  the  wire  we  get 
has  a  different  melting  point  from  that  used  by  Sosman,  and  what  that 
value  should  be. 

In  addition,  I  want  to  question  the  statement  that  a  change  in  c2 
from  14,460  to  14,350  micron  degrees  makes  anything  like  the  variation 
in  the  effective  wave-length  given  in  the  paper.  If  the  Crova  wave- 
length, which  is  the  effective  wave-length  for  a  clear  glass  or  no  glass  at 
all,  is  calculated  for  trie  same  range  (1336°  to  1828°K),  the  change  will 
only  be  about  4  parts  in  5800  for  this  change  in  c2.  For  the  effective 
wave-length,  this  change  will  be  much  smaller,  •&>  ,  less  than  1  part  in 
6000.  Furthermore,  the  change  is  in  the  opposite  direction  from  that 
stated  above;  that  is,  a  decrease  in  c2  causes  a  decrease  in  the  effective 
wave-length. 

P.  D.  FOOTE. — Dr.  Hyde  is  correct  in  his  criticism  of  our  statement  in 
regard  to  the  change  in  effective  wave-length  with  change  in  c2,  but  this 
does  not  affect  the  results  in  the  slightest  manner.  After  considering  all 
determinations  of  the  melting  point  of  palladium,  the  true  value  appears 
to  be  1552°  C.  The  value  adopted  by  the  Bureau  of  Standards  5  years 
ago  is  1550°  C.  and  until  further  work  is  done  we  do  not  feel  justified  in 
changing  to  a  new  value.  We  have  recently  procured  a  large  quantity 
of  palladium  and  Mr.  Fairchild  will  determine  the  melting  point  by  a 
crucible  method.  As  noted  in  our  paper,  a  knowledge  of  the  exact  melt- 
ing point  of  palladium,  while  most  desirable,  is  not  absolutely  essential, 
since  the  high-temperature  scale  is  reproduced  by  a  definition  of  c2  and 
the  melting  point  of  gold. 


METALS    FOR    PYROMETER    STANDARDIZATION  61 


Metals  for  Pyrometer  Standardization 

BY  CHARLES  W.   WAIDNEH*  AND   GEORGE  K.  BUKGESS,  t  WASHINGTON,    D.   C. 
(Chicago  Meeting,  September,  1919) 

IN  response  to  many  urgent  requests  for  a  concrete  realization  of  a 
series  of  standard  temperatures  that  would  be  available  to  any  one  any- 
where for  the  standardization  of  pyrometers  and  the  reproduction  of  the 
standard  scale  of  temperature  for  use  in  laboratories  and  industrial  works, 
the  Bureau,  in  1916,  made  arrangements  with  several  manufacturers  to 
prepare  a  series  of  pure  metals  that  could  best  serve  this  purpose.  After 
the  necessary  preliminary  work  of  chemical  analysis,  as  a  check  on  the 
purity,  together  with  exact  determinations  of  melting  or  freezing  points, 
the  Bureau  began,  in  1917,  the  issuing  of  'melting-point  standards  of  tin, 
zinc,  aluminum,  and  copper. 

At  the  outset,  it  was  decided  to  secure  metals  of  the  highest  purity 
and  of  American  manufacture,  and  this  was  accomplished  in  a  more 
satisfactory  manner  than  anticipated.  In  the  preparation  of  these  tem- 
perature standards,  the  Bureau  has  had  the  hearty  cooperation  of  several 
American  companies,  which  have  succeeded  in  producing  metals  highly 
satisfactory  for  the  purpose  and  of  as  high  a  state  of  purity  as  could  have 
been  obtained  anywhere.  Each  company  that  undertook  the  responsi- 
bility of  furnishing  a  metal  for  this  series  of  standards  exercised  most 
painstaking  care  to  produce  the  best  possible  product  as  to  purity  and 
uniformity.  A  representative  of  the  Bureau  witnessed  the  actual  prep- 
aration of  each  metal. 

The  300  and  400-lb.  (136  and  181-kg.)  samples  making  up  the  first 
series  were  all  used  up,  with  the  exception  of  the  aluminum,  early  in  1919. 
The  Bureau  has  taken  steps  to  replenish  its  stocks  of  metals  serving  as 
pyrometric  standards  and  to  the  list  has  been  added  lead.  The  copper 
and  lead  are  being  ordered  this  time  in  1-ton  lots  and  the  zinc  and  tin  in 
300-lb.  (136  kg.)  lots  as  before.  The  wide  dissemination  of  these  samples 
will  be  appreciated  when  it  is  noted  that  the  metals  are  distributed  by  the 
Bureau  in  quantities  of  50  cu.  cm.  each,  at  $2  a  sample  and  accompanied 
by  a  certificate  which  gives  the  melting  point  as  determined  by  samples 
from  the  same  lot.  The  manufacture  and  detailed  description  of  chemical 
analyses,  physical  measurements,  and  precautions  in  use  of  these  pyromet- 
ric standards,  are  described  in  detail  in  the  Bureau  Circular  No.  66. 

Although  an  exact  chemical  determination  of  the  residual  impurities 
remaining  in  these  metals  is  not  absolutely  essential  for  their  use  as  a 
pyrometric  standard,  it  was  thought  desirable  to  make  such  an  analysis, 
from  a  suitably  compiled  sample,  for  the  purpose  of  testing  uniformity 

*  Physicist,  Chief  of  Heat  Division,  U.  S.  Bureau  of  Standards. 

t  Physicist,  Chief  Division  of  Metallurgy,  U.  S.  Bureau  of  Standards. 


62 


METALS    FOR    PYROMETER   STANDARDIZATION 


and  also  to  demonstrate  what  could  be  done  in  American  plants  on  the 
production  of  metals  of  highest  possible  purity.  The  uniformity  of 
several  lots  of  metal  were  tested  on  several  samples  from  each  lot  by 
means  of  the  intercomparison  of  their  freezing  points,  using  as  tempera- 
ture-measuring instruments  four  electrical  resistance  thermometers  of 
pure  platinum  calibrated  at  0°,  100°,  and  444.6°  C.,  the  freezing  and  boil- 
ing points  of  water  and  the  boiling  point  of  sulfur,  respectively.  The 
temperature  scale  given  by  the  resistance  of  pure  platinum  within  the 
range  covered  by  this  series  of  metals  (0-1100°  C.)  reproduces  the  standard 
scale  of  temperatures  as  closely  as  it  can  be  determined.  The  actual 
value  to  be  used  for  the  freezing  point  of  each  metal  is  given  by  the  certi- 
ficate furnished  with  each  sample. 

In  the  accompanying  table  are  given  the  values  of  the  freezing  points 
and  purity  of  the  first  series  and  of  the  second  series,  in  so  far  as  they  have 
been  determined,  of  the  standard  pyrometric  metals  issued  by  the  Bureau. 
As  may  be  required,  and  as  opportunity  permits,  other  metals  will  be 
added  to  this  series,  including  palladium,  platinum,  and  gold  in  the  form 
of  wires;  and  it  may  be  advisable  to  include  pure  chemical  substances  of 
known  boiling  points  such  as  naphthalene,  benzophenone,  and  sulfur. 
Other  convenient  standardization  temperatures'  for  pyrometers  are  the 
freezing  point  of  sodium  chloride,  801°  C.;  the  A2  transformation  of  pure 
iron,  768°  C;  and  the  transformation  temperature  of  crystalline  quartz, 
573.3°  C. 

These  pyrometric  standards  have  been  found  of  great  use  in  testing  and 
research  laboratories  and  for  control  of  pyrometric  operations  in  many 
kinds  of  industry.  They  are  particularly  convenient  in  that  they  permit 
calibrations  to  be  carried  out  in  place  and  thus  save  time,  conserve 
equipment,  and  prevent  breakage  of  instruments  in  transit. 

TABLE  1. — Metals  for  Pyrometer  Standardization 


Source 

Lot 

No. 

Metal 

Freezing 
Point, 
Degrees  C. 

Purity, 
Per  Cent. 

Main  Impurities, 
Per  Cent. 

Aluminum      Co.      of 

1 

Aluminum 

658.68 

99.66 

Fe  0.18,  Si  0.15,  C  0.01, 

America. 

Cu  0.004. 

Raritan  Copper  Co  ... 

1 

Copper 

1083.0 

99.987 

Sb    0.004,     As    and    S 

0.0026  each. 

Raritan  Copper  Co  ... 

2 

Copper 

1083.00 

National  Lead  Co  .... 

2 

Lead 

Amer.  Smelt.  &  Refin. 

Co  

1' 

Tin 

231.88 

99.988 

Pb  0.007,  Cu  0.003,  Fe 

0.002. 

Amer.  Smelt.  &  Refin. 

Co  

2 

Tin 

New  Jersey  Zinc  Co  .  . 

1 

Zinc 

419.44 

99.993 

Fe  0.005,  Cd  0.0018,  Pb 

0.0004. 

New  Jersey  Zinc  Co.  . 

2 

Zinc 

419.39 

FUNDAMENTAL   LAWS    OF    PYROMETRY  63 


Fundamental  Laws  of  Pyrometry 

BY   C.    E.    MENDENHALL,  *   PH.   D.,    MADISOtf,    WIS. 
(Chicago  Meeting,  September,  1919) 

THE  word  temperature  has  both  a  colloquial  and  a  technical  use. 
For  everyday  purposes  of  abusing  the  weather  man,  no  very  exact 
definition  is  necessary,  but  for  the  purpose  of  giving  a  simple  summary 
of  the  physical  laws  that  form  the  basis  of  practical  pyrometry,  something 
more  precise  is  required.  Beginning,  therefore,  with  the  common  con- 
cept of  "hotness"  and  "coldness,"  we  must  agree  on  a  method  of  meas- 
uring differences  in  "hotness,"  on  the  unit  to  be  used,  and  on  the  point 
from  which  measurements  are  to  be  taken.  We  shall  then  have  a  definite 
"scale  of  temperature,"  which  can  be  used  in  all  methods  of  pyrometry. 
But,  as  in  many  similar  cases,  it  is  much  easier  to  define  or  describe  some- 
thing than  it  is  to  make  practical  application  and  use  of  the  definition ; 
so  that  much  of  our  attention  will  be  taken  up  with  practical  methods  of 
realizing  or  applying  the  scale  agreed  upon. 

It  was  early  observed  that  changes  in  temperature  produced  large 
and  easily  measurable  changes  in  gases,  which  may  be  most  simply 
separated  into  changes  in  volume  (expansion  and  contraction)  under 
conditions  of  constant  pressure,  and  changes  in  pressure  under  conditions 
of  constant  volume.  These  changes  are  much  the  same  in  magnitude  for 
the  common  gases  oxygen,  nitrogen,  and  hydrogen  and  also  for  the  rarer 
helium  and  argon.  This  relative  uniformity  in  behavior  led  to  the  sug- 
gestion of  "the  gas  thermometer"  and  "the  gas  scale"  as  the  basis  for 
all  temperature  measurements.  However,  as  methods  were  refined, 
differences  appeared  between  different  gases  and  different  ways  of  using 
gases,  so  that  Lord  Kelvin  introduced  his  more  fundamental  notion  of 
the  "absolute  thermodynamic  scale"  of  temperature,  which  he  defined 
as  follows:  Given  two  bodies,  say  two  tanks  of  water,  at  different  tem- 
peratures, to  determine  these  temperatures  on  the  "absolute  thermodyna- 
mic," or  Kelvin,  scale,  operate  .a  thermal  engine  between  these  two 
temperatures,  letting  it  take  in  heat  from  the  hot  body  and  give  out  heat 
to  the  cold,  which  therefore  corresponds  to  the  boiler  and  the  condenser 
of  a  steam  engine.  The  engine  we  may  imagine  as  a  cylinder  and  piston 
inclosing  a  gas  and  operating  with  the  well-known  Carnot  or  isothermal- 
adiabatic  cycle.  We  must  imagine  that  the  mechanical  losses  of  energy 
due  to  friction  and  the  thermal  losses  due  to  radiation  convection  and  con- 

*  Chairman  of  Section  on  Mathematics  and  Physics  of  National  Research  Council. 


64  FUNDAMENTAL  LAWS  OF  PYROMETRY 

duction  have  been  determined  and  allowed  for.  We  must  also  take  care 
to  run  the  engine  slowly  so  that  only  an  inappreciable  difference  or  tem- 
perature exists  between  it  and  the  hot  and  cold  bodies  when  it  is  absorb- 
ing or  giving  out  heat  to  them.  Such  an  engine  with  the  corrections 
applied  as  indicated,  is  called  perfect  because  it  is  of  maximum  efficiency. 
If  it  is  found  that  a  quantity  of  heat  HI  is  taken  in  at  the  high  temperature 
and  H2  is  given  out  at  the  low  temperature  we  have 


where  TI  and  772  are  the  cwo  temperatures  in  question  on  the  K.  scale. 

IT     IT  rp    rp 

From  this,   J^w     *  =  -^jf, — -;  and  from  this,  the  idea  of  an  absolute 
"i  •*  i 

zero  is  suggested  as  that  temperature  for  which  H2  =  0  for  then  T2  =  0. 
To  complete  the  definition  of  the  K.  scale,  it  is  only  necessary  to  agree 
that  the  interval  between  the  two  most  reliable  fixed  points,  namely,  the 
temperatures  of  melting  ice  and  of  boiling  water,  shall  contain  a  certain 
number  of  degrees,  100  if  we  are  working  with  the  centigrade  K.  scale. 

IT  IT  TOO 

This  condition  is  expressed  by     l—n~-    °  =  m~ •  (2) 

•"0  -I  0 

and  combining  this  with  the  general  equation  1  we  have 

1  =  n  vT  ("' 

•"100  —  -"0 

which  gives  any  temperature  on  the  centigrade  K.  scale  in  terms  of  the 
quantities  of  heat  taken  in  and  rejected.  How  can  this  scale  be  realized 
and  used  in  practice?  Obviously  not  by  means  of  any  ideal  engine  as 
outlined  above.  There  are,  however,  two  distinct  ways  in  which  it  may 
be  done,  both  depending  on  theoretical  deductions  from  the  second  law 
of  thermodynamics,  to  which  the  K.  scale  is  very  intimately  related. 

First,  by  applying  the  second  law  expressions  may  be  derived  (Cal- 
lendar-Berthelot-Buckingham)  in  which  the  pressure  of  a  gas  (volume 
constant)  is  given  in  terms  of  its  temperature  on  the  K.  scale  and  of  certain 
physical  characteristics  of  the  gas.  By  a  different  treatment  of  the  prob- 
lem, analogous  expressions  may  be  obtained  giving  the  volume  of  gas 
(pressure  constant)  in  terms  of  its  K.  temperature  and  physical  properties. 
These  expressions,  in  the  form  given  by  Buckingham,  are: 

A  (volume  constant  =  v) 

n? 

5  '    -   •         8vl  j,, 

uu 


p  —  p°  =  __  I    1 

6         Q  I    #2  LOW  ov-ig 

t/  e, 

B  (pressure  constant  =  p) 

r9 

V          Vn 

e~e0  = 

J  a 


C.    E.    MENDENHALL  65 

In  these  expressions 

6   =  temperature  on  K.  scale; 

do  =  initial  temperature  (say  ice  point)  on  K.  scale; 

cp  =  specific  heat  under  constant  pressure; 

u.  =  -----  =  under  Joule-Kelvin  conditions;  that  is,  when  expanded 
Ap 

adiabatically  through  a  porous  plug; 
po   =  pressure  at  00  and  v; 
vQ   =  volume  at  00  and  p. 

The  physical  properties  involved,  namely,  the  specific  heat  at  con- 
stant pressure,  the  pressure-  volume  relations  of  the  gas,  the  Joule-Kelvin 
coefficient  or  rate  of  change  of  temperature  with  pressure  when  expanded 
through  a  porous  plug,  should  be  known  throughout  the  temperature 
interval  that  is  to  be  determined  for  the  volumes  and  pressures  to  be 
used,  in  order  to  evaluate  these  expressions.  The  initial  volume  or 
pressure,  as  the  case  may  be,  must  also  be  known  with  especial  accuracy. 
If  these  quantities  are  known  for  a  given  gas,  the  observation  of  the 
change  in  volume  of  this  gas  at  constant  pressure  (v0  to  v)  will  enable  us  to 
compute  the  K.  temperature  at  which  the  volume  was  observed  to  be  v; 
and,  similarly,  if  the  pressure  change  is  observed  at  constant  volume, 
the  temperature  can  be  computed  from  the  pressure  change  p  —  p0. 
Unfortunately  the  experimental  knowledge  of  cp,  /*,  and  the  pv  relation 
is  very  incomplete,  being  limited  to  a  short  temperature  range,  and  great 
ingenuity  and  care  are  needed  in  handling  and  extrapolating  the  data  to 
get  the  most  reliable  results. 

The  results  of  the  thermodynamic  computations  just  referred  to  are 
usually  expressed  in  a  somewhat  different  way  by  introducing  various 
"gas  scales"  of  temperature.  For  example,  if  we  are  operating  with 
nitrogen,  we  may  conveniently  consider  two  nitrogen  scales,  one  defined 
by 


TP     \vop 

if  the  gas  is  maintained  at  constant  pressure  during  the  measurements, 
and  the  other  by 


(--}   = 

\Tj  , 


if  the  volume  is  maintained  constant.  Every  other  gas  would  have  its 
own  two  scales.  Hence  the  previous  fundamental  equations  may  be 
described  as  giving  the  differences  between  any  given  gas  scale  and  the 
K.  scale,  and  these  differences  or  corrections  maybe  tabulated  and  applied 
to  reduce  gas-thermometer  observations  to  the  K.  scale.  These  correc- 
tions have  only  been  computed  up  to  1200°  C.  and  are,  in  general,  some- 
what more  than  twice  as  large  for  the  constant-pressure  gas  thermometer 


66 

as  for  the  constant-volume,  but  there  are  other  compensating  advantages 
in  favor  of  the  former. 

Confining  our  attention  to  nitrogen  as  the  most  useful  gas  for  high- 
temperature  measurements,  it  may  be  briefly  said  that  the  correction 
to  Tp  is  about  1.70°  at  1000°  C.  and  2.15°  at  1200°  C.,  while  it  is  only 
0.96°  for  Tv  at  1200°  C.  These  errors  are  of  significance  when  it  comes  to 
determining,  by  gas  thermometry,  the  fundamental  fixed  points  of  the 
high-temperature  scale,  though  too  small  to  be  considered  in  ordinary 
work.  Above  1200°  C.  they  are  undoubtedly  larger,  but  unfortunately 
data  are  not  yet  available  for  their  computation. 

There  is  one  other  fundamental  matter  that  must  be  considered,  and 
that  is  the  value  of  the  ice  point  on  the  Kelvin  scale.  This  may  be  cal- 
culated from  the  same  basic  equations  already  given.  The  computations 
of  Buckingham  and  Berthelot  agree  very  well  in  giving  60  =  273.13;  that 
is,  for  high-temperature  work,  the  ice  point  may  be  taken  as  flat  273° 
on  the  Kelvin  scale. 

The  second  field  in  which  it  has  been  possible  to  connect  practical 
methods  of  pyrometry  with  the  Kelvin  scale  is  the  field  of  radiation. 
The  fundamental  facts  of  radiation  are  that  it  is  an  effect  of  one  body  on 
another  that  may  take  place  across  a  vacuous  space,  and  that  it  is  closely 
related  to  the  "hotness"  of  bodies  and  tends  to  equalize  their  tempera- 
ture. Without  attempting  to  distinguish  between  what  we  know  and 
what  we  merely  say,  radiation  may  be  described  in  the  usual  way  as  an 
electromagnetic  disturbance  sent  off  from  bodies,  which  may  be  analyzed 
by  spectroscopes  of  various  types,  and  shown  to  contain  waves  of  various 
lengths,  of  which  those  having  lengths  from  0.0003900  mm.  to  0.00076 
mm.  directly  affect  the  eye  and  are  called  " light"  waves;  waves  longer 
than  these  are  called  "infra-red,"  those  shorter  "ultra-violet,"  and  those 
still  shorter  x-rays.  The  entire  range  of  waves  is  called  a  complete 
spectrum,  and  various  radiating  bodies  emit,  according  to  their  nature 
and  condition,  various  characteristic  spectra;  that  is,  different  groupings 
of  wave-lengths  with  different  distributions  of  energy  among  them.  The 
radiation  from  solids  and  liquids  increases  very  rapidly  with  rising  tem- 
perature, and  their  spectra  are  similar,  and,  in  general,  continuous;  that 
is,  there  are  no  "gaps"  if  arranged  according  to  wave-lengths.  But  the  total 
energy  radiated  and  its  distribution  among  the  wave-lengths  is  very 
different  for  different  substances  at  the  same  temperature.  At  present,  it 
is  not  possible,  theoretically,  to  express  the  radiating  power  of  a  substance 
in  terms  of  its  other  physical  properties  and  temperature,  but  there  is  a 
special  form  of  radiator  which  can  be  successfully  dealt  with  both  theoret- 
ically and  experimentally,  namely,  the  perfect  or  complete  radiator,  or 
black  body.  The  original  idea  of  this  was  due  to  Kirchhoff,  and  the 
experimental  realization  to  Lummer  and  Pringsheim  and  many  later 
workers.  Given  an  inclosure  with  opaque  walls  at  a  uniform  and 


C.    E.    MENDENHALL  67 

constant  temperature,  the  fundamental  theorem  is  that  the  radiation 
inside  the  inclosure  will  be,  both  as  to  intensity  and  energy  distribution 
in  the  spectrum,  entirely  independent  of  the  material  of  the  walls  and 
dependent  only  on  its  temperature.  Since,  in  general,  bodies  that  are  good 
radiators  are  poor  inflectors  and  good  absorbers,  and  vice  versa,  it  is  quite 
reasonable  to  conceive  of  the  radiation,  so  to  speak,  accumulating  in  a 
closed  space  until  an  equilibrium  condition  is  reached  dependent  only  on 
the  temperature.  For  immediately  contiguous  to  one  part  of  the  wall  the 
equilibrium  condition  might  be  maintained  by  strong  absorption  and  cor- 
respondingly strong  emission,  while  at  another  point  the  same  condition 
might  result  from  strong  reflection  and  weak  emission.  Nevertheless,  of 
the  so-called  proofs  of  this  theorem  that  have  been  given  by  Kirchhoff , 
Pringsheim,  and  others,  none  is  entirely  satisfactory,  and  the  most  con- 
vincing evidence  that  it  is  possible  to  produce  radiation  independent  of 
the  qualities  of  special  kinds  of  matter  and  dependent  only  on  tempera- 
ture, is  furnished  by  experiment. 

The  first  form  of  perfect  radiator  used  for  experimental  purposes 
consisted  of  a  hollow  vessel  with  walls  as  uniformly  heated  as  possible 
and  provided  with  a  small  aperture  through  which  radiation  from  the 
inside  passed  out  to  be  examined  spectroscopically  and  otherwise.  If 
the  area  of  the  aperture  is  small,  compared  to  the  interior  radiating  walls, 
the  condition  of  equilibrium,  which  must  exist  in  order  that  the  interior 
radiation  should  be  complete,  will  be  very  little  disturbed  and  the  depar- 
ture from  equilibrium,  and  hence  from  completeness,  may  be  made  as 
small  as  desired  by  a  proper  choice  of  proportions.  The  experimental 
proof  referred  to  consists  in  the  fact  that  whereas  radiation  observations 
on  free  metal  or  other  surfaces  are  very  difficult  to  repeat,  that  is  to  say, 
the  radiation  from  free  surfaces  is  very  much  subject  to  changes  in  surface 
conditions,  all  observations  with  inclosures  are  much  more  uniform, 
and  it  is  possible  to  arrange  matters  so  that  the  emergent  radiation  is 
observably  independent  of  interior  surface  conditions.  Once  we  accept 
this  as  possible,  eye  observations  of  the  interior  furnish  a  most  sensitive 
test  as  to  whether  the  conditions  of  perfect  radiation  really  exist  in.  any 
given  case.  There  are,  of  course,  experimental  difficulties  in  the  way  of 
producing  sufficiently  uniform  temperature  conditions,  but  these  need 
not  be  discussed  here. 

Granted  then  that  there  is  such  a  thing  as  a  perfect  radiator,  we  must 
consider  Boltzmann's  ingenious  extension  of  the  theory.  First,  however, 
it  must  be  recalled  that  Bartolli  had  shown  that  in  order  to  be  consistent 
with  the  second  law  of  thermodynamics,  radiation  must  exert  a  pressure 
on  any  surface  on  which  it  impinges,  the  pressure  being  twice  as  great  if 
the  surface  is  perfectly  reflecting  as  if  it  is  perfectly  absorbing — being, 
in  fact,  proportional  to  the  amount  of  radiant  energy  per  unit  volume  in 
front  of  the  surface.  Maxwell  drew  the  same  conclusion  from  his 


68  FUNDAMENTAL   LAWS    OF   PYROMETRY 

electromagnetic  theory;  and  later  Lebedew,  Nicholls  and  Hull  experi- 
mentally verified  these  theoretical  deductions.  On  this  basis,  Boltz- 
mann  conceived  of  a  " radiation  engine,"  which  might  be  a  cylinder  with 
reflecting  walls  and  piston  and  a  radiating  base,  in  which  radiation  could  • 
be  "expanded,"  so  to  speak,  and  allowed  to  do  mechanical  work  through 
the  pressure  on  the  piston.  By  applying  to  this  engine  the  second  law, 
Boltzmann  showed  that  the  total  radiation  from  a  perfect  radiator  must 
vary  in  amount  as  the  fourth  power  of  the  temperature  of  the  radiator, 
measured  on  the  Kelvin  scale,  that  is 

E  =  (r04 

where  E  is  total  radiant  energy,  <r  is  a  constant,  and  0,  as  before,  is  Kel- 
vin temperature. 

This  theoretical  deduction  is  quite  simple,  is  no  more  questionable 
than  the  direct  thermo- mechanical  deductions  from  the  second  law, 
such  as  are  used  in  the  theory  of  gas  thermometry,  and  constitutes  the 
second  experimental  hold  on  the  Kelvin  scale.  It  is  known  as  the  Stefan- 
Boltzmann  law  because  Stefan  had  some  years  earlier  deduced  it  from  a 
discussion  of  bad  observations  on  imperfect  radiators,  for  which  it  does 
not  hold — a  case  in  which  two  negatives  have  apparently  been  equivalent 
to  an  affirmative,  so  to  speak. 

The  next  advance  in  radiation  theory  was  by  Wien,  the  radiation 
engine  still  being  the  basis;  but  the  arguments  are  not  quite  so  simple  and 
free  from  objection  as  in  Boltzmann's  case.  The  result  is  Wien's  dis- 
placement law,  of  which  the  usual  statement 

0Xmax.  =  constant 

is  a  special  case.  Here  6  is  the  Kelvin  temperature  at  which  a  perfect 
radiator  would  have  the  maximum  of  its  spectral  energy  curve  at  the 
wave-length  Xmax.  Wien  proceeded  further  and,  by  still  less  satisfactory 
methods,  deduced  the  equation  of  spectral  energy  distribution  known  as 
Wien's  law 


wherein 

EX  =  ordinate  at  wave-length  X  of  spectral  energy  distribution 

curve; 

8=  corresponding  Kelvin  temperature; 
d  and  c2=  constants. 

Further  work  of  Planck  led  to  the  well-known  expression 


C.    E.   MENDENHALL  69 

which  for  small  values  of  X0,  that  is,  short  wave-lengths  and  not  too  high 
temperatures,  becomes  practically  identical  with  Wien's  law.  While 
Planck's,  and  within  the  limits  just  mentioned,  Wien's  distribution  for- 
mulas have  been  experimentally  verified  with  a  fair  degree  of  accuracy 
and  for  temperatures  below  about  1500°  C.,  unfortunately  their  theore- 
tical deduction  cannot  be  regarded  as  sound.  They  do  not,  therefore, 
furnish  another  independent  connection  between  the  Kelvin  scale  and 
observable  quantities,  but  must  be  regarded  as  empirical  equations  whose 
accuracy  has  been  demonstrated  within  limits.  We  must  consider 
these  three  radiation  formulas  of  Stefan-Boltzmann  (total),  Wien  (dis- 
placement) ,  and  Planck  (distribution)  somewhat  more  in  detail  in  their 
bearing  on  pyrometry.  The  first  of  these  gives  us  a  means,  independent 
of  corrected  gas  thermometry,  of  completely  evaluating  the  Kelvin  scale, 
beginning  merely  with  the  ice  and  steam  points  and  the  assumption  of 
100°  between  the  two.  For  this  purpose  it  is  theoretically  much  simpler 
than  gas  thermometry,  but  whether  it  would  work  out  to  be  of  compar- 
able accuracy,  especially  at  low  temperatures,  cannot  be  said  as  no  one  has 
ever  attempted  to  apply  it  in  this  comprehensible  way.  For  high  tem- 
peratures, more  particularly  for  extreme  high  temperatures,  the  case  is 
clearer. .  The  limit  of  successful  gas  thermometry  is  at  present  the  palla- 
dium point  (1549°  C.),  and  above  this  region  experimental  difficulties  seem 
to  be  increasing  at  an  appalling  rate.  On  the  other  hand,  the  difficulties 
in  applying  the  fourth-power  law  in  a  sense  diminish  with  increasing 
temperatures  because  "stray"  radiation  in  general  becomes  proportion- 
ately less  compared  to  that  which  is  to  be  m'easured.  While  beginnings 
have  been  made  in  the  application  of  the  Stefan-Boltzmann  law  to  accu- 
rate pyrometry,  the  possibilities  have  not  been  in  any  sense  exhausted. 
The  law  should  be  applied  to  determine  the  gold  melting  point  (1062.6°  C.) 
and  especially  the  palladium  melting  point,  as  a  check  upon  the  determi- 
nation by  gas  thermometry,  and  there  is  room  for  more  work  in  deter- 
mining other  standard  fixed  points  in  the  range  beyond  1600°  C.,  in  which 
region  the  total  radiation  method  seems  to  be  about  the  only  hopeful 
one.  It  should  perhaps  be  pointed  out  that  in  using  this  method  no 
absolute  measurements  of  radiant  energy  are  needed — one  deals  entirely 
with  energy  ratios,  and  enough  work  has  been  done  to  prove  that  very 
considerable  accuracy  is  possible. 

Wien's  displacement  law  6  =  .—    '  has  also  been  used  in  pyrometric 

Ajnax. 

work,  but  the  disadvantages  are  several.  In  the  first  place  the  wave- 
length corresponding  to  maximum  energy  is  difficult  to  determine,  and 
as  the  inverse  wave-length  varies  only  as  the  first  power  of  the  tem- 
perature the  shift  is  not  sensitive  to  temperature  changes,  particularly  at 
high  temperatures.  We  may  therefore  dismiss  this  as  of  little  value 
either  for  fundamental  or  practical  measurements. 


70  FUNDAMENTAL  LAWS  OF  PYROMETRY 

If  we  confine  our  attention  to  a  single  wave-length,  or  rather  a  narrow 
band  of  wave-lengths  in  the  spectrum,  the  laws  of  Planck  and  Wien  give 
the  rate  of  variation  of  the  energy  in  this  band  as  a  function  of  tempera- 
ture, a  very  useful  indicator  of  temperature  change.  Within  the  range 
wherein  Wien's  law  is  valid,  it  serves  most  conveniently  to  express  this 
variation  of  partial  radiation  and  may  be  written  in  this  form 


This  equation,  which  must  be  looked  upon  as  empirical  and  which  has 
been  verified  with  a  high  degree  of  accuracy  for  temperatures  up  to  1600°  C., 
and  less  accurately  from  there  up  to  3000°  C.,  is  the  basis  of  all  optical 
pyrometry.  As  such  its  importance  warrants  still  more  careful  compari- 
son between  it  and  the  Stefan-Boltzmann  law  up  to  the  highest  possible 
temperatures.  For  while  the  Stefan-Boltzmann  equation  gives  our  best, 
if  not  our  only  sound,  connection  with  the  Kelvin  scale  and  while  it  is  easy 
to  work  very  accurately  with  this  equation  under  laboratory  conditions, 
the  equation  is  subject  to  certain  troublesome  errors  (notably  that  due 
to  absorption  by  vapors  or  smoke  and  to  windows  and  reflectors  etc.  that 
may  intervene  between  the  radiator  and  indicating  instruments)  which 
are  difficult  to  eliminate  in  practice.  Similar  errors  affect  optical  pyro- 
meters, but  perhaps  not  quite  so  seriously,  and  there  is  a  certain  safety  as 
regards  the  detection  of  trouble  in  actually  looking  through  the  instru- 
ment into  the  space  whose  temperature  is  being  determined.  However, 
excellent  instruments  have  been  devised  according  to  each  principle,  and 
the  only  object  here  is  to  point  out  that  one  may  be  regarded  as  funda- 
mental in  terms  of  which  the  other  should  be  calibrated. 

In  all  this  discussion  of  radiation  pyrometry  it  has  been  presupposed 
that  the  object  whose  temperature  was  to  be  measured  was  a  perfect 
radiator,  as  the  laws  that  have  been  used  apply  only  to  such  a  case.  For 
fundamental  measurements  this  is,  of  course,  essential;  and  for  practical 
measurements  it  is  usually  possible  and  always  desirable,  for  a  perfect 
radiator  is  the  most  definite  and  easily  reproducible.  A  tube  thrust  into 
a  furnace,  a  narrow  deep  hole  bored  into  a  large  hot  mass  —  a  crack  in  such 
a  mass  —  can  usually  be  arranged  so  that  they  will  approximate  sufficiently 
to  the  "uniformly  heated  enclosure  with  small  aperture"  that  is  desired, 
especially  if  the  surface  actually  observed  through  the  aperture  is  itself 
a  good  radiator  and  a  poor  reflector.  There  are  other  ways  of  approxi- 
mating a  perfect  radiator  by  using  multiple  reflection,  but  a  discussion  of 
these  various  methods  is  not  germane  to  the  present  subject.  It  should 
be  pointed  out,  however,  that  for  purposes  of  practical  pyrometry, 
ordinary  non-perfect  radiators  may,  and  sometimes  have,  to  be  used  ;  and 
if  the  radiation  laws  are  applied  directly  in  such  cases,  true  temperatures 
will  not  be  obtained,  but  instead  lower  values  which  are  commonly  referred 


C.    E.    MENDENHALL  71 

to  as  "black  body  (or  perfect  radiator)  temperatures,"  meaning  thereby 
the  temperatures  at  which  a  "perfect  radiator"  or  "black  body"  would 
radiate  as  the  real  body  is  observed  to  do.  If  the  real  surface  is  suffi- 
ciently definite  and  reproducible,  known  corrections  in  some  cases  may 
be  applied  to  reduce  the  observations  to  Kelvin  temperatures,  or  it  may  be 
that  black-body  temperatures  will  suffice. 

There  are  two  methods  of  temperature  measurement  still  to  be  touched 
upon — resistance  and  thermoelectric  pyrometry — but  in  neither  of  these 
is  it  possible  to  deduce  any  theoretical  connection  between  the  observed 
quantities  and  the  Kelvin  scale.  They  are  entirely  empirical  and  all 
the  instruments  must  be  calibrated  by  comparison  directly  or  indirectly 
with  gas  or  radiation  work.  There  are  certain  relations,  such  as  Cal- 
lendar's  parabolic  formula  connecting  the  resistance  of  platinum  with 
temperature,  and  various  equations  giving  thermoelectromotive  force 
as  a  function  of  temperature  that  are  extremely  useful  and  hold  with 
great  accuracy  within  certain  temperature  limits,  but  which  cannot  be 
called  fundamental  laws.  They  may,  therefore,  be  left  without  further 
discussion,  as  most  valuable  methods  both  in  the  laboratory  and  in 
practice,  but  not  contributing  anything  to  our  grasp  of  the  fundamental 
temperature  scale.  Even  as  methods,  their  range  is  much  more  limited 
than  the  radiation  processes. 

This  survey  of  the  physical  basis  of  pyrometry  is  of  necessity  super- 
ficial, and  it  may  seem  at  first  thought  that  the  underlying  idea  of  work- 
ing to  an  "absolute  scale"  is  unnecessary,  any  single  arbitrary  scale  being, 
for  all  practical  purposes,  just  as  good.  This  is  quite  true,  provided  a 
single  arbitrary  scale  could  be  agreed  upon  and  accurately  reproduced. 
But  the  general  experience  in  such  matters  has  been  that  the  more  funda- 
mental and  absolute  the  nature  of  any  scale  of  measurement,  the  more 
accurately  it  can  be  maintained  and  reproduced.  In  other  words,  the 
absolute  scale  whose  development  has  been  described  in  this  paper  is  of 
importance  in  pyrometry,  not  because  of  its  "absoluteness"  but  because 
of  its  permanence  and  ease  of  reproduction. 


72  PRESENT   STATUS    OF  RADIATION   CONSTANTS 


Present  Status  of  Radiation  Constants 

BY    W.    W.    COBLENTZ,*    PH.   D.,   WASHINGTON,   D.   C. 
(Chicago  Meeting,  September,  1919) 

THE  constants  in  question  pertain  to  the  total  radiation  and  the 
spectral  radiation  of  a  uniformly  heated  enclosure,  or  so-called  black 
body.  These  constants  have  been  determined  for  the  range  within 
which  temperatures  can  be  measured  with  thermocouples.  Conversely, 
using  these  constants  and  suitable  instruments,  such  as  an  optical  pyro- 
meter, for  example,  it  is  possible  to  determine  temperatures  far  above  the 
range  attainable  with  the  most  refractory  thermocouples. 

The  formula  of  Stefan  Boltzmann  for  expressing  the  total  radiation  of 
a  black  body  is  R  =.  kT*  in  which  k  is  the  coefficient  or  "constant" 
under  discussion.  In  view  of  the  fact  that  total  radiation  pyrometers 
are  usually  calibrated  on  an  arbitrary  scale,  there  is  no  great  demand  for 
an  exact  value  of  the  coefficient  of  total  radiation  in  absolute  value.  How- 
ever, it  is  intimately  connected  with  the  constant  of  spectral  radiation; 
hence,  an  accurate  determination  of  the  constant  k  gives  a  check  on  the 
constant  of  spectral  radiation. 

The  distribution  of  radiation  in  the  spectrum  of  a  black  body  is  repre- 
sented by  Planck's  equation,  E^  =  CiA~5  (e'/xr  -  I)-1.  The  spectral 
radiation  constant  c,  in  this  formula,  is  useful  in  optical  pyrometry  and 
in  establishing  a  high  temperature  scale.  The  numerical  value  of  the 
constant  c  has  been  determined  in  the  range  of  temperatures  measurable 
with  thermocouples  and,  also,  by  extending  the  temperature  scale  to 
higher  temperatures  by  means  of  total  radiation  measurements. 

But  little  work  has  been  done  on  the  radiation  constants  since  the 
beginning  of  the  war,  and  especially  since  1916.  In  a  summary1  of  the 
data  of  various  observers,  it  was  shown  that,  after  making  corrections 
for  atmospheric  absorption,  the  variously  obtained  values  of  the  coeffi- 
cient of  total  radiation  are  close  tok  =  5.7  X  10~12  wattcm.~2deg.~4,  which 
is  close  to  this  Bureau's  value,  k  =  5.72.  A  recalculation2  of  these  data 
gave  a  value  of  k  =  5.72  X  10~12  ±  0.  012  watt  cm.~2  deg.~4.  In  a  recent 
discussion,3  it  was  shown  that  the  determinations  by  Kahanowicz,4  when 
corrected  for  atmospheric  absorption  lead  to  a  value  of  k  =  5.7  which  is 

*  Associate  Physicist,  U.  S.  Bureau  of  Standards. 

1  Coblentz:  U.  S.  Bureau  of  Standards  Bull.  12  (1916),  553. 

2  Coblentz:  Proc.  Nat.  Acad.  Sci.  (1917)  3,  504. 

3  Coblentz:  Jril.  Wash.  Acad.  Sci.,  9,  185. 

4  Kahanowicz:  Nuovo  Cimento  (6)  (1917)  13,  142. 


W.    W.    COBLENTZ  73 

in  agreement  with  other  data.  For  some  years  this  Bureau  has  used  the 
value  k  =  5.7  X  10~12  watt  cm."2  deg.~4. 

Determinations  of  the  spectral  radiation  constant  c  have  been  made 
principally  by  the  Reichsanstalt  and  by  this  Bureau;  the  various  data 
have  been  summarized  in  a  recent  paper.5  In  this  paper  a  recalcula- 
tion of  this  Bureau's  data,  on  the  basis  of  a  revised  calibration  curve 
of  the  fluorite  prism  used  in  obtaining  the  spectral  energy  curves,  gave 
a  value  of  c  =  14,353  micron  deg.  A  recalculation  of  Paschen's  data 
have  a  value  of  c  =  14,350  to  14,370.  The  summary  of  the  extensive 
data  obtained  by  the  Reichsanstalt  indicated  values  of  c  =  14,250  to 
14,400;  and  the  adoption6  of  c  =  14,300  and  the  melting  point  of  palla- 
dium =  1557°  C.  For  some  years  this  Bureau  has  been  using  the  value 
of  c  =  14,350;  although  there  are  indications  that  probably  c  =  14,330 
would  be  a  better  value.  Using  Millikan's  value  of  the  unit  electric 
charge,  e  =  4.774  X  10~10  e.s.u.,  and  this  Bureau's  value  of  k  =  5.72,  in 
the  appropriate  formula  which1  interrelates  c  and  k,  it  is  found  that  the 
value  of  c  =  14,320. 

Another  check  upon  these  data  was  obtained  by  Hyde7  and  his  colla- 
borators from  measurements  of  the  brightness  of  a  black  body  at  the 
melting  point  of  gold  and  of  palladium,  as  determined  with  an  optical 
pyrometer.  Adopting  the  value  c  =  14,350, .consistent  results  could  be 
obtained  only  on  the  assumption  that  the  melting  point  of  palladium  is 
1555°  C.  instead  of  1550°  C.  as  previously  used.  The  latter  gives  a  value 
of  c  =  14,460,  which  is  too  high  according  to  the  best  data  available. 

A  further  check  on  the  radiation  constants  is  obtained  by  considering 
the  interrelation  between  the  spectral  radiation  constant  c  and  Planck's 
natural  constant  h.  Recent  determinations,  by  various  methods  (e.g., 
x-ray,  ionization  potential,  etc.)  indicate  a  value  of  this  constant  of  the 
order  h  =  6.55  X  10~27  erg.  sec.  This,  in  turn,  indicates  a  value  of  c  = 
14,320. 

From  the  foregoing  summary,  it  appears  that  the  radiation  constants 
are  known  to  0.3  per  cent.;  certainly  to  0.5  per  cent.  In  view  of  the  diffi- 
culties involved  it  seems  remarkable  that  all  these  constants,  including 
the  constant  h,  are  so  closely  determined.  This  is  especially  true  in 
view  of  the  fact  that  a  variation  (increase)  of  less  than  0.1  per  cent,  in 
the  value  of  e  would  produce  exact  agreement  in  the  computed  data.  In 
conclusion  it  may  be  added  that  from  a  consideration  of  the  data  avail- 
able it  appears  that  the  value  of  the  constant  of  spectral  radiation  is  close 
to  c  =  14,330  micron  degrees  and  that  the  coefficient  of  total  radiation 
is  close  to  k  =  5.72  X  10~12  watt  cm.-2  deg.~4. 

5  Coblentz:  U.  S.  Bureau  of  Standards  Bull  13  (1916),  459. 

6  Warburg:  Verh.  Phys.  Ges.  (1916),  1. 

7  See  Phys.  Rev.  (2)  (1919)  13,  48. 


74  THERMOELECTRIC    PYROMETRY 


Thermoelectric  Pyrometry 

BY   PAUL   D.    FOOTE,*    PH.    D.,    T.    B.    HARRISON,*  B.   S.,    AND    C.    O.    FAIRCHILD,*   B.    S., 

WASHINGTON,    D.    C. 

(Chicago  Meeting,  September,  1919) 

SEEBECK  discovered,  in  1821,  that  if,  in  a  closed  circuit  of  two  metals 
the  two  junctions  are  at  different  temperatures,  an  electric  current  will 
flow  in  the  circuit.  In  the  case  of  an  iron  and  a  copper  wire,  for  example, 
current  will  flow  from  copper  to  iron  at  the  hot  junction,  or  from  iron  to 
copper  at  the  cold  junction.  Two  factors  combine  to  cause  the  current. 
An  electromotive  force  exists  between  the  two  metals,  the  magnitude  of 
which  depends  upon  the  temperature  and  upon  the  metals;  this  is  called 
the  Peltier  e.m.f.  If  a  single  wire  of  homogeneous  material  is  heated 
at  one  end,  an  electromotive  force  is  developed  between  the  ends  of  the 
wire,  the  magnitude  of  which  depends  upon  the  metal  and  upon  the  differ- 
ence in  temperature;  this  is  known  as  the  Thomson  e.m.f.  The  total 
e.m.f.  acting  in  the  circuit  is  the  sum  of  the  Peltier  e.m.f.  at  the  two  junc- 
tions and  the  Thomson  e.m.f.  in  each  wire,  consideration  being  given, 
of  course,  to  the  algebraic  signs  of  the  four  e.m.f.'s.  The  total  electromo- 
tive force  thus  depends  upon  the  difference  in  temperature  of  the  two 
junctions.  If  the  temperature  of  one  junction  is  fixed,  the  temperature  of 
the  other  junction  can  be  determined  by  measuring  the  electromotive 
force  developed  in  the  circuit;  this  is  the  basic  principle  of  thermoelectric 
pyrometry.  The  electromotive  forces  developed  by  thermocouples  are 
small,  usually  a  few  thousandths  of  a  volt;  to  measure  such  small  e.m.f.'s 
special  types  of  sensitive  voltmeters  (millivoltmeters)  or  indicators  are 
required.  For  any  particular  type  of  couple,  these  instruments  may  be 
graduated  to  read  temperatures  directly  instead  of  electromotive  force. 

A  simple  thermoelectric  pyrometer  consists  of  three  parts: 

(a)  The  thermocouple  of  two  different  metals  or  alloys,  having  a  fused 
junction  which  is  inserted  into  the  furnace,  while  the  cold  junctions  pro- 
trude from  the  furnace  and  are  maintained  at  some  fixed  temperature, 
such  as  that  of  the  room  or  of  melting  ice. 

(&)  Two  lead  wires,  usually  of  copper,  running  from  the  cold  junctions 
of  the  thermocouple  to  the  indicator. 

(c)  The  indicator,  which  may  be  a  millivoltmeter,  a  potentiometer, 
or  a  special  type  of  instrument  embodying  both  of  these  principles,  and 

*  U.  S.  Bureau  of  Standards. 


PAUL   D.    FOOTE,    T.    R.    HARRISON   AND    C.    O.    FAIRCHILD  75 

may  be  graduated  to  read  either  electromotive  force,  or  temperature,  or 
both. 

Although  any  two  dissimilar  metals  might  be  employed  for  a  thermo- 
couple, certain  combinations  are  unsatisfactory  because  of  the  very  small 
e.m  f.'s  which  can  be  developed,  and  because  of  the  fact  that  with  some 
combinations  the  electromotive  force  may  first  increase,  then  decrease, 
become  zero,  and  finally  change  sign,  as  the  temperature  increases. 
Obviously  desirable  properties  for  a  thermocouple  are : 

1.  Ability  to  resist  corrosion  and  oxidation. 

2.  Development  of  relatively  large  electromotive  forces. 

3.  A  temperature-e.m.f.  relation  such  that  the  latter  increases  con- 
tinuously with  increasing  temperature  over  the  range  to  be  employed. 

For  general  work  at  the  higher  temperatures,  several  different  types 
of  couples  are  employed  in  the  United  States.  Up  to  360°  C.  for  extreme 
precision,  or  to  500°  C.  for  a  precision  of  5°  or  10°,  the  couple  may  have 
one  wire  of  copper  and  the  other  of  constantan.  Iron-constantan  or 
nichrome-constantan  couples  may  be  employed  for  technical  processes 
below  900°  C.  For  operation  below  1100°  C.  special  patented  alloys  of 
chromium  and  nickel  and  of  aluminum  and  nickel,  chromel-alumel  or 
nichrome-alumel  are  satisfactory,  even  for  continuous  service.  For  the 
range  300°  to  1500°  C.  the  Le  Chatelier  couple  should  be  employed, 
having  one  wire  of  platinum  and  the  other  of  an  alloy  containing  90  per 
cent,  platinum  and  10  per  cent,  rhodium.  Other  alloys  and  metals  may 
be  employed  for  special  work,  but  the  above  combinations  are  sufficient 
for  almost  all  technical  processes  conducted  at  less  than  1500°  C.  No 
satisfactory  c'ouple  has  been  developed  for  operation  much  above  1500° 
C.;  there  are  several  metals  and  numerous  alloys  which  melt  only  at  much 
higher  temperatures,  but  all.  so  far  known,  are  subject  to  serious  dis- 
advantages which  prevent  their  practical  application.  For  example,  a 
couple  having  one  wire  of  iridium  and  the  other  of  the  alloy  Ir  90  X  Ru 
10,  can  be  used  up  to  2000°  C.,  but  it  is  so  costly  as  to  be  prohibitive,  so 
fragile  and  brittle  that  a  slight  jar  will  fracture  it,  and  the  iridium  rapidly 
volatilizes  even  at  1200°  C.  Tungsten-molybdenum  could  be  employed 
possibly  up  to  2400°  C.,  but  both  of  these  metals  oxidize  so  readily  that 
extreme  care  must  be  taken,  by  the  use  of  hydrogen,  to  prevent  oxida- 
tion; a  satisfactory  method  for  thus  protecting  such  a  couple  for  tech- 
nical purposes  has  never  been  developed.  The  peculiar  fact  that  nickel 
is  readily  oxidized  and  made  brittle  by  heating  in  air,  but,  when  alloyed 
with  chromium  or  aluminum,  resists  oxidation  and  does  not  crystallize 
so  rapidly,  suggests  the  possibility  that  certain  similar  alloys  of  tungsten 
may  be  developed  which  will  not  suffer  the  rapid  oxidation  characteristic 
of  the  pure  metal.  Such  alloys  might  prove  of  great  value  in  thermo- 
electric pyrometry. 

The  principal  cause  of  change  in  calibration  is  inhomogeneity,  which 


76  THERMOELECTRIC    PYROMETRY 

may  develop  through  contamination  by  fumes  or  metallic  vapors  from 
the  furnace,  through  oxidation,  or  for  other  reasons.  Contamination 
may  usually  be  prevented  by  the  use  of  suitable  protecting  tubes,  and 
the  effect  of  contamination  may  be  minimized  by  using  wire  of  large 
cross-section.  Different  furnace  conditions  and  different  types  of  couples 
require  different  methods  of  protection  against  contamination ;  for  example, 
a  platinum  couple  is  usually  protected  by  refractory  porcelain  tubes,  but 
if  the  atmosphere  surrounding  the  platinum  be  reducing,  the  porcelain  may 
do  more  harm  than  good,  because  the  reducing  atmosphere  changes  the 
silica  of  the  porcelain  into  silicon,  which  readily  attacks  the  platinum. 
The  electromotive  force  of  some  couples  gradually  diminishes  with  use. 
The  platinum  and  Pt  90  +  Ir  10  couple  has  not  proved  very  satisfactory 
for  this  reason,  although  it  develops  a  much  larger  force  than  the  plati- 
num-rhodium couple;  the  iridium  gradually  distills  from  the  alloy  wire, 
especially  above  1200°  C.,  requiring  frequent  recalibration. 

When  thermocouples  are  employed  in  the  laboratory  for  scientific 
purposes,  although  desirable,  it  is  not  important  that  the  calibration 
of  couples  of  the  same  type  be  exactly  similar.  In  an  industrial  plant, 
however,  the  question  of  reproducibility  is  of  considerable  moment. 
The  indicating  instruments  are  usually  graduated  in  degrees  of  tempera- 
ture, and  the  graduation  holds  for  one  definite  temperature-e.m.f .  relation. 
If  the  calibrations  of  various  couples  of  the  same  type  are  not  similar, 
corrections  must  be  applied  to  the  readings  of  the  indicator,  and  these 
corections  will  be  different  for  every  couple.  When  several  couples  are 
operated  with  one  indicator,  and  when  the  process  is  such  as  to  require 
frequent  renewal  of  couples,  the  application  of  these  corrections  becomes 
troublesome.  For  extreme  precision  it  is  always  necessary  to  apply  such 
corrections,  but  for  most  industrial  processes,  thermocouples  which  are 
sufficiently  interchangeable  can  be  secured,  so  that  the  corrections  may 
be  omitted.  Thus  the  calibration  of  different  homogeneous  and  uncon- 
taminated  chromel-alumel  couples  should  not  vary  by  more  than  10°  or 
15°  C.,  and  of  platinum-rhodium  couples  by  more  than  2°  or  3°  C.  The 
variations  in  iron-constantan  couples  have  been  considerably  greater, 
but  rapid  progress  is  now  being  made  in  the  production  of  constantan 
having  reproducible  thermoelectric  characteristics.  No  industrial  proc- 
esses conducted  at  high  temperatures  require  such  accurate  temperature 
control  that  variations  in  the  calibration  of  new  platinum-rhodium 
thermocouples,  of  the  same  type,  warrant  consideration.  Variations 
in  the  calibration  of  different  base-metal  couples  are  frequently  corrected 
by  the  use  of  series  or  shunt  resistance;  but  most  of  the  methods  yet 
devised  are  somewhat  unsatisfactory,  and  some  of  the  compensating 
devices,  after  continued  use,  may  develop  larger  errors  than  those  arising 
from  the  variation  of  the  couple,  as  will  be  shown  later.  The  above 
remarks  as  to  reproducibility  apply  only  to  new  couples;  after  a  couple 


PAUL   D.    FOOTE,    T.    R.    HARRISON    AND    C.    O.    FAIRCHILD 


77 


has  been  used  for  some  time,  especially  a  base-metal  couple,  or  has  become 
contaminated  in  any  manner,  the  calibration  may  change  considerably. 
All  thermocouples  should  be  tested  frequently  under  operating  conditions. 
The  proper  mounting  and  protection  of  a  thermocouple  is  of  great  im- 
portance; the  correct  protection  depends  upon  the  particular  process  in 
which  the  couple  is  employed,  and  will  be  considered  in  some  detail 
later.  The  usual  rare-metal  couple  consists  of  wires  0.5  mm.  or  prefer- 
ably 0.6  mm.  in  diameter  and  from  50  to  125  cm.  in  length.  Wires  as 
small  as  0.1  mm.  and  even  less  are  frequently  used  for  special  research. 
One  or  both  oft  he  wires  are  insulated  by  threading  them  through  small 
porcelain  or  quartz  tubes.  For  measuring  temperatures  below  about 
1400°  C.,  two-hole  porcelain  tubes  are  useful,  but  for  higher  tempera- 


A 


FIG.  1. — COUPLES   MADE  BY   THWING  INSTRUMENT  Co.     A.   IRON-CONSTANT  AN 

COUPLE  IN  IRON  PROTECTING  TUBE  FOR  USEBELOW  900°  C.  B.  EXTENSIBLE  CHROMEL- 
ALUMEL  COUPLE  FOR  MOLTEN  BRASS  J  THE  EXPOSED  JUNCTION,  WHICH  IS  IMMERSED  IN 
THE  MOLTEN  BRASS  WITHOUT  PROTECTION,  IS  RENEWED  FROM  THE  MAGAZINE  OF  RESERVE 

WIRE.     C.  PORCELAIN  OR  FUSED  SILICA  PROTECTING  TUBE  AND  MOUNTING  FOR  KARE- 

METAL  COUPLES. 


tures  separate  tubes  should  be  used.  The  hot  junction  of  the  couple  is 
made  by  fusing  the  two  wires  in  an  arc  or  oxygen-gas  flame.  The  couple 
and  insulating  tube  are  inserted  in  a  small  outside  protecting  tube  of 
porcelain,  glazed  on  the  outside  only,  or  of  fused  silica,  hemispherically 
closed  at  one  end.  On  the  open  end  of  the  protecting  tube  may  be 
mounted  the  head  of  the  couple,  which  serves  as  a  handle  and  as  a 
support  for  rigidly  holding  the  wires.  The  couple  wires  frequently  extend 
beyond  the  head  so  that  their  ends  may  be  maintained  at  some  con- 
trolled cold-junction  temperature.  Usually  the  wires  terminate  in  bind- 
ing posts  on  the  couple  head,  in  which  case  the  cold-junction  temperature 
may  be  controlled  by  water  jackets,  or  may  be  allowed  to  remain  that 


78 


THERMOELECTRIC    PYROMETRY 


of  the  surroundings,  or  the  couple  may  be  fitted  with  one  of  the  various 
devices,  discussed  later,  for  the  elimination  of  cold-junction  errors. 

Base-metal  couples  for  laboratory  use  may  be  constructed  in  much 
the  same  manner,  and  may  be  made  of  wire  as  small  as  No.  20  (0.8  mm.) 
or  of  much  smaller  wire  for  certain  types  of  research  at  lower  temperatures. 
Small  wires,  however,  are  readily  and  completely  oxidized  at  high  tem- 
peratures, so  that  for  continuous  operation  in  industrial  installations  the 
couples  are  made  of  No.  8  (3.3  mm.)  or  No.  6  (4  mm.)  wire,  or  of  still 


8      D 


FIG.  2. — END  SECTION  OF  THE  WILSON- MAEULEN  PYOD.     THE  OUTER  TUBE  P, 

WHICH  IS  ONE  ELEMENT,  IS  WELDED  TO  THE  OTHER  ELEMENT  B  AT  W.  THE  INNER 
ROD  IS  INSULATED  FROM  THE  TUBE  BY  ASBESTOS  CORD. 

larger  wire  when  there  is  danger  of  contamination.  The  hot  junction  is 
fused,  and  usually  the  two  wires  are  twisted  for  a  few  turns  at  the  hot 
junction  in  order  to  give  greater  mechanical  strength  to  the  joint.  The 
two  wires  are  insulated  by  fireclay  insulating  tubes,  or  by  asbestos  sleev- 
ing or  cord,  and  are  connected  to  a  suitable  couple  head  forming  the  cold 
junction.  For  severe  use  it  is  necessary  to  encase  the  couple  in  a  pro- 
tecting tube  of  steel,  chromel,  porcelain,  fireclay,  etc. 

A  different  form  of  base-metal  couple,  known  under  the  trade  name 
of  "pyod"  consists  of  an  outer  tube  of  iron  and  an  inner  wire  or  rod  of 


FIG.  3. — INTERIOR   OF  THERMOCOUPLE   HEAD   MADE  BY   BEIGHLEE    ELECTRIC   Co., 

SHOWING    THE    COLD-JUNCTION   COMPENSATOR   DESCRIBED    UNDER    FlG.    23. 

constantan.  The  two  are  fused  at  one  end  into  a  neat  joint  forming 
the  hot  junction,  and  are  insulated  from  each  other  up  to  the  couple 
head,  or  cold  junction.  The  couple  is  thus  mechanically  stronger  than 
one  formed  of  two  wires,  and  when  used  without  an  additional  protecting 
tube,  is  somewhat  less  liable  to  contamination  than  the  bare-wire 
couple.  Pyod  couples  should  nevertheless  be  protected  by  outer  tubes 
if  subjected  to  severe  furnace  conditions.  Figs.  1  to  3  illustrate  several 
couples  and  mountings.  It  is  sometimes  desirable  to  bend  a  couple, 


PAUL   D.    FOOTE,    T.    R.    HARRISON   AND    C.    O.    FAIRCHILD  79 

usually  a  mounted  couple  will  stand  bending,  but  one  should  first 
remove  porcelain  tubes  or  insulators  liable  to  be  broken  by  the 
process. 

INDICATING  INSTRUMENTS 

The  indicating  instruments  connected  to  the  thermocouple  are  of  three 
general  types;  those  operating  upon  the  galvanometric  principle,  as  an 
ordinary  voltmeter;  those  operating  upon  the  potentiometric  principle; 
and  those  operating  upon  a  combination  of  these  two  principles.  The 
first  two  types  of  instrument  have  been  made  automatically  recording,  as 
will  be  discussed  elsewhere. 

Galvanometer  Method 

Galvanometers  for  measuring  the  electromotive  force  developed  by  a 
thermocouple  usually  operate  on  the  d'Arsonval  principle,  having  a 
moving  coil  mounted  between  the  poles  of  a  permanent  magnet.  Dif- 
ferent methods  for  mounting  the  coil  are  employed.  The  coil  may  be 
suspended  both  above  and  below  by  phosphor-bronze  suspensions,  and 
although  many  foreign  instruments  of  this  type  have  proved  delicate, 
one  of  the  latest  forms  of  American  double-suspension  galvanometer  may 
be  subjected  to  severe  handling  without  injury.  The  combination  of  an 
upper  suspension  and  a  lower  pivot  has  been  used  extensively.  A  uni- 
pivot  system  is  employed  by  one  English  and  one  American  manufacturer. 
The  pivot  is  at  the  center  of  the  coil  and  the  center  of  gravity  of  the  whole 
moving  system  is  at  the  point  of  the  pivot. 

The  scale  of  the  instrument  may  be  graduated  to  read  e.m.f.  or  tem- 
perature. By  adding  a  series  resistance,  mounted  inside  the  galva- 
nometer case,  and  an  extra  terminal,  two  scale  ranges  may  be  utilized,  one 
for  base-metal  and  the  other  for  rare-metal  couples.  Indicators  may  be 
obtained  in  either  the  switchboard  or  the  portable  type,  the  former 
being  desirable  for  permanent  installations.  Usually  high  precision  is 
not  required  of  an  instrument  of  the  switchboard  type,  so  that  the 
graduations  may  be  coarse. 

Resistance  of  Indicating  Instrument. — When  operated  at  the  highest 
safe  working  temperatures,  most  base-metal  couples  develop  a  maximum 
e.m.f.  of  less  than  50  to  70  millivolts,  and  the  LeChatelier  couple  about 
16  millivolts;  a  very  sensitive  indicator  or  milli voltmeter  is  therefore 
required.  On  the  other  hand,  the  instrument  must  be  able  to  withstand 
rough  handling,  and  these  opposing  conditions  are  difficult  to  satisfy. 
The  deflection  registered  by  the  millivoltmeter  is  approximately  propor- 
tional to  the  current  flowing  through  the  coil;  hence,  if  the  resistance  of 
the  total  thermoelectric  circuit  is  low,  relatively  large  currents  are 
obtained,  resulting  in  a  torque  high  on  the  movable  coil.  When  the 
current  is  large,  the  construction  of  the  indicator  may  therefore  be  robust; 


80  THERMOELECTRIC  PYROMETRY 

strong  springs  for  balancing  the  turning  moment  of  the  coil  may  be  em- 
ployed, and  less  attention  need  be  given  to  the  friction  of  the  pivots  in 
their  bearings.  The  torque  may  be  made  high  without  greatly  increas- 
ing the  resistance  of  the  circuit  by  using  a  great  number  of  turns  of  copper 
wire  in  the  coil.  Copper  possesses  a  large  temperature  coefficient  of 
resistance,  so  that  ordinarily  the  calibration  of  an  instrument  having 
its  entire  electrical  circuit  of  copper  would  be  affected  by  the  temperature. 
However,  by  the  use  of  shunt  and  series  resistances  of  a  certain  type  it  is 
possible  to  reduce  these  errors  to  a  negligible  amount.  Thus  it  is  not 
difficult  to  construct  a  sufficiently  sensitive  millivoltmeter  of  low  resist- 
ance. The  objection,  from  the  pyrometric  standpoint,  to  such  an  instru- 
ment used  as  a  simple  galvanometer  has  led  to  the  development  of 
indicators  having  considerable  resistance.  These  are  made  by  placing 
a  high  resistance,  of  zero  temperature  coefficient,  such  as  manganin,  in 
series  with  the  coil,  and  by  increasing  the  number  of  turns  on  the  moving 
coil  to  compensate  for  the  decrease  in  sensitivity  arising  from  the  increased 
resistance.  The  so-called  swamping  resistance,  having  zero  tempera- 
ture coefficient,  may  be  so  proportioned  with  respect  to  the  copper  that, 
due  account  being  taken  of  the  temperature  coefficient  of  elasticity  of 
the  springs,  the  instrument  as  a  whole  possesses  a  negligible  temperature 
coefficient.  The  high  resistance  greatly  diminishes  the  current  flowing 
through  the  coil  and  thus  reduces  the  deflection;  hence,  attention  must 
be  given  to  the  elimination  of  bearing  friction,  and  the  instrument  is 
necessarily  more  delicate  than  a  low-resistance  indicator  of  the  same  type. 
The  advantage  of  an  instrument  having  a  high  resistance  is  demonstrated 
by  the  following  discussion. 

The  current  flowing  in  the  circuit  is  equal  to  e  -f-  R,  e  being  the  elec- 
tromotive force  developed  by  the  couple  and  R  the  total  resistance  of  the 
circuit.  While  the  temperature  of  the  furnace  remains  fixed,  e  is  constant, 
but  the  deflection  of  the  instrument  will  be  affected  by  changes  in  resist- 
ance; hence  any  variation  in  R  which  produces  a  change  in  the  reading  of 
the  instrument  would  be  interpreted  as  a  change  in  the  temperature  of 
the  furnace.  The  total  resistance  of  the  circuit  consists  of  three  parts, 
Rg,  of  the  millivoltmeter,  RL,  of  the  line,  and  Rc,  of  the  couple.  If  these 
elements  are  properly  proportioned,  the  effect  upon  the  reading  of  the 
indicator,  due  to  any  changes  in  the  total  resistance  likely  to  occur,  can 
be  reduced  practically  to  zero.  This  condition  is  realized  when  the 
resistance  of  the  galvanometer  is  sufficiently  high  compared  with  the 
resistance  of  the  external  circuit.  Suppose  that  the  indicator  has  a  scale 
graduated  to  read  the  potential  difference  at  its  terminals.  The  relation 
between  the  reading  of  the  instrument  e0  and  the  true  e.m.f.  e  of  the 
couple  is  given  by  the  following  equation  : 

Ra 


Rg  +  Rc  + 


PAUL   D.    FOOTE,    T.    R.    HARRISON   AND   C.    O.    FAIRCHILD  81 

Thus,  if  Rg  is  large  compared  to  Rc  +  RL,  the  ratio  Rg  -f-  (Rg  +  Rc  +  RL) 
is  practically  1,  and  the  reading  of  the  galvanometer  gives  the  true 
e.m.f.  of  the  couple. 

Robust  indicators  are  now  obtainable  having  resistances  of  300  to 
1200  ohms.  Consider,  for  example,  an  installation  in  which  the  galva- 
nometer resistance  is  300  ohms,  couple  resistance  1  ohm,  line  resistance 

1  ohm. 

*•  -  B-inhnr e  -  wrrTjrr e  -  °-993e 

Kg  -\~  Kc  ~T  K>L  0\J\J  -J-    1  ~T   J- 

Thus  the  reading  of  the  instrument  gives  the  true  electromotive  force  of 
the  couple  to  within  0.7  per  cent.  Instruments  having  a  resistance  as 
low  as  10  ohms,  or  less,  are  in  extensive  use.  Suppose  a  galvanometer  of 
10-ohm  resistance  were,  used  in  the  circuit  described  above: 

Rg  10 

e0  =  p     i'p~jrp~  e  =  iTr^rrXT  =  °-83e 

Thus  the  e.m.f.  indicated  by  the  instrument  would  be  17  per  cent,  less 
than  the  true  e.m.f.  of  the  couple.  Such  large  errors  are  compensated 
by  arbitrarily  graduating  the  scale  to  read  the  true  e.m.f.  of  the  couple 
when  the  external  resistance  has  a  certain  value.  Bad  contacts,  deteriora- 
tion of  the  couple  from  oxidation,  change  in  depth  of  insertion,  tempera- 
ture coefficient  of  the  copper  lead  wires,  etc.  may  at  any  time  alter  the 
resistance  of  the  external  circuit.  Let  us  compare  the  behavior  of  the 
300-ohm  instrument  and  the  10-ohm  instrument,  assuming  both  are 
compensatingly  graduated  to  read  correctly  for  an  external  resistance  of 

2  ohms,  when  for  one  of  the  several  reasons  cited  above  the  external 
resistance  varies  slightly.     The  relation  between  the  potential  drop  e0 
across  the  terminals  of  the  galvanometer  and  the  e.m.f.  e  of  the  couple  is 
as  follows,  where  the  total  line  resistance  R'  —  Rc  +  RL- 

e0  =       R°      e 

Hence  eQ  is  always  less  than  e.  Suppose,  however,  for  a  certain  line  re- 
sistance R'o  the  scale  of  the  indicator  is  arbitrarily  graduated  so  that 
the  reading  e'  equals  the  true  e  of  the  couple.  The  relation^  bet  ween  the 
scale  reading  and  the  potential  drop  across  the  terminals  of  the  instru- 
ment must  be,  accordingly 

,  Rg  +  R'o  n 

e'  =  -  —„       e0  =  Fe0 

Kg 

where  F  is  a  constant.  The  graduations  on  the  scale  are  such  that  for 
any  deflection  of  the  pointer  the  scale  reading  is  F  times  the  potential 
drop  across  the  galvanometer  terminals.  On  substituting  this  value  of 
e0  in  the  above  equation  we  obtain: 

FRa 
e    ~*e°~R0  +  R'e 


82 


THERMOELECTRIC    PYROMETRY 


The  following  table  shows  the  percentage  error  in  the  indicated 
e',  computed  from  the  above  equation,  when  the  line  resistance  R'  has 
the  values  1,  2,  3  and  4  ohms  respectively,  when  Rg,  the  resistances  of  the 
indicators,  are  300  and  10  ohms  respectively,  and  R'0,  the  normal  line 
resistance,  is  2  ohms. 

TABLE  1. — Error  Due  to  Change  in  Line  Resistance 


Line 
Resistance, 
Ohms 

Per  Cent.  Error  in  Indicator  Reading 

Error  in  Degrees  at  1000°  C. 

300  Ohms 

10  Ohms 

300  Ohms 

10  Ohms 

1 

+0.33 

+9.1 

+3.3 

+91 

2 

0 

0 

0 

0 

3 

-0.33 

-7.7 

-3.3 

-77  . 

4 

-0.66 

—14.3 

-6.6 

-143 

Thus,  when  both  instruments  read  correctly  for  an  external  resistance 
of  2  ohms,  if  the  external  resistance  is  increased  by  1  ohm,  the  low-resist- 
ance indicator  is  in  error  by  7.7  per  cent,  or  about  77°  at  1000°  C.,  while 
the  high-resistance  instrument  still  reads  practically  correct.  This 
emphasizes  the  importance  of  using  a  galvanometer  having  a  resistance 
of  300  ohms  or  more.  In  actual  operation  the  line  resistance  may  change 
by  several  ohms  on  account  of  bad  contacts  and  deterioration  of  the 
thermocouple. 

It  must  not  be  inferred  that  all  high-resistance  indicators  are  neces- 
sarily superior  to  all  indicators  of  low  resistance.  Superior  workman- 
ship and  certain  mechanical  details  may  lead  to  the  choice  of  a  particular 
instrument  having  a  resistance  less  than  100  ohms,  especially  in  the  case 
of  recording  milli voltmeters;  nevertheless  the  resistance  of  the  instrument 
is  a  matter  of  extreme  importance,  and  a  galvanometric  indicator  of  low 
resistance  is  always  subject  to  the  errors  arising  from  small  changes  in 
the  resistance  of  the  circuit.  This  does  not  apply  to  the  compensated 
galvanometer,  or  to  semi-potentiometric  instruments  described  later. 
Figs.  4  and  5  represei  t  typical  American  indicating  galvanometers. 

Portable  Test  Set. — On  account  of  the  errors  which  may  be  introduced 
in  the  reading  of  a  galvanometer  by  variation  in  resistance  of  the  line 
or  couple,  it  is  important  to  have  some  means  for  measuring  this  resist- 
ance occasionally.  Every  plant  requiring  a  large  thermocouple  installa- 
tion with  simple  galvanometric  indicators  should  have  a  portable 
Wheatstone  bridge  or  "test  set"  for  this  purpose.  Fig.  6  illustrates  a 
simple  and  inexpensive  instrument  made  by  Leeds  &  Northrup  Company. 
Disconnect  the  pyromecer  indicator  from  the  circuit  and  connect  the  two 
line  wires  to  the  X  terminals  of  the  test  set;  note  the  measured  resistance. 
Reverse  the  +  and  --  lead  wires  at  X  and  obtain  a  new  reading;  the 


PAUL   D.    FOOTE,    T.    R.    HARRISON   AND    C.    O.    FAIRCHILD 


83 


mean  of  the  two  observations  is  the  resistance  of  the  circuit,  the  two  read- 
ings being  different  because  of  the  e.m.f.  developed  by  the  couple.  If 
two  indicators,  or  an  indicator  and  a  recorder,  are  operated  in  parallel 


FIG.  4. — MOVING  ELEMENT  OF  THE  ENGELHARD  INDICATOR.  A  DOUBLE-SUSPEN- 
SION INSTRUMENT  IN  WHICH  THE  SUSPENSIONS  ARE  KEPT  UNDER  TENSION  BY  THE 
SPRINGS  A  AND  B.  THE  TENSION  IS  SUFFICIENT  TO  MAINTAIN  AXIAL  ALIGNMENT  OF 
THE  COIL  WITHOUT  PRECISE  LEVELING.  THE  INSTRUMENT  IS  VERY  ROBUST  AND  HAS 
A  HIGH  RESISTANCE,  ABOUT  50  OHMS  PER  MILLIVOLT. 


FIG.  5. — SINGLE-PIVOT   MOVEMENT  USED   IN   WILSON-MAEULEN   INDICATORS 
PIVOT  IS  AT  THE   CENTER  OF  GRAVITY  OF  THE  MOVING  SYSTEM. 


THE 


on  the  same  circuit,  care  must  be  taken  that  both  instruments  are  dis- 
connected from  the  circuit  during  the  measurement  of  the  resistance. 
If  the  resistance  of  the  line  and  couple  is  found  to  be  much  higher  than 
that  for  which  the  indicator  was  designed,  short-circuit  the  line  at  the 


84 


THERMOELECTRIC    PYROMETRY 


cold  junction  and  determine  whether  the  fault  is  in  the  couple  or  in  the 
line;  if  in  the  former,  the  couple  usually  should  be  replaced.  By  such 
occasional  observations  serious  faults  may  be  detected  long  before  they 
would  be  suspected  from  the  low  values  in  the  indicated  temperatures. 
Galvanometer  with  Variable  Series  Resistance. — Galvanometers,  es- 
pecially those  of  low  resistance,  are  usually  calibrated  to  read  correctly 
for  a  definite  line  resistance.  Suppose  an  indicator  is  desired  for  a  line 
the  resistance  of  which  changes  from  almost  zero  to  10  ohms.  The  in- 
strument is  calibrated  to  read  correctly  for  a  line  resistance  of  10 


FIG.  6. — "TEST  SET".  OR  WHEATSTONE  BRIDGE  FOR  MEASURING  LINE    RESISTANCE. 

(Leeds  &  Northrup.) 

ohms,  and  in  the  galvanometer  case,  in  series  with  the  line,  is  a  variable 
resistance  which  can  be  adjusted  by  hand  until  the  sum  of  the  line  re- 
sistance and  the  variable  resistance  equals  10  ohms.  The  dial  of  the 
variable  resistance  is  graduated  to  read  the  amount  of  resistance  cut  out 
of  the  circuit;  hence  it  should  be  set  at  the  resistance  of  the  line  and 
couple,  which  value  may  be  determined  by  a  test  set.  This  method  is  of 
great  value  for  precision  work  with  a  galvanometric  indicator.  The 
principal  objection  to  it,  which  also  applies  to  all  galvanometric  indica- 
tors thus  far  described  when  used  for  accurate  measurements,  is  the 
necessity  for  measuring  the  resistance  of  the  line  and  couple.  This  ob- 
jection is  avoided  and  other  desirable  features  have  been  added  in  the 
instrument  next  mentioned. 

Harrison-Foote  Compensated  Indicator. — This  instrument,  manufac- 
tured by  The  Brown  Instrument  Co.,  is  illustrated  in  a  simple  form  by  Fig. 
7.  The  circuit  CDGF  is  an  ordinary  millivoltmeter  in  which  G  represents 


PAUL   D.    FOOTE,    T.    R.    HARRISON   AND    C.    O.    FAIRCHILD 


85 


the  moving  coil,  in  series  with  which  is  an  adjustable  rheostat  CB. 
The  maximum  value  r5  of  this  resistance  is  chosen  equal  to  the  maximum 
value  of  the  resistance  of  the  line  and  couple  likely  to  occur  in  practice;  a 
convenient  value  is  15  ohms.  With  the  slide  of  the  rheostat  set  for  the 
maximum  resistance,  TI  =  r5,  the  instrument  is  calibrated  in  terms  of 
the  potential  drop  across  AH.  Hence  when  the  instrument  is  con- 
nected through  the  line  having  resistance  L  to  the  couple  having  resist- 
ance T,  the  rheostat  CB  must  be  adjusted  until  TI  +  L  +  T  =  r$.  The 
scale  reading  then  gives  correctly  the  e.m.f .,  e,  of  the  couple,  or  the  correct 
temperature  if  the  scale  is  graduated  in  degrees.  This  adjustment  is 


FIG. 


HARRISON-FOOTE 


COMPENSATED    INDICATOR    (BROWN    INSTRUMENT    Co. 
IMPROVED  HEATMETER). 


made  in  the  following  manner.  By  depressing  the  key  K  a  portion  of  the 
galvanometer  resistance  rz  is  short-circuited  and  the  rest  of  the  resistance 
r3,  containing  the  moving  coil,  is  shunted  by  a  resistance  r4.  If  e'  repre- 
sents the  potential  drop  across  DF  when  the  key  is  open,  and  e"  repre- 
sents the  drop  when  the  key  is  closed,  we  obtain  : 


e'  = 


er^ 


L  +  T  +  n  +  r2  +  r3 


e"  = 


If  TI  is  so  adjusted  that  these  two  potential  drops,  and  hence  the  deflec- 
tions of  the  indicator,  are  the  same,  we  have/  on  equating, 


L  +  T  +  ri  = 


rgr4 


=  a  constant. 


If  now,  in  the  construction  of  the  instrument,  r2r4  -T-  r3  is  made  equal  to 
r5,  the  above  setting  makes  L  +  T  +  ri  =  r5,  the  adjustment  required 
in  order  that  the  reading  of  the  scale  may  give  the  true  e.m.f.  of  the  couple. 
The  ease  with  which  the  proper  setting  can  be  obtained  is  greatly 
improved  by  making  ?-3  -T-  rt  equal  to  from  5  to  10.  Suppose  it  be  made 
equal  to  9.  Then  if  TI  is  improperly  adjusted,  and  the  instrument  reads 


86  THERMOELECTRIC .  PYROMETRY 

in  error  by,  say,  8e  when  K  is  open;  on  depressing  the  key  the  reading 
is  changed  by  98e.  If  now  n  is  readjusted  with  the  key  depressed  until 
the  reading  takes  its  initial  value,  the  error  with  the  key  open  is  reduced 
to  5e-r-10.  The  process  for  operating  the  instrument  is  accordingly  as 
follows: 

1.  Read  the  instrument  with  the  key  open. 

2.  Close  the  key  and  adjust  the  rheostat  TI  until  the  instrument  reads 
approximately  the  same  as  in  1. 

3.  Repeat  1  and  2  if  necessary. 

When  r3  -r-  rt  =  9  it  is  rarely  necessary  to  make  a  second  adjustment. 
In  position  1  the  instrument  acts  as  an  ordinary  galvanometer.  The 
single  setting  in  position  2  reduces  the  error  in  the  ordinary  galvanometer 
by  the  factor  ^-fo>  which  is  usually  sufficient.  The  adjustment  for  the 
proper  external  resistance,  if  desired,  can  be  made  with  10  times  the  pre- 
cision necessary.  Variation  in  line  resistance,  which  might  give  rise  to 
very  serious  errors,  is  thus  easily  and  accurately  controlled  by  a  simple 
mechanical  adjustment. 

The  device  is  readily  applicable  to  multiple  installations  of  different 
line  resistances.  For  multiple  point  recorders  and  indicators,  as  many 
resistances  BC  may  be  employed  as  there  are  couples.  These  may  be 
inexpensive  rheostats,  having  a  resistance  of  approximately  15  ohrns  each, 
located  in  each  line  between  the  couple  and  the  selective  switch;  they 
may  be  adjusted  in  the  manner  described  whenever  convenient  or  neces- 
sary. A  suitable  proportioning  of  resistances  for  a  300-ohm  indicator 
would  be:  r2  =  135  ohms;  r3  =  150  ohms;  r4  =  150  -r-  9  =  16%  ohms; 
r6  =  15  ohms;  r2  +  r3  +  r5  =  300  ohms. 

If  the  simple  indicator  has  the  proper  ratio  of  manganin  to  copper,  its 
temperature  coefficient  is  practically  zero.  In  that  case  the  shunt  r4 
should  have  the  same  manganin  to  copper  ratio  as  the  portion  of  the 
galvanometer  resistance  comprised  by  r3,  thus  giving  the  instrument  as  a 
whole  a  zero  temperature  coefficient.  If  the  simple  indicator  does  not 
have  a  zero  temperature  coefficient  it  is  possible,  by  increasing  the  pro- 
portion of  manganin  in  r4,  to  compensate  for  the  temperature  coefficient 
of  the.  resistance  r3. 

Potentiometer  Method 

The  potentiometer  method  is  the  most  accurate  for  measuring  the 
e.m.f.  of  a  thermocouple.  The  fundamental  principle  is  illustrated  by 
Fig.  8.  A  constant  current  from  the  battery  B  flows  through  the  slide- 
wire  resistance  abc.  One  wire  of  the  couple  T  is  connected  to  the  movable 
contact  b  and  the  other  wire,  in  series  with  a  sensitive  galvanometer,  is 
connected  to  a.  The  contact  6  is  moved  until  the  galvanometer  reads 
zero,  showing  that  no  current  is  flowing  through  the  thermocouple  circuit; 
the  true  e.m.f.  of  the  couple  is  then  equal  to  the  potential  drop  across 


PAUL   D.    FOOTE,    T.    R.    HARRISON   AND    C.    O.    FAIRCHILD  87 

ab,  and  this  is  computed  from  e  =  ir,  where  i  is  the  current  flowing  through 
the  resistance  r  =  ab.  The  slide  wire  may  be  graduated  to  read  milli- 
volts or  temperature  directly,  if  the  current  is  always  adjusted  to  a  definite 
value;  various  devices  are  employed  for  this  purpose.  An  ammeter  will 
answer,  but  the  usual  method  is  to  obtain  a  preliminary  setting  by  re- 
placing the  thermocouple  by  a  constant  known  source  of  e.m.f.,  such  as 
a  standard  cell.  The  galvanometer  G  is  always  used  as  a  zero  instrument 
in  a  strictly  potentiometric  circuit;  hence  it  requires  no  calibrated  scale 
and  no  attention  need  be  given  to  the  constancy  of  its  sensitivity,  pro- 
vided it  is  always  sufficiently  sensitive  to  serve  its  purpose.  These 


/WvVWVWNM/WWWWV 

/, 


B 
FIG.  8. — SIMPLE  WIRING  DIAGRAM  FOR  POTENTIOMETRIC  INDICATOR. 

requirements  are  easily  met,  and  the  entire  potentiometer,  including 
galvanometer,  battery,  standard  cell,  slide  wires,  etc.  are  mounted  in  a 
case  not  much  larger  than  that  of  a  millivoltmeter.  The  instrument  is  as 
mechanically  robust  as  many  indicators  operating  on  the  galvanometric 
principle. 

The  potentiometer  method  has  several  important  advantages.  The 
scale  is  easily  made  very  open,  thus  permitting  accurate  readings;  the 
instrument  described  below  has  a  scale  length  of  40  cm.  The  calibration 
of  the  scale  is  in  no  way  dependent  upon  the  constancy  of  magnets, 
springs,  or  jewel  bearings,  nor  upon  the  level  of  the  instrument.  From 
the  pyrometric  standpoint,  the  greatest,  advantage  is  the  complete 
elimination  of  errors  due  to  changes  in  the  resistance  of  the  couple  or  of 
the  lead  wires;  no  matter  what  resistance  is  inserted  in  the  thermocouple 
circuit,  the  reading  of  the  potentiometer  still  gives  the  true  e.m.f.  of  the 
couple  although,  of  course,  the  sensitivity  of  the  instrument  is  decreased 
by  excessive  resistance.  The  only  objections  to  the  potentiometer  are 
its  slightly  greater  initial  cost  and  the  fact  that  usually  a  manual  adjust- 
ment must  be  made  to  obtain  a  setting.  In  the  potentiometric  recording 
instrument,  however,  all  the  various  manipulations  may  be  performed 
mechanically,  even  to  the  balancing  against  the  standard  cell. 

Fig.  9  illustrates  the  portable  potentiometer  manufactured  by  Leeds 
&  Northrup,  and  Fig.  10  shows  the  wiring  diagram.  The  galvanometer 
should  be  adjusted  to  read  zero  on  open  circuit.  At  intervals  of  a  few 


88 


THERMOELECTRIC    PYROMETRY 


hours  the  setting  on  the  standard  cell  should  be  made.  This  is  done  by 
depressing  the  key  SC  and  adjusting  the  resistance  RRr  by  turning  the 
knurled  head  on  the  left  of  the  case  until  the  galvanometer  reads  zero. 
The  thermocouple  is  connected  to  the  terminals  TC  and  the  e.m.f.  or 
temperature  is  observed  directly  on  the  dial  by  depressing  the  key  marked 


FIG.  9. — PORTABLE  POTENTIOMETER.     (Leeds  &  Northrup.) 

TC  and  turning  the  main  dial  until  the  galvanometer  indicates  zero. 
The  key  TC  and  the  key  SC  should  never  be  depressed  at  the  same  time. 
These  instruments  can  be  made  in  any  scale  range  or  with  multiple  scale 
ranges  adapted  to  various  types  of  couple. 


R       R 


Battery 


8.C.    T.C. 

FIG.  10. — WIRING   DIAGRAM   OF  LEEDS   &   NORTHRUP    PORTABLE    POTENTIOMETER. 

Semi-potentiometer  Method 

It  is  possible  by  means  of  a  single  galvanometer  or  millivoltmeter  to 
measure  the  e.m.f.  of  a  couple  potentiometrically.  Thus,  in  Fig.  8, 
by  using  a  shunted  galvanometer  first  in  the  main  circuit  abcB,  as  an 
ammeter,  the  initial  setting  for  a  standard  current  is  obtained.  Then  the 
instrument,  without  the  shunt,  is  placed  in  the  position  G  and  the  con- 


PAUL   D.    FOOTE,    T.    R.    HARRISON   AND    C.    O.    FAIRCHILD 


89 


tact  6  is  moved  along  the  graduated  slide  wire  until  a  zero  setting  is 
obtained.  The  objection  to  this  method  is  that  if  the  milli voltmeter  is 
sufficiently  sensitive  to  be  used  as  a  zero  instrument  it  is  not  likely  to  be 
reliable  as  an  ammeter,  and  vice  versa.  Various  modifications  of  this 
device,  however,  have  proved  valuable  in  thermoelectric  pyrometry. 
Northrup  Pyrovolter. — Referring  to  Fig.  11  (a),  the  dry  cell  B  con- 
tained in  the  case  of  the  instrument  sends  a  current  through  the  variable 
resistance  R  and  the  fixed  resistances  C  and  S.  The  resistance  C,  of 
copper,  is  equal  in  value  to  the  resistance  of  the  copper  coil  in  the  moving 


FIG.  11. — WIRING  DIAGRAM   OF   PYROVOLTER. 

element  of  the  galvanometer  G.  The  couple  T  is  connected,  in  series  with 
the  moving  coil  of  the  galvanometer,  across  the  resistance  S.  The 
resistance  R  is  adjusted  until  the  galvanometer  reads  zero,  by  turning  the 
knurled  head  in  the  lower  right-hand  corner  of  the  instrument.  The 
key  in  the  lower  left-hand  corner  is  then  depressed,  which  throws  the 
thermocouple  and  the  resistance  C  out  of  the  circuit,  and  replaces  C  by 
the  galvanometer  G  having  equivalent  resistance,  Fig.  11  (6).  The 
galvanometer  is  now  deflected  by  an  amount  proportional  to  the  current 
flowing  through  it,  which  in  turn  is  proportional  to  the  potential  drop 
across  S.  The  scale  of  the  instrument  is  graduated  to  read  the  potential 
drop  over  S,  and  since  this  potential  difference  was  made  equal  to  the 
e.m.f .  of  the  couple  by  the  initial  setting  for  zero  deflection,  the  galva- 
nometer indicates  directly  the  true  e.m.f.  of  the  couple.  The  initial  setting 
is  not  altered  by  introducing  resistance  into  the  thermocouple  circuit,  so 
that  the  instrument  is  really  a  form  of  potentiometer.  The  scale  may  be 
graduated  to  indicate  temperature  for  any  particular  type  of  couple,  and 
the  instrument  may  be  obtained  with  several  different  scale  ranges. 
Northrup  Continuously  Deflecting  Pyrovolter. — This  instrument  is  the 
ordinary  pyrovolter  with  the  addition  of  an.  extra  key  and  an  adjustable 
resistance.  After  the  e.m.f.  of  the  couple  has  been  determined  by  the 
pyrovolter  method  just  described,  the  galvanometer,  in  series  with  this 
resistance,  is  connected  directly  to  the  thermocouple  terminals.  The 


90 


THERMOELECTRIC    PYROMETRY 


resistance  is  then  adjusted  until  the  reading  of  the  instrument  is  the  same 
as  that  determined  by  means  of  the  pyrovolter. 

Brown  Precision  Heatmeter  (old  form,  prior  to  May,  1919). — Making 
use  of  somewhat  differently  arranged  circuits,  this  instrument  is  identical 
in  principle  with  the  Northrup  continuously  deflecting  pyrovolter.  The 
wiring  diagram  is  given  in  Fig.  12.  By  means  of  suitable  switches  the 
electrical  connections  are  thrown  successively  into  the  three  positions 
illustrated,  No.  3  being  the  final  working  position  in  which  the  couple  is 
connected  directly  to  the  millivoltmeter  through  a  definite  line  resistance. 
In  the  first  position,  the  e.m.f.  of  the  couple  is  balanced  against  the  po- 


Position  No.l  Position  Nc.2  Position  No.3 

FIG.  12. — WIRING  DIAGRAM  OP  THE  HEATMETER.     (Brown  Instrument  Co.) 

tential  drop  between  S  and  Sf  by  varying  R  and  R'  until  the  galvanome- 
ter Y  indicates  zero.  In  position  2,  the  thermocouple  circuit  is  cut  out 
and  the  galvanometer  is  connected  to  the  points  S  and  S']  the  scale  of  the 
galvanometer  is  so  divided  as  to  read  the  potential  differences  across  SS'. 
This  potential  difference  is  not  altered  by  switching  from  position  1  to 
2,  since  the  resistance  D  is  so  chosen  that  A  =  Y  +  D.  The  total  re- 
sistance of  the  galvanometer  circuit  in  position  2  is  D  +  Y  +  B.  In 
position  3  the  resistance  Rs  is  adjusted  until  the  total  resistance  of  the 
galvanometer  circuit  is  equal  to  that  of  position  2,  viz. :  D  +  Y  +  B 
=  Rc  +  RL  +  R»  +  B  +  Y.  This  adjustment  is  obtained  when  the 
reading  of  the  galvanometer  is  not  altered  by  switching  from  position 
2  to  3.  Thus  with  Rg  properly  adjusted,  the  reading  of  the  indicator  in 
position  3  gives  directly  the  true  e.m.f.  or  temperature  of  the  couple  so 
long  as  the  line  resistance  RL  +  Rc  remains  unchanged.  This  instru- 
ment is  now  superseded  by  the  Harrison-Foote  compensated  indicator, 
called  the  Brown  Improved  Heatmeter. 

Deflection  Potentiometer  Method 

A  potentiometer  is  ordinarily  used  as  a  null  instrument,  the  e.m.f  of 
the  couple  being  exactly  balanced  by  the  potential  drop  over  a  resistance 
through  which  a  current  from  a  battery  is  flowing.  The  objection  some- 


PAUL   D.    FOOTE,    T.    R.    HARRISON   AND    C.    O.    FAIRCHILD  91 

times  raised  against  the  ordinary  potentiometer  for  industrial  installations 
is  that  it  requires  a  manual  adjustment  of  a  dial  or  slide  wire  every  time 
an  observation  is  made.  This  objection  is  practically  eliminated  in  the 
deflection  potentiometer,  which  may  be  so  constructed  as  to  embody  the 
accuracy  of  the  ordinary  potentiometer  and  the  convenience  of  the 
galvanometer  indicator. 

In  the  deflection  potentiometer,  part  of  the  e.m.f.  of  the  couple  is 
balanced  against  the  potential  drop  over  a  resistance  through  which  a 
current  is  flowing,  while  the  remainder  is  indicated  by  the  deflection  of  a 
galvanometer  in  series  with  the  couple.  For  example,  the  instrument 
may  be  constructed  with  a  dial  of,  say,  16  points,  representing  potential 
differences  from  0  to  15  millivolts,  and  a  galvanometer  which  gives  full- 
scale  deflection  on  1  millivolt.  The  dial  is  set  to  the  approximate  e.m.f. 
developed  by  the  couple,  and  the  dial  reading,  combined  with  the  galva- 
nometer reading,  gives  the  true  e.m.f.  of  the  couple.  In  many  industrial 
processes  the  temperature  of  the  couple  will  vary  only  slightly  during 
several  hours,  so  that  a  new  dial  setting  is  infrequently  required.  Thus 
the  method  for  obtaining  the  e.m.f.  of  the  couple  is  nearly  as  simple  as 
when  an  ordinary  galvanometric  indicator  is  used. 

The  theory  of  the  deflection  potentiometer  has  been  developed  in 
detail  by  Brooks. l  As  applied  to  e.m.f.  measurements,  the  simple  theory 
may  be  deduced  as  follows.  It  has  been  shown  in  the  case  of  an  ordinary 
potentiometer  that  if  an  e.m.f.  e'  is  exactly  balanced  against  the  potential 
drop  in  a  resistance  wire  of  a  potentiometer,  the  value  of  e'  will  be  given 
by  the  equation 

e'  =  ~^~  (1) 

n  + r2 

where  E  is  the  e.m.f.  of  the  battery  used  to  furnish  the  current  in  the 
resistance  wire,  r\  is  the  resistance  of  this  wire,  and  r2  is  all  other  resistance 
in  the  battery  circuit.  Usually  the  value  of  e'  is  indicated  by  figures  on 
the  dials  or  slide  wire  of  the  potentiometer.  If  e'  changes  to  a  new  value 
e  and  the  potentiometer  remains  adjusted  as  before,  a  current  will  flow 
through  the  galvanometer  and  thermocouple,  causing  the  galvanometer 
to  deflect.  The  currents  now  flowing  through  r:  and  rz  are  unequal  and 
are  different  from  the  original  value.  The  currents  flowing  in  the  differ- 
ent branches  of  the  circuit  are  indicated  by  Fig.  13,  in  which  T  represents 
a  thermocouple,  G  the  galvanometer,  abcB  the  potentiometer,  R  the 
resistance  of  the  galvanometer  and  thermocouple,  e  the  e.m.f.  of  the 
couple,  and  B  the  battery.  7  represents  the  current  flowing  from  a  to  6 
through  TI,  and  i  the  current  flowing  through  the  galvanometer.  Since 


1U.  S.  Bureau  of  Standards  Sci.  Pantrs  33,  79,  172,  173. 


92 


THERMOELECTRIC    PYROMETRY 


the  sum  of  the  e.m.f.'s  and  potential  drops  around  any  closed  circuit 
must  equal  zero  the  following  two  equations  may  be  written : 

E  =  (ri  +  r2)  /  +  r2i  (2) 

e  =  rj  -  Ri  (3) 

Substituting  the  value  of  7  from  equation  (2)  in  equation  (3),  and  sub- 
tracting this  value  of  e  from  the  value  of  e'  given  by  equation  (1),  we 
obtain : 


(4) 


+ 


or  i  = 


7? 


Hence,  when  a  potentiometer  is  not  balanced,  a  current  will  flow  through 
the  galvanometer  -equal  to  the  difference  between  the  setting  of  the 
potentiometer  and  the  e.m.f.  of  the  thermocouple,  divided  by  the  total 
resistance  72  +  r^  -T-  (r\  +  r2)  in  the  galvanometer  circuit. 


FIG.  13.  —  UNBALANCED  POTENTIOMETER  CIRCUIT. 
Representing  the  resistance  within  the  potentiometer,  r^  -5-  (n  +  r2), 


by  rp,  the  resistance  of  the  galvanometer  circuit  will  be  R  +  rp.  If 
the  galvanometer  is  to  indicate  correctly  the  unbalanced  e.m.f.  e'  —  e  at 
all  values  of  e',  its  sensitivity  must  remain  constant,  which  requires  that 
R  4-  rp  remain  constant.  The  value  of  rp  will  change  as  point  6  is  moved 
nearer  to  a  or  c,  thus  altering  the  values  of  n  and  r-2.  Hence  it  is 
necessary  to  put  in  the  galvanometer  circuit  a  variable  resistance  which 
compensates  for  these  changes  in  rp. 

In  instruments  of  low  range,  suitable  for  thermocouples,  rt  is  made 
small  compared  with  r2.  Since  rp  =  nr2  -f-  (ri  +  r2)  if  rz  is  sufficiently 
large  compared  with  rb  we  may  neglect  the  term  TI  in  the  denominator 
and  the  above  equation  reduces  to  rp  =  r\.  For  such  an  instrument  the 
compensating  resistance  in  series  with  the  galvanometer  is  decreased  by 
the  value  of  r\,  at  any  dial  setting.  Instruments  of  this  type  have  been 
designed  by  Brooks  and  by  White.  The  compensating  resistance  is 
mounted  as  an  integral  part  of  the  dial,  so  that  turning  the  dial  changes 


PAUL   D.    FOOTE,    T.    R.    HARRISON    AND    C.    O.    FAIRCHILD 


93 


the  e.m.f.  setting  and  at  the  same  time  adjusts  the  compensating  re- 
sistance in  the  galvanometer  circuit  to  its  proper  value. 

Fig.  14  illustrates  a  deflection  potentiometer  for  thermocouples, 
made  by  the  Taylor  Instrument  Co.,  and  known  as  the  "range  control 
board."  The  galvanometer  G  is  provided  with  two  scales,  in  the  ranges 
0  to  500°  and  450  to  950°  respectively.  The  galvanometer  circuit  is 
connected  at  fixed  points  ,a  and  6  within  the  potentiometer,  and  when 
the  instrument  is  to  operate  in  the  lower  range  the  battery  circuit  is  opened. 
Thus  the  potentiometer  setting  e'  is  made  zero  without  changing  the 
value  of  ri.  In  this  range  the  instrument  operates  as  an  ordinary  galva- 
nometric  indicator.  If  the  temperature  of  the  thermocouple  is  above 


FIG.  .14. — RANGE  CONTROL  BOARD.     (Taylor  Instrument  Co.}    \ 

450°  or  500°  C.,  a  current  of  such  magnitude  is  made  to  flow  through  r\ 
that  the  potential  drop  e'  across  r\  balances  the  e.m.f.  developed  by  the 
couple  when  at  450°  C.  The  temperature  will  then  be  indicated  on  the 
high-range  scale. 

The  total  resistance  of  the  galvanometer  circuit  is  almost  exactly  equal 
when  operating  in  either  range,  since  when  operating  in  the  upper  range 
the  shunting  effect  of  r2  +  rz  on  r\  is  negligible.  Since  a  separate  galva- 
nometer scale  is  provided  for  each  setting  of  the  potentiometer  (0  and  e'}, 
it  is  not  really  necessary  that  the  sensitivity  be  equal  in  the  two  cases. 

For  the  high  range,  the  current  from  the  battery  is  adjusted  by  con- 
necting switch  S,  as  shown  by  the  dotted  line,  and  setting  rs  so  that  the 
galvanometer  deflects  to  a  marked  position.  Provision  must  be  made  for 
reversing  the  direction  of  the  current  from  the  battery  through  the  galva- 
nometer after  this  adjustment  has  been  made.  The  figure  does  not  show 
this,  nor  the  switch  for  opening  the  circuit  when  the  instrument  is  to  be 
used  for  the  low  range. 

Fig.  15  shows  the  Leeds  &  Northrup  instrument,  which  is  a  modifi- 
cation of  a  design  by  W.  P.  White.  When  a  range  suitable  for  thermo- 


94 


THERMOELECTRIC    PYROMETRY 


couples  is  used,  and  the  condition  of  a  balanced  Wheatstone  bridge  with 
arms  of  equal  resistance  is  never  far  departed  from,  the  resistance  of 
that  part  of  the  galvanometer  circuit  which  is  constituted  by  the  poten- 
tiometer usually  remains  constant  within  a  few  tenths  of  1  per  cent. 
The  value  of  this  resistance,  for  instruments  of  the  same  range  and  using 
the  same  battery  current,  is  considerably  higher  than  that  of  the  designs 
previously  discussed.  The  following  table  shows  suitable  values  of  the 


[     -r- 

.  —  'WVWWMA-^            •*  —  V/VWVVW  1 

a          c 
/?                '/* 

-    •   ll   i        AAAA/VWW  

«(; 

D 

V 

1     |       /vy^vvvvvv 

vwvwvw^—  wwwvw^- 

FIG.  15. — THERMOCOUPLE  DEFLECTION  POTENTIOMETER.     (Leeds  &  Northrup  Co.) 

different  resistances  which  will  give,  to  a  satisfactory  degree,  conditions 
of  a  nearly  balanced  Wheatstone  bridge  with  equal  arms: 

Resistance  e  to  a / 475  ohms 

Resistance  a  to  c 70  ohms 

Resistance  c  to  d 455  ohms 

Resistance  e  to  / 475  ohms 

Resistance  /  to  d 525  ohms 

Resistance  dBe.  .  Immaterial 


497 


10 


20  30  40  00 

Slide  Wire  Setting 


FIG.  16. — CHANGE  OF  rp  WITH  SLIDE-WIRE  SETTING  OF  THE  LEEDS    &  NORTHRUP 

DEFLECTION    POTENTIOMETER. 


These  values  may  be  divided  or  multiplied  by  any  number,  in  order 
to  obtain  any  desired  range  of  currents  and  a  proper  critical  damping 
resistance  for  the  galvanometer;  the  above  design  allows  a  range  of  70 
millivolts  over  the  slide  wire.  The  curves  in  Fig.  16  show  the  manner  in 


PAUL   D.    FOOTE,    T.    R.    HARRISON   AND    C.    O.    FAIRCHILD  95 

which  the  resistance  rp  of  that  part  of  the  galvanometer  circuit  consisting 
of  the  parallel  paths  within  the  potentiometer  varies  with  different  set- 
tings of  the  slide  wire,  ranging  from  0  to  70  millivolts,  and  with  resistance 
TS,  in  the  outside  battery  circuit  dBe,  varying  from  zero  to  infinity. 
Actually  r3  would  not  be  likely  to  vary  beyond  the  limits  100  and  500 
ohms.  An  average  value  for  rp  may  be  obtained  by  setting  the  slide  wire 
to  read  about  7  millivolts,  and  the  variation  of  rp  from  this  value  will 
generally  be  less  than  0.1  per  cent.,  whatever  the  setting  of  the  slide  wire 
on  the  battery  resistance  rz.  Therefore,  if  the  galvanometer  is  calibrated 
when  the  slide  is  set  to  read  7  it  will  be  more  nearly  accurate  with  varied 
settings  and  adjustments  of  the  instrument.  Since  rp  constitutes  only 
part  of  the  galvanometer  circuit,  the  galvanometer  sensitivity  will 
remain  constant  within  proportionally  less  than  0.1  per  cent. 

This  instrument  is  designed  primarily  so  that  the  slide  wire  S  may 
be  set  to  read  the  exact  temperature  required.  The  galvanometer 
G  accordingly  indicates  the  departure  of  the  actual  temperature  from 
the  temperature  desired.  It  thus  serves  as  a  very  convenient  guide 
to  the  operator  of  a  furnace,  who  can  see  at  a  glance  by  how  many 
degrees  the  temperature  at  any  time  differs  from  the  temperature  at 
which  the  furnace  should  be  operated. 

The  Beighlee  Electric  Company  makes  a  deflection  potentiometer 
which  is  a  modification  of  its  Wheatstone  bridge,  cold-junction  com- 
pensating instrument.  By  altering  the  ratio  of  the  coils  (Fig.  23)  in 
suitable  steps,  the  e.m.f.  of  the  couple  is  opposed  by  potential  drops  of 
different  values,  the  indicator  showing  the  unbalanced  portion  of  the 
thermocouple  e.m.f.,  as  in  the  other  instruments  described. 

Graduation  for  Reading  Temperature  Directly. — The  preceding  discus- 
sion has  assumed  that  the  scale  of  the  galvanometer  may  be  graduated 
to  read  either  e.m.f.  or  temperature.  If  the  thermocouple  has  a  linear 
relation  between  e.m.f.  and  temperature,  the  theory  outlined  is  just  as 
applicable  to  a  scale  and  dial  graduated  in  terms  of  temperature  as  in 
terms  of  e.m.f.  If  the  temperature  e.m.f.  relation  of  the  couple  is  not 
linear,  a  given  temperature  change  corresponds  to  different  changes  in 
e.m.f.,  depending  upon  the  temperature  of  the  couple.  Hence,  in  order 
that  the  scale  of  the  galvanometer  may  be  graduated  in  degrees,  it  is 
necessary  to  modify  its  sensitivity  in  the  various  temperature  ranges,  by 
means  of  series  resistance.  The  change  in  thermoelectric  power  of  a 
couple  through  the  temperature  range  represented  by  the  scale  of  the 
galvanometer  is  usually  small. 

In  the  "range  control  board"  the  above-mentioned  difficulty  is 
avoided  by  the  provision  of  a  separate  scale  for  each  range,  but  it  would 
not  be  practical  to  apply  this  method  to  the  ordinary  deflection  potenti- 
ometer, which  may  have  10  to  20  different  scale  ranges.  In  the  deflection 
potentiometer  made  by  Leeds  &  Northrup  (Fig.  15)  the  ratios  of  the 


96 


THERMOELECTRIC    PYROMETRY 


resistances  may  be  so  modified  as  to  produce  very  closely  the  proper  com- 
pensation in  rp,  for  any  type  of  couple  used  industrially. 

TEMPERATURE  OF  THE  COLD  JUNCTIONS  OF  THERMOCOUPLES 

The  e.m.f .  developed  by  a  thermocouple  depends  upon  the  tempera- 
ture of  the  cold  junctions  as  well  as  upon  that  of  the  hot  junction.  For 
certain  base-metal  couples  having  a  nearly  linear  relation  between  tem- 
perature and  e.m.f.,  the  latter  is  approximately  proportional  to  the  dif- 
ference in  temperatures  between  hot  and  cold  junctions;  with  such  a  couple 
a  change  of  50°  in  the  temperature  of  the  cold  junctions,  unless  allowed 
for,  would  cause  an  error  of  50°  in  the  observed  temperature. 

If  a  couple  is  calibrated  with  a  cold-junction  temperature  of  t0°  C., 
but  is  used  with  a  cold-junction  temperature  of  t'0°  C.,  the  true  temperature 
of  the  hot  junction  is  obtained  by  adding  to  the  observed  temperature  the 
value  (t'0  —  to)  K,  where  K  is  a  factor  depending  upon  the  particular 
couple  employed  and  upon  the  temperature  of  the  hot  junction.  K 
varies  from  1.5  to  0.3,  but  for  rough  work  may  be  assumed  as  1.0  for 
base-metal  couples,  and  0.5  for  rare-metal  couples.  Table  2  gives  the 
cold-junction  factors  for  several  different  types  of  couple. 

The  corrections  may  be  applied  directly,  without  computing,  by 
setting  the  pointer  of  the  galvanometer  to  read  the  cold-junction  tem- 
perature on  open  circuit;  this  is  done  by  turning  the  zero-adjustment 
screw  of  the  indicator  when  the  couple  is  disconnected.  This  method  of 

TABLE  2. — Cold-junction  Correction  Factors 


Engelhard 
"LeChatelier  " 

Johnson-Matthey 
"LeChateliei  " 

Copper-constantan 

Iron-constantan 

Temp., 
Degrees  C. 

K* 

Temp., 
Degrees  C. 

K* 

Temp., 
Degrees  C. 

K* 

Temp., 
Degrees  C. 

K* 

265-450 
450-650 
650-1000 

1000-1450 

0.65 
0.60 
0.55 

0.50 

250-400 
400-550 
550-900 

.   900-1450 

0.60 
0.55 
0.50 

0.45 

0-50 
50-80 
80-110 

110-150 

150-200 
200-270 
270-350 

1.00 
0.95 
0.90 

0.85 

0.80 
0.75 
0.70 

0-100 
100-600 
600-1000 

1.00 
0.95 
0.85 

Chromel-alumel 

0-800 
800-1100 

1.00 
1.05 

*  Based  on  calibration  with  ta  =  0°  C. 

correcting  is  accurate,  but,  of  course,  requires  new  settings  whenever  the 
temperature  of  the  cold  junction  is  altered.  Indicators  of  the  poten- 
tiometric  type  frequently  have  a  movable  slide  on  the  temperature  scale, 
or  an  auxiliary  dial  (see  discussion  below),  which,  when  set  to  the  tern- 


PAUL   D.    FOOTE,    T.    R.    HARRISON    AND    C.    O.    FAIRCHILD 


97 


perature  of  the  cold  junction,  gives  perfect  compensation  at  all  tempera- 
tures of  the  hot  junction;  these  two  methods  also  require  new  settings 
whenever  the  temperature  of  the  cold  junction  is  altered.  With  large 
and  permanent  installations  the  applying  of  corrections  for  the  tem- 
perature of  the  cold  junction  by  any  of  the  above  methods  is  frequently 
troublesome,  since  the  temperature  may  vary  considerably  within  a  few 
hours.  There  are  several  methods  for  obviating  this  necessity.  The 
head  of  the  couple  may  be  fitted  with  a  water-jacket,  maintained  at 
practically  constant  temperature;  copper  wires  lead  from  the  terminals 
of  the  couple,  inside  the  water  jacket,  to  the  indicator,  the  pointer  of 
which,  on  open  circuit,  is  set  to  read  the  mean  temperature  of  the  water. 

Compensating  Leads 

The  use  of  compensating  lead  wires  from  the  couple  to  the  indicator  is 
the  most  generally  satisfactory  method  for  minimizing  cold-junction 
errors  in  industrial  installations.  For  base-metal  couples,  these  lead 


FIG.  17. — THERMOSTATED  COLD-JUNCTION  BOX. 


wires  are  of  nearly  the  same  materials  as  those  employed  in  the  couple, 
small  stranded  wires  being  used  for  flexibility.  Thus  the  cold  junction 
is  transferred  from  the  head  of  the  couple,  where  the  temperature  varies, 
to  a  point  at  some  distance  from  the  furnace,  where  the  temperature  is 
reasonably  constant,  and  from  this  point  copper  wires  lead  to  the  indicator. 
The  compensating  wires  may  terminate  in  a  thermostated  cold-junction 
box,  as  illustrated  by  Fig.  17,  or  may  be  buried  underground.  At  a 
depth  of  10  ft.  beneath  the  floor  of  a  large  building,  the  temperature 
remains  constant  to  within  2°  C.  throughout  the  year;  usually  this  mean 
temperature  is  about  12°  C.  for  temperate  climates,  but  may  differ  some- 


y»  THERMOELECTRIC    PYROMETRY 

what  in  the  immediate  vicinity  of  a  large  furnace.  To  apply  this  method 
of  control,  an  iron  pipe  of  the  proper  length,  closed  at  the  bottom,  is 
driven  into  the  ground,  and  the  two  cold  junctions,  well  soldered  and 
carefully  insulated,  are  threaded  to  the  bottom  of  the  pipe  in  such  manner 
as  to  be  conveniently  removable  when  necessary.  The  top  of  the  pipe 
may  be  plugged  with  asbestos  or  waste,  and  covered  with  pitch  to  keep 
water  away  from  the  insulation.  The  scale  of  the  indicator  is  set  to  read 
the  mean  temperature  of  the  bottom  of  the  tube.  It  is  convenient  to 
have  an  extra  pair  of  compensating  leads,  or  an  extra  thermocouple  with 
its  junction  at  the  bottom  of  the  pipe,  to  measure  this  temperature 
occasionally.  Usually  the  compensating  leads  of  a  base-metal  couple  are 
marked,  or  are  equipped  with  one-way  terminals,  so  that  they  are  easily 
connected  properly  to  the  head  of  the  couple.  If  reversed  at  the  couple, 
the  leads  will  cause  an  error  double  the  amount  of  the  compensation. 
When  compensating  leads  of  a  base-metal  couple  are  properly  connected 
to  the  couple,  no  deflection  of  the  indicator  is  registered  by  heating  the 
head  of  the  couple. 

The  high  cost  of  platinum  prevents  the  use  of  compensating  leads  of 
that  metal,  but  inexpensive  wires  of  copper  and  nickel-copper  alloy  are 
now  available  for  use  with  the  platinum  and  platinum-rhodium  couple. 
These  lead  wires  do  not  compensate  individually,  but  taken  together  they 
compensate  to  within  5°  C.  for  a  variation  of  200°  C.  at  the  junctions  of  the 
couple  and  lead  wires.  Both  terminals  on  the  head  of  the  couple  should 
be  kept  as  nearly  as  possible  at  the  same  temperature.  The  copper 
compensating  lead  is  connected  to  the  platinum-rhodium  wire  of  the 
couple,  and  the  copper-nickel  wire  is  connected  to  the  platinum  wire  of 
the  couple,  i.e.,  alloy  wire  to  pure  metal  in  each  case.  The  cold  junction 
is  then  located  at  the  indicator  end  of  the  compensating  leads,  and  its 
temperature  may  be  controlled  by  one  of  the  methods  described ;  coppei 
wires  run  from  this  point  to  the  indicator. 

Potentiometric  Compensation  Methods 

The  wiring  diagram  for  the  Leeds  &  Northrup  portable  potentiometer, 
equipped  with  a  hand-adjusted  cold-junction  compensator,  is  given  in 
Fig.  18.  The  e.m.f.  of  the  thermocouple  H  is  balanced  against  tho 
potential  drop  across  DG,  a  condition  obtained  when  the  galvanometer 
reads  zero.  Dial  G  is  calibrated  to  read  the  temperature  of  the  hot 
junction  of  the  couple.  If  this  temperature  remains  constant  while 
the  temperature  of  the  cold  junction  T  increases,  the  e.m.f.  of  the  couple 
diminishes  and  the  point  G  would  have  to  be  moved  nearer  D  to  obtain  a 
balance.  If  this  were  done,  however,  the  temperature  indicated  on  the 
scale  would  be  too  low.  Hence,  instead  of  moving  G  the  contact  D  is 
turned  nearer  G  by  an  amount  depending  upon  the  temperature  of  the 


PAUL   D.    FOOTE,    T.    R.    HARRISON   AND    C.    O.    FAIRCHILD 


99 


cold  junction.  A  portion  of  the  slide  wire  DGE,  containing  the  contact  D, 
is  mounted  as  a  separate  dial,  empirically  graduated  for  any  particular 
type  of  couple,  to  read  the  temperature  of  the  cold  junction.  The 
pointer  on  this  dial  is  set  at  the  cold-junction  temperature,  by  doing 
which  the  contact  D  is  moved  the  proper  amount  for  exact  compensation. 
The  balance  is  then  made  in  the  usual  manner  by  adjusting  the  contact 
G.  The  temperature  now  indicated  on  the  main  dial  is  the  correct  tem- 


FlG. 


-HAND-ADJUSTED   COLD-JUNC- 
TION COMPENSATOR. 


FIG.  19. — HAND-ADJUSTED  COLD-JUNC- 
TION COMPENSATOR. 


perature  of  the  hot  junction  of  the  couple.  Fig.  18,  while  illustrating 
the  principle  of  this  method  of  Compensation  does  not  permit  e.m.f. 
measurements  to  zero;  the  temperature  scale  on  the  main  dial  must 
start  at  the  highest  temperature  on  the  cold-junction  dial.  Fig.  19  illus- 
trates the  wiring  system  more  usually  employed,  the  cold-junction  dial 
being  in  parallel  instead  of  in  series  with  the  main  dial;  this  permits  set- 
tings on  the  main  dial  to  zero. 

The  above  principle  has  been  applied  in  an  automatic  compensator 
which  has  been  used  satisfactorily  with  the  Leeds  &  Northrup  recording 


FIG.  20. — AUTOMATIC  COLD-JUNCTION  COMPENSATOR. 

indicators  (Fig.  20).  The  wiring  system  is  similar  to  the  one  just  de- 
scribed, except  that  the  contact  D  of  Fig.  19  is  mechanically  fixed  between 
the  two  resistances  M,  having  a  zero  temperature  coefficient,  and  N, 
of  nickel,  having  a  high  temperature  coefficient.  The  resistance  N  is 
located  near  the  cold  junction  of  the  couple  so  that  its  temperature  and 
that  of  the  cold  junction  are  identical.  If  this  temperature  increases, 
the  e.m.f.  developed  by  the  couple  decreases,  but  the  accompanying 


100 


THERMOELECTRIC   PYROMETRY 


increase  in  the  resistance  N  automatically  produces  the  same  effect  as 
moving  the  contact  D  toward  G  in  Fig.  18.  The  circuit  is  more  simply 
represented  by  Fig.  21.  Let  e  =  e.m.f.  developed  by  the  couple  when  the 
hot  junction  is  at  a  temperature  t°  and  e0  =  the  e.m.f.  developed  for  a 
hot-junction  temperature  t0,  the  cold- junction  temperature  being  0° 
in  both  cases.  Then  the  e.m.f.  developed  by  the  couple  when  its  hot 
junction  is  t°  and  cold  junction  is  tQ°  is  e  —  e0.  Neglecting  the  slight  effect 


V\A/WVWWWWNAAAA/VWW\AAA/WI 


K 


M 

AAAAAAAAAAAAAAAA/W 


D 


N 
^VWWW 


FIG.  21.  —  AUTOMATIC  -COLD-JUNCTION   COMPENSATOR. 

of  variation  in  resistance  N  with  the  temperature,  the  potential  drop 
from  KtoE  due  to  the  battery  B  is  constant,  e'.  Whence,  the  potential 
difference  between  G  and  D  when  the  galvanometer  in  the  thermocouple 
circuit  indicates  zero  is  simply  derived  as  follows,  and  is  equal  to  the  e.m.f. 
of  the  couple  : 


-c      Cu 
~ 


For  exact  compensation,  the  position  G  must  be  independent  of  the  cold 
junction  temperature  t0',  hence,  on  differentiating  the  above  expression 
we  obtain  the  following  as  a  condition  which  must  be  satisfied : 

de0  _    t       M         dN 
dto  =  e  (M  +  AT)*  5*o 

With  proper  proportioning  of  M  and  N,  this  condition  is  fairly  well 
satisfied  by  the  nickel  coil  for  either  base-metal  or  rare-metal  couples, 
provided  the  temperature  range  for  the  cold  junction  is  small. 

Compensation  by  a  Shunt 

The  use  of  a  resistance,  having  a  high  temperature  coefficient,  shunted 
across  the  terminals  of  the  couple  at  the  cold  junction  was  suggested  by 
Foote2  in  1913  as  a  possible  method  of  partly  correcting  the  cold-junction 
errors.  A  modification  of  this  method  has  since  been  patented  by  Mertel- 
meyer3  and  is  used  by  the  Bristol  Co.  As  the  temperature  of  the  cold 


1  Foote:  U.  S.  Bureau  of  Standards  Sci.  Paper  202,  12. 

8  Mertelmeyer.  assignor  to  Bristol  Co.,  U.  S.  Patent  No.  1228803,  1917. 


PAUL  D.  FOOTE,  T.  R.  HARRISON  iND  0.  <>.' 


101 


junction  increases,  the  e.m.f.  developed  by  the  couple  diminishes.  If, 
however,  the  resistance  of  the  coil  shunted  across  the  couple  increases 
with  the  temperature,  the  potential  drop  over  the  coil  tends  to  increase, 
and  by  properly  proportioning  the  constants  of  the  circuit  a  fair  degree 
of  compensation  is  obtained.  Fig.  22  illustrates  the  method  of  com- 
pensation for  potentiometric  measurements. 


TI  =  resistance  of  the  thermocouple. 
r2  =  series  resistance  having  a  zero  temperature  co- 
efficient (manganin). 
TZ  =  resistance  of  the  shunt  at  0°. 
r'3  =  resistance  of  shunt  at  temperature  <0°. 
a  =  temperature  coefficient  of  resistance  of  the  shunt, 

reckoned  from  0°. 
0  =  thermoelectric   power  of  the  couple   (assumed 

constant). 

t  =  any  temperature  of  the  hot  junction. 
t0  =  any  temperature  of  the  cold  junction,  and  of  r's. 
t'  =  temperature  of  hot  junction  for  which  perfect 

compensation  is  desired. 
t'o  and  0°  =  temperatures  of  cold  junction  for  which  perfect 

compensation  is  desired. 
e'  =  potential  drop  across  r's,  hot  junction  being  at 

t°,  and  cold  junction  at  t0°. 
e  —  e0  =  p  (t  —  <0)  =  e.m.f.  developed  by  couple. 


To  Potentiometer 


-WWV-  ^o 


Ve 

FIG.  22.  —  COLD- 
JUNCTION  COMPENSA- 
TION BY  SHUNT  AND 
SERIES  RESISTANCE. 


In  order  to  reduce  to  a  minimum  the  effect  of 
variations   in   the   resistance  r\  of  the  couple,  a 
relatively  high  resistance  r2  of  manganin  is  mounted 
in  series  with  the  couple.     If  r2  is  sufficiently  high 
compared  with  ri,  we  may  neglect  consideration  of 
the  latter  in  the  present  discussion.     The  potential  drop  e'"  across  r's 
for  a  cold-junction  temperature  t'0°  and  a  hot-junction  temperature  t'° 
is  as  follows: 

Q111    =  (1) 

For  a  cold-junction  temperature  of  0°,  and  a  hot-junction  temperature 
t'°,  the  potential  drop  e"  is: 

"TJ-  (2) 


/>' '     ^— 


For  exact  compensation,  e"  must  equal  e'";  hence  from  (1)  and  (2) 

rs       a(t'  -  «'0)  -  1 


(3) 


; '.  ~rz  ~        1  +  crf'o 

For  a  cold-junction  temperature  fo°,  and  a  hot-junction  temperature  t°, 
the  potential  drop  across V3  is: 


e'  = 


-f 


(4) 


102 


PYROMETRY 


For  a  cold-junction  temperature  of  0°,  and  a  hot-junction  temperature 
t°,  the  potential  drop  across  r'z  is: 


(5) 


m 


The  error  in  the  compensation,  expressed  in  degrees,  at  any  hot-junction 
temperature  t  and  cold-junction  temperature  t0  is: 


Error,  in  degrees  = 


t  =  t  - 


'2 


On  substituting  the  value  of  r3  -f-  r2  from  equation  (3),  we  obtain: 

t'  -  Kt\ 


where 


„          ...                ,  (t '  -  Kt\ 
Error,  in  degrees  =  t0(-.f ^,  ) 

\t    —  A  to' 

1  +  at' 


(6) 


(7) 


K  = 


Suppose  an  iron-constantan  couple  is  employed  and  perfect  compen- 
sation is  desired  for  a  hot-junction  temperature  of  800°  C.  when  the  cold- 
junction  temperatures  are  0°  or  50°  C.  The  potentiometer  indicator  is 
graduated  to  read  correctly  all  temperatures  of  the  hot  junction  when 
the  temperature  of  the  cold  junction  is  0°.  If  the  shunt  is  constructed  of 
nickel  wire,  we  have  the  following  data:  a  =  0.006;  j8  =  0.05  millivolts 
per  degree;  i'  =  800°  C.;  t'0  =  50°  C. 

On  substituting  these  values  in  equations  (3)  and    (7)  we  obtain: 

r3  -*-  rz  =  2.7 

/800  +  4.8  tu  -  I 
Error,  in  degrees  =  ta  (  —^p—* 

The  ratio  of  the  resistance  of  the  nickel  shunt  to  that  of  the  manganin 
should  be  2.7,  at  0°.     Table  3  shows  the  magnitude  of  the  errors. 


TABLE  3. — Error  in  Compensation  by  Shunt  Method 


Hot  Junction,  t, 
Degrees  C. 


700°  C. 


800°  C. 


900°  C. 


Cold 

junction,  t0 

0 

0 

0 

0 

10 

-0.7 

-2.3 

-3.9 

20 

-0.3 

-3.3 

-6.3 

30 

+  1.1 

-3.2 

-7.5 

40 

+3.5 

-2.0 

-7.6 

50 

+6.7 

0.0 

-6.7 

60                               +9.7 

+2.6 

-4.5 

PAUL   D.    FOOTE,    T.    R.    HARBISON   AND    C.    O.    FAIRCHILD  103 

Thus,  over  a  range  of  200°  C.  in  the  temperature  of  the  hot  junction  and 
of  60°  C.  in  the  temperature  of  the  cold  junction,  the  method  compensates 
to  within  10°  C. 

The  above  method  may  be  applied  even  more  satisfactorily  when  a 
galvanometer  is  used  instead  of  a  potentiometer.  •  By  carrying  through 
a  series  of  computations  for  the  more  complicated  circuit  with  a  galva- 
nometer, having  resistance  R,  a  relation  may  be  obtained  between  R,  r2, 
r3,  a  and  /3.  A  wiring  system  according  to  the  data  in  Table  4  will  give 
slightly  better  compensation  around  800°  C.  than  that  of  the  potenti- 
ometer, as  in  Table  3. 

TABLE  4. — Shunt  Compensation  with  Galvanometric  Indicator 


Galvanometer  Resistance,                        Series  Resistance 
Ohms                                         (Manganin),  Ohms 

Shunt  Resistance 
(Nickel),  Ohms 

100 

126 

150 

100 

59 

100 

100 

23 

50 

It  is  evident  that  this  method  of  compensation  has  certain  advantages 
in  the  control  and  maintaining  of  furnace  temperature.  It  should  not 
be  employed  when  the  variation  in  temperature  of  the  furnace  is  much 
greater  than  100°  C.  There  are  few  processes  in  which  the  use  of  compen- 
sated leads  with  a  cold- junction  box  or  a  buried  cold  junction  is  not 
preferable.  The  shunt  method  has  been  described  at  some  length  because 
it  has  not  been  discussed  elsewhere,  and  in  certain  restricted  applications 
the  method  offers  desirable  features. 

Wheatstone  Bridge  Compensation 

This  method  of  compensation  for  the  temperature  of  the  cold  junc- 
tion, as  applied  by  the  Beighlee  Electric  Co.,  is  illustrated  in  Fig.  23. 
The  switch  S  is  first  thrown  to  position  t.  The  fixed  resistances  A  and 
B  are  equal,  so  that  if  T  were  equal  to  D  the  galvanometer  would  show 
no  deflection ;  the  resistance  of  T  is  actually  considerably  greater  than  that 
of  D.  By  varying  the  resistance  VR,  the  pointer  of  the  galvanometer 
may  be  adjusted  to  some  definite  mark  on  the  scale.  This  preliminary 
setting  fixes  the  amount  of  current  flowing  in  the  main  circuit  due  to  the 
battery  E. 

For  a  temperature  measurement,  the  switch  S  is  thrown  to  position 
1 .  The  couple  and  the  resistances  X  +  C  now  constitute  an  arm  of  the 
bridge.  The  resistance  C  has  a  high  temperature  coefficient  and 
is  located  at  the  cold  junction  of  the  couple.  Suppose  the  apparatus  is 
standardized  for  a  cold-junction  temperature  of  0°;  at  that  temperature 
X  +  C  =  D,  and  the  galvanometer  would  show  nti  deflection  if  the  tern- 


104 


THERMOELECTRIC    PYROMETRY 


perature  of  the  hot  junction  were  0°.  As  the  temperature  of  the  hot 
junction  rises,  the  bridge  is  thrown  out  of  balance,  causing  a  deflection 
of  the  galvanometer.  The  scale  of  the  instrument  is  accordingly  em- 
pirically graduated  to  read  the  temperature  of  the  hot  junction  when  the 
cold  junction  is  at  0°,  and  when  the  proper  current  is  flowing,  through 
the  main  battery  circuit  as  determined  by  the  preliminary  adjustment. 
When  the  temperature  of  the  cold  junction  rises,  the  e.m.f.  of  the  couple 
diminishes,  but  the  resistance  C  increases.  An  increase  in  the  resistance 
C  tends  to  increase  the  deflection  of  the  galvanometer,  while  a  decrease  in 


FIG.  23  — COLD-JUNCTION  COMPENSATION  BY  WHEATSTONE  BRIDGE  METHOD. 

the  e.m.f.  of  the  couple  tends  to  decrease  the  deflection.  For  the  reason 
that  the  thermoelectric  power  of  a  couple  and  the  temperature-resistance 
coefficient  of  the  coil  C  are  nearly  constant,  or  vary  similarly  with  tem- 
perature, over  a  small  range,  by  properly  proportioning  the  various 
resistances  of  the  circuit,  the  increased  deflection  due  to  increased 
resistance  of  C  compensates,  for  all  practical  purposes,  for  the  diminished 
deflection  due  to  the  reduction  in  e.m.f.  of  the  couple  as  the  temperature 
of  the  cold  junction  rises. 

CORRECTION  FOR  IRREPRODUCIBILITY  OF  COUPLES 

Platinum  and  platinum-rhodium  suitable  for  thermocouples  are 
refined  in  this  country  by  Engelhard  and  in  England  byJohnson-Matthey. 
The  temperature-e.m.f.  relations  of  the  couples  obtained  from  these  two 
sources  differ  somewhat  from  each  other,  but,  as  stated  elsewhere  in  this 
paper,  the  reproducibility  of  either  of  these  general  types  of  Le  Chatelier 
couple  is  highly  satisfactory.  The  e.m.f.  of  base-metal  couples  of  any 


PAUL   D.    FOOTE,    T.    R.    HARRISON    AND    C.    O.    FAIRCHILD  105 

given  type,  under  the  same  temperature  conditions,  may  differ  by  5  per 
cent,  or  more;  expressed  in  temperature,  these  differences  may  amount  to 
50°  at  1000°  C.  If  the  manufacturer  of  the  couples  exercises  special 
care  in  the  choice  of  the  wire,  these  differences  may  be  considerably 
reduced.  For  example,  a  certain  length  of  chromel  wire  and  another 
of  alumel  wire  may  be  selected  as  representative  of  the  standard  couple 
for  which  the  scale  of  the  pyrometer  indicator  is  graduated.  The  various 
stock  coils  of  alumel  are  tested  thermoelectrically  against  the  standard 
alumel  wire  and  the  stock  coils  of  chromel  against  the  standard  chromel 
wire.  If  no  e.m.f.  is  developed  by  heating  the  junction  of  the  standard 
wire  and  the  wire  under  test,  this  indicates  that  the  two  are  similar. 
Suppose,  however,  that  an  e.m.f.  of  0.5  millivolt  is  observed,  the  standard 
alumel  wire  being  positive.  This  coil  of  alumel  wire  should  accordingly 
be  used  with  a  coil  of  chromel  wire  to  which  the  standard  chromel  wire 
tested  0.5  millivolt  positive.  By  carrying  through  a  series  of  such  tests 
on  many  coils  of  wire,  always  heating  to  the  same  temperature,  various 
pairs  of  chromel  and  alumel  coils  may  be  selected,  of  which  the  tempera- 
ture-e.m.f.  relations  are  nearly  the  same  as  that  of  the  standard  couple. 
When  these  pairs  of  coils  are  made  into  couples,  any  differences  in  cali- 
brations from  that  of  the  standard  couple  will  be  due  mainly  to  hetero- 
geneity of  the  wire  itself.  Usually  a  coil  of  wire  is  drawn  from  a  single 
ingot,  and  its  variations  in  thermoelectric  properties  are  likely  to  be 
much  smaller  than  the  differences  between  two  coils  of  wire.  Any  re- 
maining variations  are  usually  of  small  practical  importance,  and  some 
manufacturers  consider  couples  made  from  wire  thus  selected  to  be 
sufficiently  reproducible  for  industrial  purposes. 

Compensation  by  Series  Resistance 

It  is  obviously  impractical  to  graduate  the  scale  of  the  indicator  for 
every  individual  couple,  especially  as  several  couples  are  frequently 
connected  to  the  same  indicator.  If  a  couple  shows  a  higher  e.m.f.  than 
the  standard  couple  for  which  the  indicator  is  graduated,  a  sufficient 
amount  of  resistance  may  be  placed  in  series  with  the  galvanometer. 
Usually,  if  a  base-metal  couple  shows  an  e.m.f.,  say,  2  per  cent,  high  at 
1000°,  it  will  be  2  per  cent,  high  at  all  other  temperatures,  so  that  this 
method  of  correction  is  satisfactory  for  all  temperature  ranges.  The 
series  resistance  must  be  located  at  the  couple  and  not  inside  the  galva- 
nometer. If  a  couple  shows  an  e.m.f.  2  per  cent,  high,  for  example,  the  extra 
series  resistance  must  be  2  per  cent,  of  the  total  resistance  of  the  circuit; 
thus,  with  a  300-ohm  galvanometer  and  negligible  line  resistance,  6  ohms 
of  manganin  is  placed  in  series  and  is  mounted  on  a  spool  inside  the 
terminal  head  of  the  couple.  If  the  e.m.f.  of  a  couple  is  low,  resistance 
must  be  taken  out  of  the  circuit.  The  scale  of  the  instrument  is  accord- 


106 


THERMOELECTRIC   PYROMETRY 


ingly  designed  for  the  normal  couple  plus  a  certain  normal  series  resistance, 
the  latter  being  sufficient  to  permit  adjustment  for  couples  showing  low 
e.m.f. 

This  method  of  compensation  is  open  to  objection,  especially  from  the 
standpoint  of  the  pyrometer  manufacturer,  in  that  the  resistance  of  the 
series  coil  required  for  exact  compensation  depends  upon  che  resistance 
of  the  indicator.  Table  5  shows  the  values  of  the  series  resistances 
required  for  indicators  of  various  resistances,  in  order  to  provide  for  a 
maximum  variation  of  ±  2  per  cent,  in  the  e.m.f.  of  different  couples.  The 
resistance  of  the  line  proper  (i.e.,  lead  wires  and  couple  without  series 
coil)  is  assumed  negligible. 

TABLE  5. — Series  Resistance  Required  for  Compensation 


E.m.f.  of  Couple 

Resistance  of  Indicator 

500  Ohms 

250  Ohms 

125  Ohms 

50  Ohms 

25  Ohms 

10  Ohms 

2  per  cent,  low  

0 

5.0 
7.5 
10.1 
12.7 
15.2 
20.3 

0 
2.4 

3.7 
5.0 
6.3 
7.6 
10.1 

0 
1.2 
1.9 
2.5 
3.1 
3.8 
5.0 

0 
0.50 
0.75 
1.01 
1.27 
1.52 
2.03 

0 
0.24 

0.37 
0.50 
0.63 
0.76 
1.01 

0 
0.10 
0.15 
0.20 
0.25 
0.30 
0.40 

1  per  cent,  low  

0  5  per  cent  low          .  .  . 

Normal  

0  5  per  cent,  high  

1  per  cent,  high  

2  per  cent,  high  

The  development  in  pyrometry  during  the  past  five  years  has  tended 
toward  the  making  of  indicators  having  higher  resistance,  with  the  object 
of  minimizing  the  errors  arising  from  variations  in  line  resistance.  As  a 
result,  instruments  having  resistances  ranging  from  5  to  600  or  even  1200 
ohms  are  on  the  market.  Hence,  when  ordering  couples  for  replacement, 
the  customer  must  state  the  resistance  of  the  indicator,  and  the  manufac- 
turer must  carry  in  stock  an  almost  endless  assortment  of  couples  com- 
pensated and  calibrated  to  fit  all  the  instruments  he  has  manufactured 
possibly  in  the  past  ten  years.  A  large  plant  which  has  purchased  in- 
struments for  five  years  may  have  an  assortment  of  perhaps  100  indicators 
having  resistances  from  50  to  600  ohms,  all  calibrated  to  read  correctly 
for  the  normal  couple.  Since  the  compensated  renewing  couples  are  no 
longer  interchangeable,  this  plant  would  be  required  to  carry  a  stock  of 
couples  for  every  indicator.  The  chances  for  confusion  of  records  and 
the  mixing  of  couples,  and  the  extra  cost  of  such  a  complete  stock,  are 
serious  items. 

The  possible  error  when  a  compensated  couple  is  used  with  the  wrong 
indicating  instrument  is  illustrated  by  the  following  example.  Suppose 
the  couple  originally  read  2  per  cent,  high  and  that  it  is  compensated 
to  read  correctly  with  a  500-ohm  indicator.  The  couple,  by  mistake, 


PAUL   D.    FOOTE,    T.    R.    HARRISON   AND    C.    O.    FAIRCHILD  107 

is  connected  to  a  50-ohm  instrument.  The  series  resistance  for  the  500- 
ohm  instrument  is  20.3  ohms  (see  Table  5)  while  that  for  the  50-ohm 
indicator  is  only  2.03  ohms.  The  introduction  of  20.3  ohms  in  series 
with  a  50-ohm  galvanometer,  when  only  2.03  ohms  should  be  used,  makes 
the  instrument  read  26  per  cent.  low.  Hence  if  the  temperature  of  the 
furnace  were  1000°  C.,  the  error  would  be  260°.  If  no  compensation  what- 
ever had  been  employed,  and  if  the  instruments  were  graduated  for  the 
normal  couple  with  no  resistance  in  series,  each  instrument,  when  con- 
nected to  the  couple  showing  e.m.f.  2  per  cent,  high,  would  be  in  error 
by  only  20°  at  1000°  C. 

One  way  to  prevent  such  mistakes  is  to  use  instruments  all  having 
the  same  resistance.  Indicators  having  a  practically  fixed  preassigned 
resistance  are  made  by  certain  manufacturers;  all  parts  of  the  galvanom- 
eters are  constructed  according  to  rigid  specifications.  The  swamping 
resistance  is  finally  adjusted  until  the  total  resistance  of  the  instru- 
ment has  the  preassigned  value  found  by  experiment  to  be  satisfactory. 
Slight  differences  may  still  exist  in  the  calibration  of  different  instruments; 
these  can  be  corrected  by  several  methods,  such  as: 

(a)  Calibrate  the  instrument  by  direct  experiment  and  make  a  hand- 
drawn  scale.  Different  instruments  will  have  slightly  different  tem- 
perature ranges. 

(6)  Use  printed  scales  and  adjust  the  sensitivity  of  the  galvanometer 
by  means  of  a  magnetic  shunt. 

(c)  Use  printed  scales  in  several  different  temperature  ranges  and 
select  the  one  which  best  fits  the  instrument. 

(d)  Shunt  the  moving  coil  to  give  a  specified  deflection  on  a  specified 
current;  then  adjust  the  external  resistance  until  the  total  resistance  has 
the  proper  value.     Use  printed  scales. 

The  adoption  of  a  standard  instrument  is  likely  to  discourage  develop- 
ment on  the  part  of  the  manufacturers.  Furthermore,  instruments 
having  all  resistances  and  scale  ranges  are  in  daily  use  in  the  industries, 
and  they  are  giving  satisfactory  service;  it  is  impossible  to  consider 
discarding  all  except  those  having  a  certain  definite  resistance  in  order 
that  a  convenient  method  of  compensation  by  series  resistance  may  be 
adopted.  The  objection  to  compensation  by  series  resistance  is  further 
emphasized  when  indicators,  or  an  indicator  and  a  recorder,  are  operated 
in  parallel  on  the  same  couple.  Suppose  a  couple  reading  normally 
2  per  cent,  high  is  installed  with  a  500-ohm  indicator;  the  series  resistance 
in  the  head  of  the  couple  (Table  5)  is  20.3  ohms.  It  is  desired  to  operate 
another  similar  indicator  in  parallel.  The  resistance  of  two  500-ohm 
indicators  in  parallel  is  250  ohms;  the  two  indicators  accordingly  act  as  a 
single  indicator  having  a  resistance  of  250  ohms.  Hence  the  .series  re- 
sistance must  be  10.1  instead  of  20.3  ohms.  If  the  couple  is  to  be  used 
with  both  indicators  it  can  never  be  used  with  the  indicators  separately. 


108  THERMOELECTRIC  PYROMETRY 

If  a  500-ohm  indicator  and  a  125-ohm  recorder  are  operated  in  parallel, 
the  two  instruments  act  as  a  single  indicator,  having  a  resistance  of  100 
ohms.  For  a  couple  reading  2  per  cent,  high,  the  series  resistance  must 
be  20.3  ohms  when  the  500-ohm  indicator  is  used,  5.0  ohms  when  the 
125-ohm  recorder  is  used,  and  about  4  ohms  when  both  are  employed  in 
parallel.  When  instruments  of  low  and  different  resistances  are  operated 
in  parallel,  the  problem  of  choosing  the  proper  compensation  for  the 
couple  becomes  very  complicated.  In  fact,  it  is  frequently  necessary 
to  use  cut-out  switches  to  throw  the  recorder  out  of  the  circuit  when  the 
indicator  is  read,  and  vice  versa.  A  final  objection  to  the  method  of 
compensation  by  series  resistance  is  that  it  has  no  effect  when  a  potentio- 
metric  or  semi-potentiometric  indicator  or  recorder  is  employed.  A 
couple  showing  e.m.f.  2  per  cent,  high,  and  compensated  to  read  cor- 
rectly on  a  500-ohm  indicator,  will  still  read  2  per  cent,  high  when  the 
measurements  are  made  with  a  potentiometer.  Since  the  use  of  potent- 
iometric  instruments  is  becoming  more  extensive  every  year,  especially 
for  laboratory  and  checking  work  and  for  recorders,  this  last  objection  to 
compensation  by  series  resistance  deserves  consideration. 

Compensation  by  Shunt  Resistance 

In  this  method  the  thermocouple  is  shunted  usually  by  a  small  re- 
sistance. If  the  couple  normally  reads  high  the  resistance  of  the  shunt 
is  decreased:  if  low,  the  resistance  is  increased.  An  ordinary  base-metal 
couple,  for  industrial  purposes,  has  a  resistance  of  0.1  to  0.3  ohm  at  room 
temperature.  It  has  been  the  practice  to  shunt  this  couple  with  a  resist- 
ance of  about  the  same  magnitude;  the  shunted  couple  may  accordingly 
be  used  with  a  potentiometer  or  with  a  galvanometer  having  almost  any 
resistance  from,  say,  10  ohms  up. 

Various  couples  are  thus  perfectly  interchangeable  and  may  be  used 
with  instruments  in  parallel,  when  desired;  hence  none  of  the  objections 
to  series  resistance  apply  to  the  shunted  couple.  A  serious  objection, 
however,  may  be  raised  against  the  shunted  resistance  in  that  the  resist- 
ance of  the  shunt,  for  proper  compensation,  depends  upon  the  resistance 
of  the  couple.  The  latter  is  subject  to  change  with  temperature  and 
depth  of  immersion,  depending  upon  the  temperature-resistance  co- 
efficient of  the  two  alloys,  and  is  altered  by  deterioration  of  the  couple. 
The  following  example  illustrates  the  error  which  may  be  expected  from  a 
slight  change  in  resistance  of  the  couple  when  the  resistance  of  the  shunt 
is  low. 

Suppose  the  couple  having  a  resistance,  at  room  temperature,  of  0.2 
ohm  is  shunted  by  0.2  ohm  of  manganin.  If  e  is  the  e.m.f.  developed 
by  the  couple,  the  potential  drop  over  the  shunt  is  ^e.  The  scale  of 
the  galvanometer  is  arbitrarily  graduated  to  take  account  of  this  reduc- 


PAUL   D.    FOOTE,    T.    B.    HARRISON   AND    C.    O.    FAIRCHILD  109 

tion  of  e.m.f.  Suppose  the  resistance  of  the  couple  changes  from  0.2 
to  0.25  ohm.  The  potential  drop  across  the  0.2-ohm  shunt  on  a  couple 
of  0.25  ohm  is  0.44e  instead  of  0.5e  for  which  the  galvanometer  was  gradu- 
ated. The  galvanometer  accordingly  reads  12  per  cent,  low,  or  in  error 
by  about  120°  at  1000°  C.,  which  is  many  times  greater  than  any 
error  which  would  be  introduced  on  account  of  irreproducibility  of  the 
couples  if  no  compensating  device  were  employed.  The  use  of  a  low- 
resistance  shunt  should  be  discontinued;  the  method  is  satisfactory, 
however,  when  the  shunt  has  a  resistance  many  times  that  of  the  couple. 
As  the  resistance  of  the  shunt  is  increased,  the  range  for  adjustment  of 
different  couples  is  diminished;  however,  it  is  not  difficult  to  secure 
matched  wire  which  is  thernioelectrically  reproducible  to  ±2  per  cent., 
and  this  variation  can  be  compensated  by  shunts  of  fairly  high  resistance. 
For  generality,  the  method  is  discussed  under  the  following  heading. 

Compensation  by  Shunt  and  Series  Resistance4 

Let  TI  =  resistance  of  the  couple  plus  a  small  resistance  (if  necessary) 
in  series  with  it;  r2  =  resistance  of  the  shunt;  R  =  resistance  of  the  gal- 
vanometer. The  potential  drop  E'  across  the  shunt  is  given  by  the 
following  equation,  where  e  is  the  e.m.f.  developed  by  the  couple  at 
a  given  temperature. 


In  case  the  potential  drop  is  measured  by  a  potentiometer: 

E  =  —^  (2) 

The  shunt  resistance  r2  is  adjusted  to  compensate  for  the  variation  in 
e.m.f.  of  the  different  couples.  On  account  of  the  term  rLr2/R  in  equation 
(1),  galvanometers  having  different  resistances  will  be  differently  affected 
by  variations  in  r2  from  couple  to  couple.  If,  however,  the  term  rirz/R 
is  small  enough  compared  with  (ri  +  r2)  this  effect  is  negligible  and 
the  potential  drop  across  r2  will  be  practically  the  same  for  all  values 
of  R  and,  with  a  potentiometer,  for  different  couples  regardless  of  the 
values  of  r2.  Let  us  impose  the  condition  that  the  values  of  TI,  r2,  and 
R  must  be  such  that  the  reading  with  a  potentiometer  shall  never  differ 
from  that  with  a  galvanometer  by  more  than  0.5  per  cent.,  (i.e.,  5°  at 
1000°  C.).  This  condition  is  expressed  by: 

—^  ^  0.005  (r,  +  r2)  (3) 

We  desire  to  compensate  for  couples  showing  e.m.f. 's  differing  from  the 
normal  couple  by  less  than  2  per  cent.  For  convenience  in  making  the 

4  Zimmerschied:  U.  S.  Letters  Patent  No.  776252,  1915. 


110 


THERMOELECTRIC    PYROMETRY 


adjustments  on  the  shunt  it  is  better  to  allow  a  little  more  variation  for 
couples  showing  low  e.m.f.;  we  will  make  the  computations  so  that  a 
couple  reading  3  per  cent,  low  could  be  compensated  by  using  a  shunt  of 
infinite  resistance — that  is,  with  no  shunt  at  all.  If  e  is  the  e.m.f.  of 
any  couple,  at  some  fixed  temperature,  and  e'  is  the  e.m.f.  of  a  couple  3 
per  cent,  below  normal,  we  have  from  equation  (2)  for  compensation : 


r2e 


r2 


-f 


=  e  ,  since  r  2  =  <» 


Hence  substituting  in  (3)  the  value  of  r2  found  from  (4) : 
R    e^        R     _  e.m.f .  of  any  couple 

Tl  ^  200  e  ~~ 


(4) 


(5) 


200   e.m.f.  of -couple  3%  low 

It  is  of  advantage  to  make  n  as  large  as  possible;  this  can  be  done  by 
making  R  large,  but  the  value  of  R  must  be  small  enough  to  provide  for 
all  galvanometers  likely  to  be  employed.  If  we  denote  by  R0  the  lowest 
galvanometer  resistance  for  which  compensation  is  required,  the  maximum 
desirable  resistance  of  the  couple  is : 
R 


~  200 
Substituting  this  value  of 


-,  =  total  resistance  of  couple. 


in  (4)  we  obtain: 

RO 


•i" 


=  shunt  resistance 


(6) 


(7) 


Table  6  shows  the  values  of  the  shunt  resistances  and  couple  resistances 
for  the  minimum  galvanometer  resistances  100,  80,  60,  and  40  ohms. 
The  compensation  is  better  the  more  the  resistance  of  the  galvanometer 
exceeds  these  minimum  values,  and  in  no  case  does  the  error  in  compen- 
sation amount  to  more  than  0.5  per  cent,  (i.e.,  3°  at  600°  or  5°  at  1000°  C.). 

TABLE  6. — Compensation  by  Shunt  and  Series  Resistance 


Series  Resistance,  Couple 

n  =  0.5  Ohm, 
Shunt 
ohms 

n  =  0.4  Ohm, 
Shunt 
ohms 

n  =  0.3  Ohm, 
Shunt 
ohms 

n  -  0.2  Ohm, 
Shunt 
ohms 

3  per  cent,  low  

00 

CO 

oo 

00 

2  per  cent,  low  

48.50 

38.80 

29.10 

19.40 

1  per  cent,  low  

24.25 

19.40 

14.55 

9.70 

Normal  

16  17 

12  94 

9  70 

6.47 

1  per  cent,  high  

12.12 

9.70 

7.27 

4.85 

2  per  cent  high. 

9  70 

7  76 

5  82 

3  88 

Minimum  galvanometer  resist- 
ance, ohms  

100 

80 

60 

40 

In  applying  this  method  of  compensation  the  standard  galvanometer 
scale  is  graduated  in  the  usual  manner  for  the  couple  which  reads  3  per 


PAUL   D.    FOOTE,    T.    R.    HARRISON   AND    C.    O.    FAIRCHILD  111 

cent.  low.     Thus  if  we  have  a  table  of  e.m.f .-temperature  for  the  normal 
couple,  we  decrease  all  the  e.m.f.  values  in  the  table  by  3  per  cent. 

It  was  noted  in  the  case  of  shunting  a  0.2-ohm  couple  by  0.2  ohm 
that  if  the  resistance  of  the  couple  increased  from  0.20  to  0.25  ohm,  the 
galvanometer  would  be  in  error  by  120°  at  1000°  C.  On  referring  to 
Table  6,  the  greatest  error  which  a  change  in  the  couple  resistance  from 
0.20  to  0.25  ohm  can  produce,  when  properly  shunted,  occurs  with  the 
couple  reading  normally  2  per  cent,  high,  in  which  case  the  shunt  has  a 
resistance  of  3.88  ohms;  the  error  due  to  this  change  amounts  to  1.2  per 
cent,  or  about  12°  at  1000°  C.  instead  of  120°.  For  the  normal  couple  the 
error  is  only  7°,  and  for  the  couple  reading  2  per  cent,  low,  only  3°.  The 
error  thus  diminishes  as  the  resistance  of  the  shunt  increases,  showing 
that  it  is  preferable  to  use  a  couple  having  a  resistance  of  0.5  ohm;  if  the 
resistance  of  the  couple  alone  is  only  0.2  ohm,  0.3  ohm  of  manganin  may  be 
placed  in  series,  and  the  shunt  connected  over  the  total  of  0.5  ohm.  If 
the  resistance  of  the  couple  is  now  altered  from  0.50  to  0.55  ohm,  the 
couples  reading  2  per  cent,  high,  normal,  and  2  per  cent,  low  will  be  in 
error  by  only  5°,  3°,  and  1°  respectively  at  1000°  C.  It  is  evident  that 
this  method  of  compensation  is  far  superior  to  the  use  of  a  shunt  of  low 
resistance. 

Summary  on  Reproducibility  of  Couples 

The  object  of  this  section  has  been  primarily  to  call  attention  to  the 
difficulties  encountered  by  the  pyrometer  manufacturer  when  he  attempts 
to  correct  for  small  variations  in  the  calibration  of  different  couples.  The 
U.  S.  Bureau  of  Standards  has  calibrated  chromel-alumel  couples  submitted 
for  test,  which  deviated  from  the  normal  couple  by  20°  at  1000°  C.  On 
the  other  hand,  it  has  purchased  chromel-alumel  wire  at  different  times 
for  which  the  maximum  deviation  from  the  specified  temperature-e.m.f. 
relation  was  only  4°  C.  The  variations  with  iron-constantan  are  usually 
greater  but  still  are  not  serious.  It  is  certainly  possible,  if  necessary,  to 
hold  the  manufacturer  to  within  ±  10°  C.  of  the  specifications  for  an 
uncompensated  couple.  There  are  few  industrial  processes,  however, 
using  base-metal  couples  which  require  an  accuracy  of  even  20°  at  1000°  C., 
and  there  are  still  fewer  processes  in  which  temperatures  are  measured 
or  would  probably  be  measured  to  this  accuracy  even  if  a  perfectly  com- 
pensated couple  were  secured.  The  users  of  pyrometers  have  forced 
these  compensation  methods  upon  the  manufacturer,  by  insisting  upon 
greater  precision  than  is  really  necessary.  In  so  doing  the  user  obtains 
a  couple  which,  when  installed  in  a  certain  precise  manner  and  frequently 
checked,  may  give  satisfactory  results,  but  usually  the  compensation 
device  is  a  source  of  more  serious  error  than  would  be  occasioned  by  the 
slight  irreproducibility  of  the  uncompensated  couples. 

A  solution  of  the  problem;  from  the  manufacturing  point  of  view,  is 


112  THERMOELECTRIC  PYROMETRY 

to  secure  as  well  matched  wire  as  possible,  do  away  with  compensation 
devices,  and  sell,  at  different  prices,  two  grades  of  couples,  one  guaranteed 
to  +  10°  and  the  other  to  ±  20°  C.  Possibly  later,  closer  specifications 
could  be  adopted.  If  any  industrial  process  requires  greater  precision 
than  this,  the  couples  should  be  individually  calibrated  and  correction 
curves  prepared  similar  to  those  furnished  with  high-grade  voltmeters 
and  other  electrical  instruments.  If  exact  reproducibility  and  higher 
accuracy  are  both  required,  the  rare-metal  couples  should  be  employed. 
In  objecting  to  compensating  methods,  we  refer  only  to  those  devices 
which  are  supposed  to  correct  for  variations  in  the  thermoelectric 
characteristics  of  the  couple  wire;  compensation  methods  for  eliminating 
cold-junction  errors,  ordinary  "compensating  lead  wires,"  etc.  are  of 
course  necessary  and  must  not  be  confused  with  the  methods  discussed 
in  this  section,  which  serye  an  entirely  different  purpose. 

.  '•  •  » 

THERMOCOUPLE  INSTALLATIONS 

The  installation  of  a  large  thermocouple  equipment  requires  the 
services  of  competent  electricians.  As  much  attention,  if  not  more, 
should  be  given  to  the  wiring,  switches,  switchboards,  etc.,  as  in  the  case 
of  ordinary  power  installations.  Proper  fixtures  should  be  used  to 
mount  the  couple  in  the  furnace.  Lead  wires  should  have  a  weather- 
proof covering  and  should  be  run  in  a  metal  conduit,  except  for  a  short 
length  of  flexible  cable  at  the  ends  of  the  conduit;  the  conduit  should  be 
grounded  to  prevent  leakage  from  power  installations  or  lighting  circuits. 
All  joints  in  the  lead  wires  should  be  soldered  and  taped;  when  indicators 
or  recorders  of  low  resistance  are  employed  it  is  of  the  greatest  impor- 
tance to  have  a  well  constructed  electrical  installation  to  insure  a  constant 
line  resistance.  Since  instruments  of  low  resistance  are  usually  calibrated 
for  a  low  line  resistance  of  definite  value,  the  size  of  copper  wire  required 
for  a  long  line  may  be  as  large  as  No.  12  or  10  (2mm.  or  2.6mm.).  Special 
attention  must  be  given  to  contact  resistances  at  switches.  Frequently 
switches  rated  at  100  amperes  are  required  although  the  actual  thermo- 
electric current  is  only  a  few  milliamperes.  If  the  indicator  is  of  high 
resistance,  or  operates  upon  the  potentiometric  or  semi-potentiometric 
principle,  the  necessity  for  low  line  resistance  is  not  so  pressing,  but  the 
wiring  should  be  well  installed,  for  the  psychological  effect  at  least. 
Stationary  indicating  and  recording  instruments  usually  should  be 
mounted  upon  switchboards,  with  suitable  selective  or  commutating 
switches  when  several  couples  are  used  with  one  indicator.  When 
the  head  of  the  couple  is  exposed  to  severe  conditions,  a  weather-proof 
terminal  head  should  be  provided,  consisting  of  an  outside  casing  which 
fits  over  both  binding  posts.  Lead  wires  should  be  carried  from  the 
couple  to  the  indicator  through  as  cool  rooms  as  conveniently  possible; 
copper  has  a  high  temperature-resistance* coefficient  and  the  frequent 


PAUL   D.    FOOTE,    T.    R.    HARRISON   AND    C.    O.    FAIRCHILD  113 

practice  of  running  wires  over  the  top  of  a  long  row  of  furnaces  may  cause 
large  variations  in  line  resistance. 

The  indicator  or  recorder  should  be  conveniently  located  and  should 
be  mounted  where  vibration  or  excessive  dirt  and  dust  will  not  injure 
the  mechanism;  in  almost  all  industrial  installations,  protecting  cases 
are  required.  Special  devices  are  employed  to  dampen  vibration  when 
this  is  serious,  as  in  the  neighborhood  of  a  trip  hammer  or  rolling  mill. 
Frequently  the  instruments  are  suspended  by  spiral  springs.  One  con- 
venient method  suitable  for  heavy  instruments,  such  as  a  recorder,  is  to 
mount  the  instrument  on  a  board  which  is  supported  on  a  pier  by  four 
tennis  balls,  one  at  each  corner. 

When  fixing  the  couples  in  the  furnace,  the  primary  consideration  is 
to  locate  the  hot  junction  at  the  exact  point  the  temperature  of  which  is 
desired;  at  the  same  time,  the  lead  wires  should  be  conveniently  situated. 
The  space  between  the  protecting  tube  of  the  couple  and  the  furnace 
wall  should  be  tightly  plugged  with  refractory  cement  so  that  cold  air 
cannot  be  drawn  in,  thus  cooling  the  hot  junction.  The  cold-junction' 
box  should  be  so  located  as  to  reduce  the  necessary  amount  of  compen- 
sating lead  wire  to  a  minimum,  since  this  wire  is  somewhat  costly  and 
should  not  be  employed  extravagantly;  great  lengths  of  compensating 
wire  also  increase  the  line  resistance,  since  its  conductivity  is  much 
lower  than  that  of  copper. 

In  case  the  cold  junction  is  buried  underground,  it  must  not  be 
located  too  near  a  large  furnace;  either  the  distance  from  the  furnace, 
or  the  depth  at  which  the  junction  is  buried,  must  be  increased.  A 
depth  of  10  ft.  and  at  least  10  ft.  from  a  large  furnace  is  usually 
satisfactory. 

Common  Return 

The  use  of  a  common  return  wire  for  a  multiple  installation  is  gener- 
ally unsatisfactory,  because  short  circuits  are  so  likely  to  occur.  At 
the  same  time,  leakage  from  a  power  installation  affects  the  reading  of 
every  couple  connected  to  the  return,  and  as  a  leakage  through  a  high 
resistance  may  alter  the  readings  of  every  couple  by  the  same  amount, 
the  presence  of  such  leaks  is  not  always  readily  detected.  It  is  also 
possible,  by  leakage  from  different  couples  to  the  ground,  to  obtain  very 
erratic  and  erroneous  readings  when  the  common  return  is  employed. 
Base-metal  couples  are  frequently  constructed  with  the  hot  junction 
welded  to  the  end  of  the  iron  protecting  tube  in  order  to  reduce  thermal 
lag.  Even  when  this  welded  junction  is  not  made,  the  hot  junction 
usually  touches  the  protecting  tube,  and  is  in  good  electrical  contact 
with  it,  especially  at  high  temperatures,  when  insulation  resistance 
becomes  very  low.  Suppose  that  the  iron  tubes. of  two  chromel-alumel 
couples  are  grounded  to  the  iron  casing  of  a  furnace,  the  two  hot  junc- 

8 


114 


THERMOELECTRIC    PYROMETRY 


tions  thus  being  connected  by  a  circuit  of  iron  (Fig.  24).  The  actual 
result  is  a  chromel-alumel  couple  one  leg  of  which  is  shunted  by  an 
alumel-iron-alumel  differential  couple.  So  long  as  the  temperatures  of 
the  hot  junctions  of  both  couples  are  the  same,  this  differential  couple 
produces  no  effect,  but  it  will  alter  the  reading  of  the  indicator  whenever 
the  temperatures  differ;  both  of  the  chromel-alumel  couples  will  accord- 
ingly give  erroneous  results.  With  indi- 
vidual returns,  the  iron  circuit  produces 
no  effect.  When  grounds  occur  further 
back  from  the  hot  junction,  for  example 
between  the  common  return  and  the  other 
lead  wire  of  a  single  couple,  every  couple 
on  the  common  return  has,  in  addition  to 
its  own  e.m.f.,  an  impressed  potential  drop 
due  to  the  current  flowing  in  the  shunted 
couple,  which  may  cause  a  large  error  in 
every  reading.  The  common  return  is  ex- 
tensively utilized  in  the  industries,  but  it  is 
a  dangerous  practice  and  should  be  avoided 
as  far  as  possible. 

Wiring  Diagrams  oj  Thermocouple 
Installations 


Indicator 

Indicator 

•o 

(8 

V 

c 

o 

E 

E 

o 

0 

B 

+ 

+ 

* 

Chromel 

Alumel 
Alumel 

Chromel 

' 

"3 
5 

•o 

a 

I 

\/ 

Iron 

\/ 

\t 

\^ 

V 

Grounds 

FIG.  24. — ILLUSTRATING  THE  OB- 
JECTION TO  COMMON  RETURN. 


Fig.  25  illustrates  a  simple  thermoelec- 
tric installation  for  a  rare-metal  couple,  the 
couple  being  properly  protected  by  a  por- 
celain or  quartz  tube  and,  if  necessary,  by 
an  outer  tube  of  iron,  chromel,  fireclay,  etc.  From  the  head  of  the 
couple,  compensating  lead  wires  are  carried  to  the  bottom  of  a  pipe 
driven  10  ft.  underground,  with  copper  wires  leading  to  the  indicator. 

Fig.  26  illustrates  a  multiple  installation  for  five  thermocouples; 
in  this  case  a  common  return  is  employed,  whereby  four  lengths  of  copper 
wire  have  been  saved,  and  the  commutating  switch  made  simpler.  The 
indicator  for  the  operator  of  the  furnaces  and  the  recorder  for  the  super- 
intendent's office  are  mounted  in  parallel.  The  indicator  or  recorder 
may  be  connected  to  any  desired  couple  by  setting  the  commutating 
switch.  Such  an  installation  can  be  utilized  only  when  the  instruments 
have  a  high  resistance.  The  recorder  and  the  indicator,  when  connected 
to  the  same  couple  at  the  same  time,  act  as  shunts  on  each  other,  which 
tends  to  make  both  instruments  read  low:  whereas  if  the  two  instruments 
are  calibrated  to  read  correctly  in  parallel,  they  will  both  read  high  when 
connected  to  different  couples.  An  example  illustrates  this  point.  Sup- 
pose the  line  and  couple  resistance  for  each  circuit  is  3  ohms,  and  the 
resistance  of  the  recorder  and  indicator  500  ohms  each.  Assume  that 


PAUL   D.    FOOTE,    T.    R.    HARRISON   AND    C.    O.    FAIRCHILD 


115 


both  instruments  are  calibrated  to  read  correctly  when  connected  sepa- 
rately to  any  couple.  The  potential  drop  E  across  the  terminals  of  either 
instrument  bears  the  following  relation  to  e,  the  e.m.f.  of  the  couple: 


Thermocouple 


as  Pipe  Driven  in 
Ground 


Junction  of  Extension  Wire  and 

Copper  Leads  to  Instrument 

Soldered  and  Insulated 


FIG.  25. — SIMPLE    THERMOCOUPLE    INSTALLATION. 

E  =  (500  -T-  503)  e.     The  scale  of  the  instrument  is  graduated  to  take 
account  of  this  reduction  in  e.m.f.     When  the  two  instruments  are  in 


Recording 

Instrument 

in  Office 


FIG.  26. — SIMPLE    S-COUPLE    INSTALLATION. 

parallel,  the  potential  drop  across  the  indicator  and  recorder  is  given  by 
the  following  equation,  where  R  =  resistance  of  indicator  or  recorder,  and 
r  =  line  resistance: 


116  THERMOELECTRIC  PYROMETRY 

,          Re          500 

~  R  +  lr  ~  506  e 

Accordingly  the  e.m.f.  is  reduced  by  the  factor  500  -f-  506.  As  each 
instrument  is  calibrated  for  a  reduction  of  500  -r-  503,  the  error  resulting 
from  the  parallel  connection  is  thus  0.6  per  cent.,  or  about  6°  at  1000°  C. 
Hence  if  the  operator  of  the  furnace  switches  a  couple  onto  the  indicator 
when  the  same  couple  is  also  connected  to  the  recorder,  both  instruments 
will  read  low;  but  this  error  is  usually  insignificant. 

A  similar  example  will  be  considered  for  an  indicator  and  a  recorder 
of  low  resistance.  Let  the  line  resistance  r  be  3  ohms,  as  before,  and  the 
resistances  R  of  the  indicator  and  the  recorder  be  10  ohms  each.  The 
potential  drop  E  across  the  terminals  of  either  instrument  bears  the  fol- 
lowing relation  to  e,  the  e.m.f.  of  the  couple: 

Re         10 


When  the  two  instruments  are  in  parallel,  the  potential  drop  across  the 
indicator  and  recorder  is: 

R  10 

*    -fi+Tr'-Te6 

The  instruments  are  calibrated  to  read  correctly  when  used  separately, 
that  is  for  a  reduction  in  e.m.f.  by  the  factor  10  -j-  13;  when  in  parallel 
the  reduction  factor  is  10  -f-  16,  the  resulting  error  thus  being  19  per  cent., 
or  about  190°  at  1000°  C.  For  this  reason,  low-resistance  instruments  can 
not  be  alternately  operated  separately  and  in  parallel  on  the  same  couple  ; 
they  must  always  be  used  either  separately  or  in  parallel.  Cut-out 
switches  are  frequently  employed,  so  designed  that  when  the  indicator 
is  connected  with  a  couple,  this  couple  is  automatically  thrown  out  of  the 
recorder  circuit.  The  paralleling  of  simple  galvanometric  instruments 
having  a  resistance  of  300  ohms  and  more  when  the  line  resistance  is 
less  than  3  ohms,  or  of  potentiometric  instruments,  is  a  safe  practice; 
the  paralleling  of  instruments  having  lower  resistances  requires  specially 
graduated  scales  or  special  wiring  circuits.  Low-resistance  instruments 
designed  for  parallel  operation  should  not  be  used  separately,  unless 
protected  by  cut-out  switches. 

Fig.  27  illustrates  a  simple  installation  in  an  oil-fired  furnace.  The 
thermocouple  is  protected  from  mechanical  shocks  and  breakage  by  an 
additional  metal  sheath.  Compensating  lead  wire  is  carried  to  the 
indicator,  the  cold  junction  being  located  at  the  indicator  and  not  ther- 
mostatically controlled.  The  indicator  is  set  to  .read  the  room  or 
cold-junction  temperature  on  open  circuit;  otherwise  correction  for 
"cold-junction  error"  must  be  applied. 


PAUL   D.    FOOTE,    T.    R.    HARRISON   AND    C.    O.    FAIRCHILD 


117 


Fig.  28  shows  a  thermocouple  imbedded  in  the  floor  of  an  oil-fired 
furnace,  thus  occupying  no  space  in  the  heating  chamber.     The  cold 


FIG.  27. 


CROSS   SECTION  A-B 

SHOWING   LOCATION 

OF   THE  THERMO-COUPLE  STEM 


FIG.  28. — THERMOCOUPLE  IN  OIL-FIRED  FURNACE. 


junction  is  water-jacketed.     Fig.  29  shows  a  method  of  installing  a  couple 
in  the  wall  of  a  large  furnace. 


118 


THERMOELECTRIC    PYROMETRY 


Fig.  30  shows  a  method  of  installing  a  couple  in  a  galvanizing  tank  or 
in  a  pot  of  stereotype  metal,  babbitt,  or  tin. 


FIG.  29. — METHOD  OF  MOUNTING  COUPLE  IN  FURNACE. 

Fig.  31  illustrates  a  multiple  thermocouple  installation  connected  to 
a  single  indicator.  Compensating  lead  wires  are  carried  from  the  couples 
to  a  conveniently  located  cold-junction  box,  the  temperature  of  which  is 


Tbermo- 
/couple 


Plug 


FIG.  30. — THERMOCOUPLE  IN  LEAD  BATH. 

thermostatically  controlled.  From  the  cold-junction  box  copper  wires 
run  to  the  terminal  block  and  selective  switch.  A  common  return  has 
been  employed  between  the  cold-junction  box  and  the  switchboard. 


PAUL   D.    FOOTE,    T.    R.    HARRISON   AND    C.    O.    FAIRCHILD 


119 


The  switchboard  is  designed  for  six  couples;  by  pressing  one  of  the  buttons 
any  desired  couple  is  connected  directly  to  the  indicator. 

Commutating  Switches 

In  one  type  of  multiple  rotary  switch,  by  turning  the  dial  to  the 
proper  position  any  one  of  twelve  couples  may  be  connected  to  the  indi- 
cator. The  commutating  brushes  are  laminated  phosphor-bronze,  diag- 
onal wiping,  and  have  a  long  spring  action  to  follow  up  all  possible  wear. 
In  a  switch  of  different  design,  but  similar  in  principle,  double  points 
of  contact  are  required  when  individual  return  wires  are  employed. 
The  positive  wires  of  each  couple  are  connected  to  the  outer  ring  of 
contacts  and  the  negative  wires  to  the  inner  ring.  The  galvanometer 
is  connected  across  the  two  solid  rings.  Commutating  switches  are 
designed  so  that  variable  contact  resistance  is  reduced  to  a  minimum. 

For  large  installations  several  hundred  couples  may  be  connected 
to  a  switchboard,  which  is  frequently  designed  somewhat  similar  to  an 
ordinary  telephone  switchboard.  Often  in  these  large  installations 
communication  between  the  operator  of  the  switchboard  and  the  opera- 
tor of  the  furnace  is  maintained  by  a  system  of  colored  electric  lamps. 

Junction  Box  and  Zone  Box 

Fig.  32  is  a  wiring  diagram  for  a  multiple -couple  installation  which 
has  the  advantage  of  saving  compensating  lead  wire,  and  thus  reducing 


Thermocouples 

FIG.  31. — MULTIPLE-COUPLE    INSTALLATION    WITH    THERMOSTATED    COLD-JUNCTION 
BOX  AND  COMPENSATING  LEADS. 

the  cost  and  the  resistance  of  the  line.  The  junction  box  is  a  cast-iron  box 
such  as  is  used  for  underground  telephone  wiring;  it  is  not  thermostated, 
since  a  constant  and  measured  temperature  is  not  required.  The  e.m.f. 
developed  at  the  junctions  of  compensating  leads  and  copper  leads  is 
corrected  by  a  common  junction,  in  the  opposite  direction,  inserted 


120 


THERMOELECTRIC    PYROMETRY 


between  the  selective  switch  and  the  indicator  or  recorder.  A  common 
cold  junction  is  also  placed  here;  in  Fig.  32  this  is  shown  located  in  a  pipe 
buried  10  ft.  underground.  The  selective  switch  and  recorder  or  indica- 
tor are  usually  mounted  in  a  single  case.  The  common  cold  junction  and 
the  junction-box  compensating  couple  are  connected  at  the  recorder 
between  the  switch  and  the  binding  post  terminals  of  the  instrument,  as 
illustrated.  The  cold  junction  is  placed  near  the  junction  box  and  the 
recorder  or  indicator  (with  switch  if  desired)  may  be  any  distance  away 
since  only  copper  leads  are  used  from  this  point  to  the  junction  box. 


Selective  Switch 


FIG.  32. — ^ILLUSTRATING  USE  OF  JUNCTION  BOX  WITH  CONTROLLED  COLD-JUNCTION 

TEMPERATURE. 

This  method  is  especially  useful  where  separate  cold  junctions  would 
require  too  long  compensating  leads.  Suppose  the  temperature  of  a 
coke  oven,  20  by  40  by  150  ft.,  is  measured  by  nine  couples  inserted  in  the 
top.  The  indicator  is  at  the  ground  level,  40  ft.  away,  and  the  buried 
cold  junctions  are  20  ft.  in  front  of  the  oven.  When  the  junction  box  is 
not  employed,  the  amount  of  compensated  lead  wire  required  to  reach  the 
buried  cold  junctions  is  as  follows: 

Three  couples  at  rear  of  furnace,      3  X  (150  +  20  +  20  +  10)  ft. 

Three  couples  at  center  of  furnace,  3  X  (  75  +  20  +  20  +  10)  ft. 

Three  couples  at  front  of  furnace,                3  X  (20  +  20  +  10)  ft. 
Total  compensating  cable 1125  ft. 


PAUL   D.    FOOTE,    T.    R.    HARRISON   AND    C.    O.    FAIRCHILD 


121 


When  a  junction  box  is  located  on  top  of  the  furnace,  at  the  center 

Three  couples  at  rear  of  furnace,  3  X  75  ft. 
Three  couples  at  center  of  furnace,  2  X  10  ft. 
Three  couples  at  front  of  furnace,  3  X  75  ft. 

From  box  to  cold  junction (75+20  +  20  +  10)  ft. 

Total  compensating  cable 595  ft. 

By  means  of  the  junction  box  we  effect  a  saving  of  some  500  ft.  of  com- 
pensating cable,  and  need  to  bury  only  one  pair  of  junctions,  the 
installation  being  just  as  satisfactory.  When  installing  a  large  multiple- 
couple  equipment  with  a  junction  box,  it  is  important  to  insure  that  the 
common  cold-junction  couple  is  connected  with  the  correct  polarity,  as 
illustrated.  Although  a  common  cold  junction  has  been  used  for  all 
couples,  the  objectional  common  return  has  not  been  employed. 

In  case  the  recorder  is  placed  where  the  temperature  is  fairly  uniform 
from  day  to  day,  the  use  of  a  buried  cold  junction  or  thermostated  cold- 
junction  box  is  not  absolutely  essential.  The  e.m.f.  generated  at  the 


Fia.  33. — ILLUSTRATING  USE  OP  JUNCTION  BOX  WITHOUT  COLD-JUNCTION  CONTROL. 

junction  box  in  Fig.  32  is  then  compensated  by  running  one  pair  of  com- 
pensating leads  from  the  recorder  to  the  junction  box,  taking  care  to 
connect  the  negative  lead  to  the  negative  terminal  of  the  recorder  and 
the  positive  lead  to  the  selective  switch.  A  simple  installation  of  this 
kind  is  illustrated  by  Fig.  33;  here  only  one  couple  is  shown,  but  as  many  as 
desired  may  be  connected  to  the  multiple-pole  selective  switch.  The 
compensating  lead  wires  are  soldered  together  inside  the  junction  box  and 
the  auxiliary  couple  formed  by  the  compensating  leads  is  in  series  with 
the  couple  connected  in  by  the  selective  switch.  The  cold  junction  is 
accordingly  actually  at  the  recorder,  where  the  temperature  is  fairly 
constant;  changes  in  temperature  of  the  distributing  or  junction  box  thus 
will  not  affect  the  reading  of  any  couple. 


122 


THERMOELECTRIC   PYROMETRY 


Fig.  34  illustrates  the  Wilson-Maeulen  zone  box,  to  which  the  couple 
yz  is  connected  directly.  Two  pairs  of  wires  lead  from  the  zone  box,  one 
pair,  of  copper,  being  connected  to  the  main  line  and  indicator;  the  other 
pair  YZ  goes  to  the  cold-junction  box  or  is  buried  underground.  For 
base-metal  couples  Z  and  y  are  of  the  same  material,  and  also  Z  and  z. 
For  rare-metal  couples,  Z  and  Y  are  respectively  copper  and  a  copper-nickel 
alloy.  The  zone  box  thus  saves  running  an  extra  pair  of  copper  lead  wires 
to  the  bottom  of  the  cold-junction  well.  If  this  principle  is  adopted  for 
a  multiple  installation,  so  as  to  save  compensating  leads,  a  selective  switch 
may  be  mounted  between  the  zone  box  and  the  different  couples;  however, 


Cupjur  Leads 


FIG.  34. — WILSON-MAEULEN  ZONE  BOX. 

this  requires  that  the  selective  switch  be  mounted  near  the  furnace,  unless 
a  complicated  interlacing  electrical  circuit  is  employed,  whereas  it  is 
always  desirable  to  have  the  switch  at  the  indicator.  It  appears,  therefore, 
that  for  large  installations  the  junction  box  previously  described  is  pref- 
erable so  far  as  economy  in  compensating  lead  wire  is  concerned.  In 
case  of  a  single  couple,  or  where  separate  cold-junction  wells  are  installed 
for  different  couples  in  multiple  installations,  the  zone  box  accomplishes 
practically  the  same  results  as  the  junction  box,  affording  a  small  saving 
in  copper  and  frequently  a  more  desirable  wiring  system.  The  main 
advantage  of  the  junction  box  is  the  saving  of  compensating  lead  wire  in  a 
multiple-couple  installation;  the  main  advantage  of  the  zone  box  is  the 
simplicity  with  which  the  cold  junction  is  extended  to  some  point  .at 
which  the  temperature  is  constant  or  can  be  controlled. 

Determination  of  Temperature  of  Buried  Cold  Junction 

The  simplest  method  is  to  use  a  thermocouple  consisting  of  the  com- 
pensating leads.  Insert  this  in  the  well,  connect  it  to  a  portable  indicator, 
and  measure  the  secondary  cold-junction  temperature  at  the  indicator 


PAUL   D.    FOOTE,    T.    R.    HARRISON   AND    C.    O.    FAIRCHILD  123 

with  a  thermometer.  With  a  well  10  ft.  deep  or  more  and  properly 
located,  it  is  necessary  to  measure  the  temperature  only  once  a  month. 
Another  method  is  to  lower  a  thermometer  into  the  well,  wrapped  with 
a  few  layers  of  cloth  but  leaving  the  stem  exposed  near  the  expected 
reading;  it  should  be  left  in  the  well  for  30  min.  All  buried  leads  to  the 
cold  junction  should  be  water-proof  insulated,  and  the  junction  well 
should  be  water  tight.  The  compensating  leads,  particularly  those 
for  base-metal  couples,  will  generate  a  large  voltaic  e.m.f.  if  they 
become  wet. 

Depth  of  Immersion  of  Couples 

Thermocouples  immersed  in  furnaces  of  the  various  industrial  types 
must  be  carefully  protected.  Heavy  iron  tubes  and  frequently  larger 
auxiliary  protecting  tubes  of  various  materials  are  employed.  There  is 
no  certainty  that  the  temperature  indicated  by  the  thermocouple  is 
actually  that  of  the  furnace,  because  of  conduction  along  the  protecting 
tubes.  Conduction  losses  may  be  reduced  and  even  eliminated  by 
allowing  a  sufficient  depth  of  insertion,  but  it  is  not  always  possible  to  do 
this  and  it  is  difficult  to  determine  when  the  depth  is  sufficient.  The 
general  practice  is  to  make  the  depth  as  great  as  convenient  and  trust 
that  this  is  sufficient.  Two  methods  may  be  suggested  for  investigating 
this  question,  but  neither  is  very  conclusive.  First:  Remove  the  couple 
alone  from  the  fixed  installation,  leaving  all  protecting  tubes  in  place. 
If  it  is  impossible  to  remove  the  iron  tube  from  the  couple  use  a  similar 
tube  in  the  fixed  installation  without  the  couple.  Explore  the  temperature 
inside  the  protecting  tube  with  an  unprotected  couple.  If  the  tempera- 
ture for  several  centimeters  along  the  inner  end  is  practically  uniform, 
the  depth  is  sufficient;  if  the  temperature  falls  rapidly  in  that  region,  the 
depth  is  not  enough.  Second:  The  couple,  previously  standardized,  is 
mounted  complete  in  the  fixed  installation  and  compared  with  a  checking 
couple  mounted  at  its  side.  The  checking  couple  must  have  a  small  cross- 
section,  and  must  be  either  unprotected  or  protected  by  an  extremely 
thin  protection  tube,  in  order  to  minimize  the  loss  of  heat  by  conduction. 
The  hot  junction  of  this  couple  and  that  of  the  couple  under  test  are 
brought  closely  together,  but  not  in  contact.  The  checking  couple 
should  indicate  the  furnace  temperature  more  closely  than  the  fixed 
couple;  if  the  difference  is  large  a  greater  depth  should  be  adopted.  These 
methods  are  complicated  by  local  variations  in  the  temperature  of  the 
furnace,  but  checks  of  this  nature,  although  somewhat  unsatisfactory, 
are  better  than  none. 

It  is  frequently  desirable  to  purposely  immerse  the  couple  to  an 
insufficient  depth.  In  many  processes  the  furnace  is  operated  at  such 
a  high  temperature  that  a  thermocouple  or  protecting  tube  cannot  with- 


124  THERMOELECTRIC  PYROMETRY 

stand  the  severe  conditions  to  which  it  may  be  subjected.  In  this  case 
the  couple  may  be  immersed  only  part  way  through  the  furnace  wall, 
or  to  a  distance  flush  with  the  inner  wall  of  the  furnace.  The  tem- 
peratures indicated  by  couples  installed  in  this  manner  are  always  lower 
than  those  of  the  furnace  interior,  but  they  bear  a  fairly  definite  rela- 
tion to  the  temperature  of  the  furnace,  and  hence  the  method  is  satis- 
factory for  temperature  control  and  reproduction  of  furnace  conditions 
from  day  to  day. 

PROTECTION  TUBES  FOR  THERMOCOUPLES 

The  choice  of  a  proper  protection  tube  for  a  thermocouple  is  nearly 
as  important  as  the  selection  of  the  material  for  the  couple.  Among 
others,  the  following  properties  of  a  protection  tube  should  be  con- 
sidered : 

(a)  Slight  porosity  to  gases;  many  tubes  become  very  porous  at 
high  temperatures  and  furnace  gases  usually  attack  the  couple. 

(6)  Low  volatility;  certain  metal  tubes  are  undesirable  at  high 
temperatures  because  the  metal  distills  upon  the  couple  and  alters  its 
calibration. 

(c)  Ability  to  withstand  high  temperatures. 

(d)  Ability  to  withstand  sudden  changes  in  temperature. 

(e)  Ability  to  withstand  mechanical  shocks  and  strains. 

(/)  High  rigidity  or  viscosity;  protecting  tubes  frequently  deform  and 
exhibit  the  phenomenon  of  plastic  flow  at  high  temperatures. 

(gr)  Thermal  conductivity;  high  thermal  conductivity  is  frequently 
desirable  when  rapidly  changing  temperatures  are  measured;  usually, 
however,  low  conductivity  is  desired  so  that  the  flow  of  heat  along  the 
tube  shall  be  small. 

(h)  Ability  to  resist  corrosion  from  molten  metals  or  furnace  gases. 

Excellent  protecting  tubes  are  obtainable  for  many  different  industrial 
processes,  but  for  certain  others,  satisfactory  tubes  have  not  yet  been 
developed;  this  applies  particularly  to  industries  dealing  with  molten 
metals,  especially  iron  and  brass. 

Fused  Quartz. — Fused  quartz  affords  good  protection  up  to  1050°  C. 
in  an  oxidizing  atmosphere  free  from  alkalies.  The  material  is  some- 
what pervious  to  hydrogen  and  probably  to  other  reducing  gases,  but  at 
this  temperature  is  not  pervious  to  oxygen  or  carbon  dioxide.  Any 
reducing  gas  within  the  protecting  tube  of  a  rare-metal  couple  is  disas- 
trous, particularly  when  the  tube  contains  silica;  the  silica  is  reduced  to 
silicon,  which  is  readily  absorbed  by  platinum.  Above  1050°  C.,  and  even 
at  lower  temperatures  after  prolonged  heating,  quartz  devitrifies  and 
crumbles.  Quartz  tubes  withstand  sudden  changes  of  temperature 
without  breaking.  Heavy  sintered  quartz  tubes,  with, walls  1  or  2  cm. 


PAUL   D.    FOOTE,    T.    R.    HARRISON   AND    C.    O.    FAIRCHILD 


125 


thick,  are  sometimes  used  for  extra  protection,  for  example,  against  acid 
fumes. 

Porcelain. — Porcelain  is  used  primarily  for  protection  of  rare-metal 
couples.  Previous  to  1914  a  highly  refractory  porcelain,  known  as  Mar- 
quardt,  was  imported  from  Germany.  A  better  grade  of  this  material 
was  developed  through  the  research  work  of  the  U.  S.  Bureau  of  Standards, 
and  is  now  manufactured  by  Stupakoff  under  the  name  "Usalite," 
and  by  Engelhard  under  the  name  "Impervite."  These  two  porcelains 
have  a  melting  point  above  that  of  platinum.  However,  only  when 


FIG. -35. — PROTECTING  TUBES  FOR  COUPLES. 

glazed  are  they  impervious  to  gasefe.  The  softening  point  of  the  American 
glaze  is  about  1300°  C. ;  that  on  the  German  tubes  softens  at  1200°  C.  If 
the  tubes  are  glazed  only  on  the  outside  they  are  serviceable  as  pyrometer 
protection  tubes  up  to  1500°  C.;  the  insulating  tubes  are  not  glazed. 
An  unprotected  porcelain  tube,  suddenly  thrust  into  a  furnace  at  1000°  C., 
will  usually  break,  but  not  if  it  is  inserted  very  slowly. 

In  permanent  installations,  quartz  and  porcelain  tubes,  and  also  the 
iron  or  chromel  tubes  of  base-metal  couples,  are  frequently  further 
protected  by  heavy  outer  tubes  of  fireclay,  carborundum,  graphite,  etc. 
The  outer  tube  is  usually  cemented  in  place  in  the  furnace  wall,  forming 
a  well  into  which  the  couple  is  inserted.  In  case  the  outer  tube  intro- 
duces too  large  a  temperature  lag,  or  if  the  temperature  of  the  hot  June- 


126 


THERMOELECTRIC    PYROMETRY 


tion  of  the  couple  is  likely  to  be  diminished  by  conduction  of  heat  through 
the  heavy  tube,  the  latter  is  ma'de  open  at  both  ends;  the  couple  and  its 
smaller  protecting  tube  are  then  so  mounted  that  the  hot  junction  pro- 
jects a  few  centimeters  beyond  the  end  of  the  outer  protecting  tube. 


TABLE  7. — Calibration  Data  of  Representative  Couples 
Cold-junction  Temperature,  0°C.     E.m.f.  in  millivolts 


Engelhard 

Johnson-Matthey 

Copper- 

Iron-constantan                Chromel- 

"Le  Chatelier" 

"Le  Chatelier" 

constantan 

alumel 

E.m.f. 

Temp., 
Degrees 
C. 

E.m.f. 

Temp., 
Degrees 
C. 

E.m.f. 

Temp., 
Degrees 

E.m.f. 

Temp., 
Degrees  C. 

E.m.f. 

Temp., 
Degrees 
C. 

B 

L 

0 

0 

0 

0 

0 

0          0 

0 

0 

0 

0 

1 

147          1 

146 

1 

25           5       105 

95 

5 

122 

2 

265          2 

260 

2 

49         10 

204 

186 

10 

243 

3 

374          3         364 

3           72         15       299 

277 

15 

363 

4 

478 

4         461 

4           94        20       392 

367 

20 

482 

5 

578          5         553           5         115        25 

483 

457 

25 

601 

6             675 

5         641 

6         136         30 

574 

546 

30 

721 

7 

770 

7         725 

7 

156        35 

662 

632 

35 

844 

8 

861 

8 

806 

8 

175 

40 

749 

713 

40 

970 

9 

950          9 

884 

9 

194 

45 

836 

792 

45 

1100 

10 

1037 

10 

959 

10         213 

50 

924 

871 

11 

1122 

11 

1032 

11 

232         55     1011 

950 

. 

12           1206         12       1103         12         250        60 

1030 

13    .        1290         13       1173         13         268 

14           1373         14       1242 
15           1455         15       1311 

B  represents  mean  calibration 
302      by  U.  S.  Bureau  of  Standards  of 

16       1379 

iron-constantan  couoles  from  all 

17       1447 

17         336 

sources.     L  represents  mean  cali- 

18        353 

bration   of   Leeds    &    Northrup's 

iron-constantan  couple. 

Carborundum. — Carborundum  is  Used  for  outer  protecting  tubes 
(Fig.  35,  No.  1).  It  has  a  high  thermal  conductivity,  about  twice  that  of 
silica,  a  low  coefficient  of  expansion,  about  one-half  that  of  fused  alumina, 
and  great  mechanical  strength.  When  heated  in  an  oxidizing  atmosphere, 
oxidation  begins  at  about  1200°  C.  At  1500°  C.  the  silica  formed  on  the 
surface  of  the  tube  fuses  and  protects  the  tube  from  further  oxidation. 
Gases,  except  chlorine,  do  not  act  on  carborundum,  but  basic  slags  attack 
it  readily.  Carborundum  reacts  at  high  temperatures  with  practically 
all  metals,  wherefore  platinum  must  be  thoroughly  protected  from  the 
carborundum  by  a  gas-tight  inner  tube.  Silfrax,  which  is  pure  finely 
crystalline  carborundum,  is  highly  satisfactory  for  ordinary  furnace 
work;  it  is  sometimes  used  in  molten  glass  and  open-hearth  slag. 


PAUL    D.    FOOTE,    T.    R.    HARRISON    AND    C.    O.    FAIRCHILD  127 

Nichrome  or  Chromel. — Cast  nichrome  or  chromel  tubes  are  exten- 
sively used  for  protection  of  both  base-metal  and  rare-metal  couples 
(Fig.  35,  No.  2).  These  tubes  resist  oxidation  remarkably  well  and, 
although  much  more  costly  than  iron  tubes,  their  longer  life  warrants 
and  requires  their  use  in  many  processes.  Chromel  "A"  may  be  used 
continuously  to  1200°  C.  It  has  not  yet  been  found  possible  to  draw 
these  alloys  into  tubes.  The  material  can  be  machined  with  difficulty 
but  threads  may  be  cut  for  pipe  fittings.  In  order  to  economize  in 
these  materials,  iron  tubes  may  have  short  sections  of  nichrome  or 
chromel  welded  to  them,  at  the  place  where  exposed  to  the  furnace. 
In  processes  carried  out  at  low  temperatures,  where  either  iron,  chromel, 
or  nichrome  may  be  utilized,  experiments  should  be  made  to  determine 
the  relative  life  of  these  tubes  in  hours  per  dollar  of  cost;  practically 
no  data  are  available  on  this  subject.  In  heat-treating  and  carburizing 
furnaces,  chromel  "A"  is  often  used;  chromel  "C"  and  nichrome  last 
many  months  in  lead  baths.  Chromel  and  nichrome  do  not  volatilize 
so  readily  as  iron;  base-metal  couples  are  thus  better  protected  by  these 
tubes  than  by  iron  or  steel.  Chromel  "A"  contains  practically  no  iron, 
while  chromel  "C"  and  nichrome  contain  considerable. 

Graphite. — Graphite  affords  an  excellent  protection  to  quartz  or  porce- 
lain tubes  on  rare-metal  couples,  and  is  frequently  used  with  base-metal 
couples  for  molten  metals  (Fig.  35,  No.  3).  Porcelain  encased  in  a 
sheath  of  graphite  can  be  used  in  molten  aluminum.  Platinum  couples 
must  be  thoroughly  protected  against  the  vapors  distilled  from  graphite 
or  carbon,  and  from  the  reducing  atmosphere  present  near  heated 
graphite. 

Fireclay. — Outer  tubes  of  fireclay  are  used  for  protection  in  kilns, 
glass  and  steel  furnaces,  annealing  ovens,  etc.  (Fig.  35,  No.  4).  Usually 
they  are  mounted  vertically  in  the  top  of  the  furnace  and  may  be  ce- 
mented in  place.  Small  fireclay  insulating  tubes  are  used  on  base-metal 
couples. 

Corundite. — Corundite  consists  of  emery  with  a  plastic  clay  binder 
(Fig.  35,  No.  5).  It  is  used  in  ceramic  and  glass  industries  for  outer  pro- 
tection tubes. 

Alundum. — Natural  corundum  usually  contains  a  large  amount  of 
iron.  The  artificial  product,  fused  AlzO3,  or  alundum,  is  practically 
free  from  iron,  and  is  very  desirable  for  protecting  rare-metal  couples 
and  also  as  outer  protecting  tubes.  The  tubes  are  made  from  pulverized 
alumina  mixed  with  a  clay  binder.  The  inner  protecting  tubes  are  glazed, 
in  order  to  reduce  porosity,  and  the  glaze  is  coated  with  an  outer  layer  of 
alundum;  this  method  permits  the  tubes  to  be  used  at  temperatures 
above  the  softening  point  of  the  glaze,  being  serviceable  up  to  1400°  C. 
Outer  protecting  tubes,  without  glazing,  withstand  temperatures  up 


128  THERMOELECTRIC  PYROMETRY 

to  1550°  C.,  .and  even  higher.  Alundum  is  mechanically  strong  and  resists 
temperature  changes  much  better  than  porcelain. 

Nickel. — Pure  nickel  is  frequently  used  in  cyanide  baths.  In  an 
oxidizing  atmosphere  a  thick,  tough  coating  of  oxide  forms  and  affords 
protection  against  further  corrosion. 

Steel  and  Iron. — Seamless  steel  and  wrought-iron  tubes  are  usually 
furnished  with  base-metal  couples.  They  are  satisfactory  for  many 
processes  up  to  800°  or  900°  C,  for  example.,  in  a  muffle  furnace. 

Colorized  Iron. — Calorizing  is  a  process  by  which  the  surface  of  a 
wrought-iron  tube  is  impregnated  with  me'tallic  aluminum.  Calorized 
tubes  resist  oxidation  better  than  the  pure  iron  or  steel. 

Duriron. — Duriron,  a  high-silicon  iron  alloy,  is  sometimes  used,  at 
lower  temperatures,  as  a  protection  against  acid  fumes.  When  sub- 
jected to  sudden  temperature  changes  the  material  may  fracture. 

DISCUSSION 

C.  B.  THWING,  Philadelphia,  Pa. — It  has  become  so  customary  to 
refer  to  the  potentiometer  method  as  the  ideal  method  which  fits  every- 
where that  the  impression  is  general  that  this  method  has  no  limitation. 
The  faults  of  the  galvanometer  method  are  so  well  known  that  a  word 
should  occasionally  be  said  on  the  other  side.  There  has  been  a  marked 
improvement  in  the  construction  of  the  galvanometers  and  little  in  the 
construction  of  the  potentiometers. 

At  the  time  we  began  to  build  recording  instruments,  milli voltmeters, 
we  called  our  75-ohm  instrument  a  high-resistance  instrument;  and 
it  was  high  in  comparison  with  the  5-ohm  instruments  then  in  use. 
At  the  present  time  we  build  a  galvanometer  with  800  to  1000  ohms 
resistance  that  has  just  as  good  control.  Mr.  Foote  mentioned  300  ohms 
as  the  desirable  resistance  for  a  good,  rugged  millivoltmeter.  If  he  had 
been  writing  5  years  ago,  he  would  have  said  30  ohms.  It  is  just  as  easy 
now  to  make  them  800,  or  1000  ohms,  that  is,  on  the  50-millivolt  basis. 

A.  O.  ASHMAN,  Palmerton,  Pa.: — Unquestionably  the  instruments  most 
used  in  pyrometric  work  are  of  the  galvanometer  type.  We  are  so 
accustomed  to  thinking  in  terms  of  high-resistance  instruments  that, 
I  believe,  today  we  have  gone  to  the  extreme.  We  are  all  well  acquainted 
with  the  evils  of  fche  low-resistance  instrument  and  think  the  cure  is  to 
make  an  extremely  high-resistance  instrument.  There  are,  from  a 
practical  point  of  view,  several  objections  to  this.  One  is  the  large  zero 
shift  in  these  high-resistance  instruments  or  the  highly  sensitive  instru- 
ments. This  zero  shift,  on  a  200  division  scale,  in  my  experience,  has 
amounted  to  5  and  6  and  sometimes  10  divisions.  It  is  very  hard  to 
compensate  for  this  in  practice,  although  it  can  be  done. 


DISCUSSION 


129 


Another  thing,  the  tendency  toward  greater  delicacy  makes  these 
instruments  less  robust.  We  have  found  that  the  higher  the  resistance, 
the  more  often  are  the  instruments  in  error,  due  to  dropping  and  accidents, 
and  usually  these  errors  are  greater  than  the  accuracy  gained  by  using  the 
high-resistance  instrument. 

C.  H.  WILSON,*  New  York,  N.  Y.  (written  discussionf) . — On  page 
122,  the  authors  say,  that  if  the  zone-box  principle  of  connections  be- 


Indicator 


Copper  Wire 
Compensating  Couple 


FIG.  36. 


tween  primary  couple,  compensator  couple,  and  copper  leads,  which 
method  in  its  application  to  a  single  couple  is  shown  in  Fig.  34,  is 
adopted  for  a  multiple  installation,  so  as  to  save  compensating  leads, 
a  selective  switch  may  be  (and  the  inference  is  that  it  must  be)  inserted 
between  the  zone  box  and  the  different  couples,  putting  the  switch  at  an 


Wilson- Maeulen  Co. 


t  Received  Sept.  20,  1919. 


130  THERMOELECTRIC  PYROMETRY 

inconvenient  point,  or  putting  up  with  some  complicated  interlacing  of 
connections.  Wilson-Maeulen  Co.  has  employed  the  zone-box  method 
extensively  for  several  years  and  in  none  of  the  many  multiple  installa- 
tions has  either  of  those  alternatives  been  resorted  to.  Instead  there  has 
been  used  a  method  of  wiring  with  the  switch  at  the  indicator  and  involv- 
ing no  more  complications  than  the  junction-box  method  shown  in 
Fig.  32.  A  diagram  of  this  method  is  given  in  Fig.  36.  The  preference 
between  the  zone  box  and  the  junction  box  is  not  of  principle  but  is 
dependent  on  relative  location  of  furnaces,  indicator,  and  cold-junction 
point. 

J.  T.  LITTLETON,  JR.,*  Corning,  N.  Y.  (written  discussionf). — This 
discussion  will  add  little  that  has  not  been  brought  out  but  will  show  how 
the  problems  encountered  in  the  Corning  Glass  Works  were  overcome. 
The  chief  points  of  consideration  in  any  equipment  are,  first,  the  work 
the  equipment  is  called  upon  to  do;  second,  the  installation;  and,  third, 
the  man  who  will  use  it.  These  three  points  should  determine  the  type 
of  apparatus  adopted.  If  only  rough  measurements,  merely  a  little 
better  than  the  eye,  are  desired,  the  cost  of  the  instruments  should  be 
considered;  but  if  accurate  temperature  control  is  necessary,  the  cost  of 
the  equipment  deserves  only  small  consideration.  There  is  no  question 
that  the  real  problem  for  any  factory  is  the  installation,  making  sure  that 
the  elements  really  give  the  temperatures  desired;  also,  the  instruments 
must  be  so  designed  that  any  man  can  use  them. 

Platinum-rhodium  thermocouples  are  necessary  for  all  glass-melting 
operations.  As  the  change  in  calibration  of  these  elements  is  a  very 
serious  factor,  each  element  should  be  tested  for  change  at  least  twice 
a  week  when  used  for  continuous  high-temperature  service.  The  new 
couples  should  also  be  calibrated,  as  couples  differing  from  the  standard 
by  10°  C.  at  1200°  C.  are  often  met  with.  For  the  majority  of  commercial 
work  it  is  not  necessary  to  know  the  actual  temperature  of  the  substances 
treated  but  it  is  necessary  to  control  this  temperature.  Experience  will 
show  that  a  certain  temperature  reading  on  a  given  installation  gives 
the  desired  results. 

The  authors  of  the  paper  set  plus  or  minus  10°  C.  as  a  practical  limit 
of  accuracy;  this  variation  is  too  great.  Glasses  at  the  standard  melting 
temperature  vary  in  viscosity  about  20  per  cent,  for  such  a  temperature 
change.  The  relaxation  time  in  annealing  will  vary  by  a  factor  of  4 
for  such  a  range.  Besides,  greater  accuracy  of  control  can  be  obtained. 
The  curves  shown  prove  this  point.  They  are  from  a  regular  factory 
chart  for  a  24-hr,  run  on  a  large  glass  pot  furnace  and  record  that  a  plus 
or  minus  5°  variation  is  not  too  much  to  demand  or  set  as  a  standard. 
Accordingly  if  an  instrument  or  couple  should  fail,  previous  conditions 

*  Physical  Laboratory,  Corning  Glass  Works.  f  Received  Oct.  8,  1919. 


DISCUSSION 


131 


should  be  reproducible  to  within  that  degree  of  accuracy  at  least.     That 
demands  a  standard  testing  equipment.  • 

Test  standardizing  equipment  at  the  Corning  Glass  Works  consists 
of  a  primary,  a  secondary,  and  a  factory  standard  thermocouple.  The 
primary  standard  is  a  Bureau  of  Standards  couple  that  is  checked  against 
the  secondary  standard  about  twice  a  year  and  against  the  melting  point 
of  standard  metals  furnished  by  the  Bureau  of  Standards.  The  sec- 
ondary standard  is  checked  at  the  gold  point  whenever  thought  desirable. 
The  factory  standard  element  is  checked  against  the  secondary  standard 


1350 


13101 


4  9 

Time,  in  Hours 

FIG.  37. 

at  regular  intervals  by  means  of  a  Leeds  &  Northrup  precision  potenti- 
ometer with  a  Bureau  certified  standard  cell.  The  difference  between 
the  couples  is  read  directly  by  connecting  them  in  opposition.  A 
ni chrome-wound  furnace  is  used  as  a  standardizing  furnace  and  corrections 
above  the  limit  of  this  furnace  are  obtained  by  extrapolation.  Each 
factory  couple  is  mounted  in  a  double-bore  hard-clay  tube  and  twice  a 
week  the  factory  standard  is  placed  in  the  hole  beside  the  couple  under 
test  and  the  correction  obtained.  Records  are  kept  of  these  corrections 
and  the  actual  temperature  of  the  element  tube  may  be  known  at  any 
time  to  within  about  3°  C.  The  schedule  is  modified  to  meet  all 
calibration  changes.  This  merely  amounts  to  using  a  slightly  different 
temperature  unit.  It  would  be  unfair  to  many  manufacturers  to  give 
the  results  obtained  with  particular  instruments,  as  what  will  best  suit 
one  set  of  conditions  will  not  suit  another. 


132  THERMOELECTRIC  PYROMETRY 

An  instrument  that  requires  two  settings  before  taking  a  temperature 
reading  is  not  suitable  for  some  operations.  If  the  temperature  is 
changing  rapidly  the  instrument  lag  may  be  sufficient  to  cause  trouble. 
Also,  when  the  instrument  is  in  the  hands  of  an  unskilled  operator,  as  is 
nearly  always  the  case,  the  chances  of  error  are  increased.  If  many 
temperatures  are  taken,  the  time  involved  may  be  a  factor.  Line-re- 
sistance and  cold-junction  changes  should  be  given  all  the  consideration 
possible.  Water-cooled  cold  junctions  are  used  on  all  platinum-rhodium 
elements.  About  170,000  gal.  of  water  a  year  are  used  on  each  couple. 
This  costs  about  $6  a  year,  including  overhead  and  installation  deprecia- 
tion; the  installation  cost  is  about  $25  per  couple. 

Line  resistance,  due  to  faulty  connections  and  deteriorated  base- 
metal  couples,  has  at  times  caused  a  difference  as  great  as  200°  C.  By 
using  a  500-ohm  resistance  deflection-type  suspension  millivoltmeter, 
together  with  a  potentiometer  recorder,  all  such  changes  are  instantly 
picked  up  due  to  failure  of  the  two  instruments  to  check.  Serious  damage 
might  have  been  incurred  had  the  furnace  had  to  wait  until  the  line  re- 
sistance could  be  checked  before  the  change  was  discovered. 

On  base-metal  couples,  a  partial-deflection  potentiometer  is  used; 
this  will  indicate  a  high  resistance  when  the  partial-deflection  readings 
do  not  check  the  balanced  settings.  The  ordinary  operator  will  not 
notice  a  change  in  sensitivity  of  the  potentiometer. 

The  chief  point  of  advantage  of  the  dual  system  is  that  it  gives  the 
desirable  features  of  both  types  of  instruments  without  the  disadvantage 
of  a  second  setting  common  to  the  types  which  combine  the  two  in  one 
instrument.  It  may  be  said  that  trouble  has  been  experienced  in  getting 
operators  to  control  by  a  recorder. 

The  general  installation  is  the  real  factory  problem.  With  the  proper 
installation  most  of  the  high-grade  instruments  will  give  good  service. 
The  location  of  the  couple,  so  that  it  gives  a  control  temperature  similar 
to  the  substance  treated,  is  extremely  important.  With  the  proper 
central-station  control,  it  is  easily  possible  to  overemphasize  the  robust- 
ness of  the  instrument.  One  advantage  of  the  deflection  type  is  that  it 
may  be  mounted  in  a  dust-proof  box  and  need  never  be  touched.  The 
average  operator  will  not  write  on  his  record  false  readings  but  if  he  can, 
so  to  speak,  fool  the  instrument  by  throwing  two  couples  in  parallel  or 
by  any  other  means  smooth  out  his  record,  he  might  do  so.  Any  oppor- 
tunity for  him  to  get  at  the  wiring  should  be  avoided. 

Switches  are  sources  of  continuous  annoyance.  The  protection  tubes 
used  are  all  manufactured  in  the  ceramic  laboratory  of  the  Corning  Glass 
Works  and  are  satisfactory,  though  improvements  are  always  desirable. 
The  Corning  Glass  Works  has  had  thermoelectric  equipment  for  about 
15  years  and  the  present  system  is  the  result  of  much  experimentation 
and  work.  Satisfactory  results  are  obtained  but  there  are  problems  still 
ahead. 


DISCUSSION 


133 


EWART  S.  TAYLERSON,*  Pittsburgh,  Pa.  (written  discussion f). — 
The  writers  of  this  paper  are  to  be  congratulated  on  being  the  first 
to  publish  a  comprehensive  collection  of  thermocouple  wiring  diagrams; 
but  it  is  well  to  point  out  the  difference  between  the  zone-box  and  cold- 
junction-box  systems.  A  zone  box,  as  its  name  implies,  is  a  zone  of 
uniform  temperature  which,  however,  need  not  remain  constant.  In 
the  cold-junction  box  the  temperature  must  either  remain  constant  or 
allowance  must  be  made  for  its  variations.  In  thermoelectric  work, 
the  zone  box  is  used  to  eliminate  the  thermoelectric  effect  of  similar  junc- 
tions connected  in  opposition  in  the  same  circuit  by  keeping  them  at  the 
same  temperature;  their  resultant  voltage  is  thus  negligible.  This  prin- 
ciple has  been  successfully  applied  for  many  years  in  the  design  of  elec- 
trical resistances,  especially  those  of  low  values  constructed  of  constantan. 


lustrutueut 

with 

Automatic 
Cold  Junction 
Compensator 


Copper  Lead 
Extension  Leads 


FIG.  38. 


The  junctions  with  the  copper  circuit  are  brought  into  close  prox- 
imity to  secure  uniformity  of  temperature  and  avoid  thermoelectric 
errors.  It  should  be  definitely  understood,  however,  that  this  zone- 
box  principle  is  not  limited  to  only  two  opposed  junctions;  it  can  be 
applied  to  any  number  of  opposed  junctions,  as  shown  in  Figs.  32,  33  and 
34,  in  which  all  the  junction  boxes  are  in  principle  zone  boxes.  Whether 
these  opposed  junctions  are  brought  into  the  zone  box  as  extension  leads, 
as  in  Fig.  32,  or  as  auxiliary  couples,  as  shown  partly  in  Fig.  33,  depends 
entirely  on  such  factors  as  economy  of  material  and  convenience  in 
wiring.  If  these  points  are  kept  in  mind,  the  criticism  of  the  zone-box 
system  on  p.  122  is  certainly  not  justified,  as  no  complicated  interlacing 
circuit  is  ever  necessary  and  the  switch  can  be  placed  at  any  desired 
position  by  the  use  of  ordinary  copper  wire.  The  copper  circuit  to  the 
switch  and  instrument  can  be  of  any  suitable  design  as  long  as  it  is  homo- 
geneous and  finally  reenters  the  zone  box. 

The  auxiliary  couple  shown  in  Fig.  33  is  said  to  bring  the  cold  junction 
to  the  recorder,  whereas  the  diagram  shows  two  junctions,  one  at  the 
switch  and  one  at  the  recorder.  This,  however,  can  be  easily  corrected 
by  extending  both  wires  of  this  couple  to  the  recorder  and  connecting 
the  free  wire  to  the  switch  by  a  copper  lead. 

*  Research  Laboratory,  American  Sheet  &  Tin  Plate  Co.     f  Received  Oct.  17,  1919. 


134 


THERMOELECTRIC    PYROMETRY 


For  the  sake  of  completeness,  other  useful  applications  of  this  principle 
are  shown  in  the  diagrams  here  shown.  The  first  system,  Fig.  38.  is  used 
with  an  instrument  having  an  automatic  cold-junction  compensator, 
such  as  some  of  the  Leeds  &  Northrup  potentiometers  or  the  instrument 
with  bimetallic  zero  control  developed  by  C.  R.  Darling  and  recently 
revived  by  Bristol.  This  method  is  sometimes  more  convenient  than 
that  shown  in  Fig.  33,  though  theoretically  they  fulfill  the  same  purpose. 


Switch 

Copper     

Auxiliary. \. 

Couples 


Cold  J 


Be.,        M 

unction  * 


FIG.  39. 


Fig.  39  shows  a  zone-box  system  that  uses  only  auxiliary  couples,  thus 
avoiding  the  controversy  over  patents  that  was  in  progress  at  the  time 
this  method  was  developed. 

PAULD.  FOOTE,  T.  R.  HARRISON,  AND  C.  O.  FAIRCHILD. — Mr.  Tayler- 
son  arbitrarily  defines  the  "zone  box"  as  equivalent  to  the  junction  box 
already  described  by  us  whereas  actually  the  "zone  box"  is  designed  for 
use  with  a  single  couple.  This  is  illustated  by  Fig.  34  taken  from  the 
Wilson-Maeulen  catalog.  Accordingly  his  Fig.  38  is  the  same  as  our  Fig. 
33,  describing  the  use  of  a  junction  box,  except  that  he  employs  an  addi- 
tional and  unnecessary  pair  of  copper  leads  between  the  junction  box 
and  the  switch  and  recorder,  which  are  usually  located  close  together 
and  hence  are  at  the  same  temperature.  If  for  any  reason  the  switch 
and  recorder  are  at  different  temperatures,  Mr.  Taylerson's  method  of 
using  the  junction  box  will  correct  for  such  a  difference,  but  a  simpler 
method  is  to  use  the  wiring  diagram  of  Fig.  33,  except  that  a  positive  com- 
pensating lead  is  substituted  for  the  copper  lead  irom  the  switch  to 
recorder. 

In  his  Fig.  39,  a  combination  of  the  zone  box  and  junction  box  is  em- 
ployed. In  general  the  use  of  the  zone  box  in  such  an  installation  com- 
plicates matters  and  requires  additional  wiring.  However,  it  may  be  of 
advantage  under  the  following  condition.  If  some  of  the  couples  in 
Fig.  33  or  34  are  so  situated,  geometrically,  that  they  lie  between  the 
junction  box  and  switch,  a  zone  box  may  be  used  at  each  of  these  couples, 
copper  leads  from  the  switch  to  the  zone  boxes,  and  compensating  leads 
or  auxiliary  couple  from  the  zone  boxes  to  the  junction  box,  the  method  of 
connection  through  the  zone  box  being  that  illustrated  by  Fig.  34  except 
that  the  auxiliary  couple  terminates  in  the  junction  box  instead  of  the 


DISCUSSION  135 

ground.  This  system  saves  lengths  of  copper  leads  from  these  couples 
to  the  junction  box.  This  paragraph  applies  also  to  the  discussion  by 
Mr.  Wilson  on  page  129. 

W.  P.  WHITE,*  Washington,  D.  C.  (written  discussion  f). — From  the 
description,  convenient  working  of  the  Harrison-Foote  compensated 
indicator  involves  increasing  the  circuit  resistance  to  ten  or  more  times 
that  of  the  thermocouple.  This  is  no  disadvantage  if  a  relatively  high 
resistance  galvanometer  has  already  been  decided  upon.  The  instru- 
ments depending  on  the  potentiometer  principle  do  not  suffer  to  the  same 
extent,  as  Williamson  and  Roberts  have  pointed  out  in  their  paper  on 
thermocouple  installation  in  annealing  kilns.  I  have  found  that,  in 
some  cases,  where  one  type  of  reading  instrument  was  said  to  be  superior 
to  another,  the  real  difference  lay  in  the  quality  of  the  instrument  and 
not  in  the  principle  at  all.  I  must  disclaim  all  credit  for  the  deflection 
potentiometer  shown  on  p.  94.  The  split  circuit  here  is  employed  in  a 
different  way  from  that  which  I  had  proposed,  and  with  a  different  purpose. 

It  seems  possible  that  the  2°  variation  of  temperature  of  a  point  10 
ft.  underground,  determined  in  England,  may  be  less  than  it  would 
usually  be  in  the  more  variable  climate  in  most  parts  of  this  country. 
This  statement  is  merely  to  correct  a  possible  misapprehension.  If 
readings  of  the  temperature  are  taken  from  month  to  month,  the  con- 
stancy seems  likely  to  exceed  all  ordinary  requirements. 

Nichrome  wires  and  other  alloys  containing  nickel  can  frequently  be 
wound  in  fairly  close  coils  without  any  other  insulation  than  the  layer  of 
tarnish  which  they  ordinarily  possess.  It  seems  that  this  fact  might 
sometimes  be  useful  in  thermocouple  construction,  although  the  trouble 
and  expense  of  porcelain  insulation  would  usually  be  preferable  to  taking 
any  chances.  I  have  found  that  a  furnace  wound  with  No.  32  nichrome 
wire  worked  well  without  any  special  insulation,  and  the  efficiency  of  the 
oxide  layer  would  be  enormously  greater  with  the  very  large  wires  used 
for  commercial  base-metal  couples.  Apparently  it  would  not  do  to  trust 
the  oxide  layer  in  a  reducing  atmosphere. 

The  methods  and  apparatus  here  described,  although  intended  for 
pyrometry,  are  likely  to  be  applied  to  work  of  higher  precision.  It 
therefore  seems  in  order  to  call  attention  to  an  error  which  may  come 
in  such  work  regarding  the  cold  junction  where  this  is  different  from 
the  room  temperature,  as  it  may  often  be,  especially  where  ice  is  used. 
Since  copper  is  a  very  much  better  conductor  of  heat  than  most  of  the 
metals  used  for  thermocouples,  its  conductivity  may,  if  precautions  are 
not  taken,  falsify  the  cold-junction  temperature.  Even  wire  as  small  as 
No.  18  has  been  known  to  do  this  to  a  marked  degree. 


Physicist,  Geophysical  Laboratory.  f  Received  Sept.  25,  1919. 


136  THERMOELECTRIC    PYROMETRY 

T.  R.  HARRISON  (written  discussion*). — Regarding  the  objection  that 
the  Harrison-Foote  instrument  requires  the  use  of  high  resistance  in  series 
with  the  galvanometer,  thus  reducing  the  sensitivity  of  a  given  instru- 
ment, as  actually  manufactured,  this  instrument  makes  use  of  resistances 
(sometimes  called  swamping  resistances)  placed  in  series  with  the  moving 
element  for  the  accomplishment  of  desirable'  purposes,  other  than  that 
referred  to  (such  as  reducing  temperature  coefficient  and  eliminating  the 
necessity  of  making  too  frequent  readjustment  of  the  rheostat  to  com- 
pensate for  minor  changes  of  resistance) .  This  relatively  high  resistance 
has  been  made  possible  for  thermocouple  work  through  the  develop- 
ment of  galvanometers  of  relatively  high  sensitivity.  Through  this 
feature,  a  construction  of  the  compensating  instrument  is  possible  whereby 
accurate  adjustment  may  be  obtained  with  little  care.  The  principle 
may  be  applied,  however,  by  using  a  much  lower  swamping  resistance 
than  is  usually  employed;  this  involves  a  more  careful  adjustment  of  the 
rheostat  in  order  to  realize  e.m.f .  readings  of  a  given  accuracy. 

If  a  swamping  resistance  value  equal  to  the  maximum  resistance  of 
couples  to  be  used  with  the  instrument  is  adopted,  adjustment  must  be 
made  with  a  precision  equal  to  that  required  in  the  final  e.m.f.  reading. 
Further  reduction  of  the  swamping  resistance  nets  no  gain  in  precision  of 
e.m.f.  observations,  as  the  increased  sensitivity  is  offset  by  the  necessity 
of  proportionately  increased  accuracy  of  adjustment.  Thus,  any  galva- 
nometer provided  with  swamping  resistance  as  great  as  the  maximum 
allowable  resistance  of  the  couple  can  be  converted  into  a  Harrison-Foote 
compensated  instrument  without  the  addition  of  any  resistance  whatever 
to  the  circuit.  Evidently  the  possible  sensitivity  increases  as  the  maxi- 
mum allowable  couple  resistance  is  reduced. 

*  Received  Oct.  27,  1919. 


POTENTIOMETERS    FOR   THERMOELEMENT    WORK  137 


Potentiometers  for  Thermoelement  Work 

BY    WALTER  P.    WHITE,*    WASHINGTON,    D.    C. 
(Chicago  Meeting,  September,  1919) 

THE  measurement  of  the  reading  of  a  thermoelement  is  the  measure- 
ment of  an  electromotive  force  extraordinarily  small  compared  to  those 
generally  used  in  commercial  work.  Of  the  various  possible  methods 
of  measuring  such  a  quantity,  the  most  advantageous  is  always  a  delicate 
galvanometer.  Since  a  galvanometer  measures  current,  it  is  necessary 
to  adopt  some  scheme  by  which  its  current  reading  shall  indicate  electro- 
motive force. 

There  are  two  general  methods  of  doing  this.  One  is  to  use  the  deli- 
cate galvanometer  as  a  "direct  reader,"  that  is,  to  let  the  electromotive 
force  of  the  thermoelement  furnish  the  power  and  produce  the  current 
measured.  If  the  resistance  is  constant  the  reading  will,  after  a  suitable 
calibration,  give  the  electromotive  force  of  the  thermoelement  which, 
by  means  of  a  table  or  of  a  suitable  scale  in  the  galvanometer  itself,  will 


rC              """)        ^ 

,*               / 

,* 

M                    N 

A     A     A     A 

FIG.  1. — SIMPLIFIED  POTENTIOMETER.     POTENTIAL  DROP  ACROSS  THK  PART  M N  OF 

CIRCUIT  OF  BULL  CELL  BALANCES  THAT  OF  THERMOELEMENT  TC  WHEN  GALVANOMETER 
G  READS  ZERO. 

give  the  temperature.  The  other  method,  that  of  the  potentiometer, 
is  to  balance  the  electromotive  force  of  the  thermoelement  by  means  of 
another  electromotive  force,  using  the  galvanometer  to  tell  when 
the  balance  is  made.  The  usual  way  of  getting  this  other  electromotive 
force  is  to  have  a  battery,  the  "bull"  cell,  send  a  current  through  a  series 
of  resistances,  which  may  in  a  special  case  be  the  resistance  of  a  single 
wire.  The  fall  of  potential  along  this  circuit  will  then,  by  Ohm's  Law, 
be  proportional  to  the  resistance;  the  potential  difference  between  any 
two  points,  proportional  to  the  resistance  between  the  two  points.  This 
is  represented  in  Fig.  1,  where  M  and  N  are  the  two  points  in  the  circuit 
AZ  of  the  battery.  If  M  or  N,  or  both,  is  movable  so  that  the  resistance 

*  Physicist,  Geophysical  Laboratory,  Carnegie  Institution  of  Washington. 


138  POTENTIOMETERS   FOR    THERMOELEMENT   WORK 

MN  can  be  varied,  the  electromotive  force  between  M  and  N  may  be 
made  equal  to  that  of  the  thermocouple  TC.  When  this  has  been  done, 
there  will  be  no  deflection  of  the  galvanometer  in  the  subordinate  circuit 
MNGTC.  The  reading  MN  then  gives  the  reading  of  the  thermoele- 
ment. Since  there  is  no  current  flowing  in  the  subordinate  circuit,  the 
resistance  of  the  thermoelement  is  of  no  importance,  but  the  current  from 
the  bull  cell  as  well  as  the  resistance  'MN  must  be  known.  The  usual 
way  of  determining  this  current  is  to  adjust  it  until  the  potential  drop 
along  another  part  of  the  resistance,  say,  AB,  just  balances  the  elec- 
tromotive force  of  a  standard  cell.  The  potentiometer  thus  compares 
thermocouple  electromotive  force  with  that  of  the  standard  cell  by  means 
of  the  ratio  of  two  resistances,  here  AB  and  MN,  using  an  adjustable  and 
reasonably  constant  current  as  an  essential  agent  in  the  comparison. 

Direct-reading  Galvanometer. — In  comparing  these  two  methods,  the 
direct  reader  and  the  potentiometer,  it  is  evident  that  the  direct  reader 
excels  in  simplicity  of  apparatus  and  also  in  quickness,  since  no  adjust- 
ments are  necessary.  This  latter  advantage  is  not  important  in  simple 
readings,  but  in  making  a  recording  instrument  the  greater  simplicity 
of  the  direct  reader  is  especially  evident,  since  otherwise  the  recorder 
must  not  only  record  but  actualy  adjust  the  position  of  a  contact  corre- 
sponding to  M  or  N.  The  disadvantages  of  the  direct  reader,  however, 
are  serious.  Chief  among  these  is  the  requirement  of  constant  resist- 
ance, while  contacts,  unless  great  care  is  used,  are  apt  to  introduce 
uncertain  and  treacherous  resistances.  Nickel-plated  contacts  are  par- 
ticularly bad  in  this  respect;  less  oxidizable  than  brass,  they  are  more 
apt  to  be  neglected,  and  frequently  look  in  excellent  condition  when 
their  conducting  power  has  been  almost  destroyed  by  tarnish. 

Delicacy  of  Galvanometer. — This  resistance  difficulty  is  complicated 
by  one  of  the  restrictions  of  the  galvanometer.  The  electromotive  force 
corresponding  to  1°,  with  the  most  sensitive  thermocouples,  is  seldom 

much  more  than  50  microvolts,  or  ^rnnn  nnn  of  the  smallest  commercial 

^,UUU,UUU 

voltage  ordinarily  used,  hence  extreme  delicacy  is  required  in  the  galva- 
nometer. Now,  the  lower  the  resistance  of  the  circuit  the  larger  is  the 
current  for  a  given  voltage,  hence  the  less  delicate  does  the  galvanometer 
need  to  be.  It  is,  therefore,  unfortunate  that  diminishing  the  circuit 
resistance  increases  the  proportionate  effect  of  the  troublesome  accidental 
changes  of  resistance.  Whether,  in  view  of  this  detrimental  effect  of 
low-circuit  resistance,  it  is  well  to  make  the  galvanometer  "robust"  by 
diminishing  resistance,  is  a  question  that  has  been  much  discussed.  In 
general,  the  requirement  of  extreme  delicacy  in  the  galvanometer  has 
acted  as  an  obstacle  to  the  use  of  electrical  methods.  Hence  its  discus- 
sion here  seems  in  order. 

There  are  three  ways  of  overcoming  the  difficulty  that  arises  from  the 


WALTER   P.   WHITE  139 

delicacy  of  a  galvanometer.  The  first  is  by  improving  the  robustness  of 
delicate-reading  instruments,  and  the  manufacturers  have  done  much  in 
this  direction.  Portable  galvanometers  capable  of  being  read  (by  esti- 
mation of  tenths)  to  1  microvolt  can  now  be  had,  and  greater  robustness 
can  be  given  to  the  more  sensitive  reflecting  instruments  as  soon  as  de- 
mand becomes  more  effective.  In  particular,  an  enormous  improvement 
could  very  easily  be  made  by  replacing  the  often  worse  than  useless 
levels  now  furnished. 

A  second  way  is  for  the  users  to  overcome  their  extreme  objections 
to  the  sensitive  galvanometer.  Those  who  have  worked  with  these 
instruments  know  that  the  objections  are  largely  psychological.  A  few 
years  ago,  it  was  given  as  a  reason  against  using  electrical  instruments  that 
they  must  ultimately  be  put  in  the  hands  of  the  dollar-a-day  man,  who 
could  not  be  expected  to  handle  them.  Evidence  is  continually  accumu- 
lating that  if  the  20-dollar-a-day  man  knows  as  much  about  the  in- 
stallation as  he  ought  to,  there  is  very  little  difficulty  in  getting  the 
lower  paid  employee  to  use  them  as  well  as  he  needs  to.  The  amount  of 
care  and  skill  required  to  use  a  delicate  galvanometer  does  not  compare 
with  that  put  forth  by  a  good  machinist  in  half  the  things  he  does.  Yet 
even  the  machinist  is  often  offended  by  the  galvanometer.  He  is  used 
to  taking  pains  in  certain  directions  and  the  different  precautions  needed 
by  the  galvanometer  may  appear  unreasonable.  With  better  understand- 
ing and  familiarity,  this  subjective  difficulty  may  be  expected  gradually 
to  disappear  and  we  may  have  a  number  of  important  measurements 
made  electrically  that  at  present  are  scarcely  known  outside  of  a  few 
laboratories.  Of  course  these  will  seldom  be  with  pyrometers,  on  which 
such  delicate  measurements  would  usually  be  wasted,  and  they  would 
not  necessarily  be  in  the  unfavorable  situations  where  many  pyrometers 
must  now  be  used.  In  particular,  a  very  unsteady  support  has  for  deli- 
cate galvanometers  a  detrimental  effect  the  overcoming  of  which  may 
be  so  expensive  as  to  prohibit  the  use  of  the  sensitive  reflecting  galva- 
nometer in  such  locations. 

A  third  way  of  overcoming  the  difficulties  arising  from  the  delicacy 
of  a  galvanometer  is  to  increase  the  current  by  decreasing  the  resistance , 
a  method  whose  limitations  have  already  been  discussed. 

Lack  of  Relative  Precision  in  Direct-reading  Galvanometer. — A  second 
objection  to  the  direct  reader  is  its  lack  of  relative  precision.  Opinions 
may  differ  as  to  whether  such  instruments  can  generally  be  relied  upon 
to  1  part  in  500  of  their  deflection,  or  to  1  part  in  2000,  but  the  limit  is 
certainly  reached  at  a  point  far  short  of  that  for  even  a  simple  potenti- 
ometer. Of  course,  for  reading  temperatures  below  1000°,  in  cases  where 
a  precision  of  10°  is  sufficient,  anything  reading  to  1  per  cent,  is  adequate, 
but  it  is  the  cases  of  greater  precision  that  most  need  discussion  so  far  as 
auxiliary  apparatus  is  concerned. 


140 


POTENTIOMETERS    FOR   THERMOELEMENT   WORK 


Simple  Potentiometer. — The  difficulties  of  the  direct  reader,  except 
those  due  to  delicacy  of  the  galvanometer,  are  entirely  absent  in  the  po- 
tentiometer. The  objections  to  the  potentiometer  are  its  greater  cost 
and  complexity,  the  protection  needed  by  the  standard  cell,  and  the  atten- 
tion that  must  be  given  to  the  constant  bull  cell.  The  greater  time  re- 
quired to  make  a  setting  is  ordinarily  almost  inappreciable,  because 
once  the  setting  is  made  small  divergences  from  it  can  be  read  directly 
on  the  galvanometer.  Thus  the  quickness  of  the  direct  reader  and  the 
precision  of  the  potentiometer  are  combined.  To  thus  use  the  potenti- 
ometer as  a  "deflection  potentiometer,"  of  course,  demands  that  the 
galvanometer  sensitiveness  shall  have  an  exact  value,  though  this 
requirement  is  much  less  severe  than  where  the  galvanometer  handles  the 
whole  quantity  to  be  measured.  Those  who  do  not  care  to  keep  their  gal- 
vanometer in  this  condition  may  have  the  advantage  of  being  entirely 
independent  of  its  calibration  at  the  cost  of  a  little  more  time  in  making 
the  settings. 

Devices  to  Avoid  Use  of  Standard  Cell. — Considerable  effort  has  been 
expended  to  avoid  the  use  of  the^tandard  cell.  Some  time  ago  a  poten- 


FIG.  2. — PTROVOLTER.     FIRST  POSITION,  POTENTIAL  DROP  M N  is  MADE  EQUAL  TO 

ELECTROMOTIVE  FORCE  OF  THERMOELEMENT;  SECOND  POSITION,  CURRENT  IS  MEASURED 
BY  SAME  GALVANOMETER. 

tiometer  was  designed  at  the  Reichsanstalt  in  which  the  current  was  kept 
constant  by  the  reading  of  an  ammeter.  This  instrument  had  two  galva- 
nometers, namely,  the  ammeter  and  the  more  delicate  balance-galva- 
nometer, and  yet  the  final  precision  was  no  greater  than  that  of  the 
direct-reading  ammeter.  A  simpler  arrangement  for  the  same  general 
purpose  is  illustrated  in  Fig.  1  of  a  paper  by  E.  D.  Williamson  and  H.  S. 
Roberts.1  Here  the  current  is  variable  and  the  resistance  corre- 
sponding to  MM  of  the  present  paper  (see  Fig.  1)  is  constant,  so  that 
the  reading  is  made  on  the  ammeter  scale  instead  of  on  a  number  of 
resistance  dials. 

The  Pyrovolter. — A  further  improvement  is   made  possible  by  the 
principle  of  the  pyrovolter,2  Fig.  2,  in  which  the  same  galvanometer  is 

1  This  volume,  p.  468. 

z  Manufactured  by  the  Pyrolectric  Instrument  Co.,  of  Trenton,  N.  J. 


WALTER    P.    WHITE 


141 


ingeniously  employed  to  fulfil  both  functions;  first,  to  establish  a  balance, 
and  second,  to  measure  the  current.  A  resistance  R2  replaces  the  galva- 
nometer when  that  is  not  in  the  main  circuit,  so  that  the  transfer  of  the 
galvanometer  does  not  alter  the  current.  In  this  case,  as  in  Williamson 
and  Roberts'  Fig.  1,  the  current  is  not  kept  constant  as  in  the  orthodox 
potentiometer,  but  is  adjusted  to  give  a  suitable  drop  over  a  constant, 
resistance  MN.  There  are  thus  no  resistances  to  be  measured.  A  vari- 
able rheostat  R i,  which  is  not  read,  adjusts  the  current  and  the  second, 
or  final,  reading  depends,  as  in  the  direct  reader,  on  the  calibration  of  the 
galvanometer.  The  advantage  is  in  eliminating  resistance  difficulties 
in  the  thermoelement.  The  operation  of  the  instrument  involves,  first, 
an  adjustment  to  a  zero  reading  of  the  galvanometer,  as  with  a  regular 


FIG.  3. — BROWN    IMPROVED   HEATMETER.     DEPRESSION  OP  KEY  DOES  NOT  CHANGE 
DEFLECTION  WHEN  CIRCUIT  RESISTANCE  IS  ADJUSTED  TO  PROPER  VALUE. 


potentiometer,  but  following  that,  instead  of  reading  the  resistance  setting 
the  ammeter  deflection  is  produced  and  that  is  read  just  as  with  a  direct 
reader. 

It  is  easy  to  show  that  with  this  instrument,  and  any  given  galva- 
nometer sensitiveness,  the  zero  setting  is  less  sensitive  than  the  final 
reading,  and  this  difference  becomes  very  large  if  the  galvanometer 
resistance  or  the  thermocouple  resistance  is,  accidentally  or  otherwise, 
much  higher  than  the  resistance  of  the  fixed  coil.  In  some  instruments 
this  difficulty  is  overcome  by  .varying  the  current  sensitiveness  of  the  gal- 
vanometer. The  difficulty  should  also  be  small  if  the  thermocouple  re- 
sistance is  not  greater  than  that  for  which  the  instrument  was  designed. 

Harrison  and  Foote  Instrument. — The  same  advantage,  namely,  avoid- 
ance of  error  from  uncertain  thermocouple  resistance,  is  obtained  in  a  new 
instrument  designed  by  Harrison  and  Foote  (the  Brown  Improved  Heat- 
meter,  Fig.  3)  in  which  the  potentiometer  principle  is  not  used  at  all. 
The  circuit  resistance  is  adjusted  to  compensate  for  any  changes  or  differ- 
ences in  the  thermoelement,  and  in  this  the  thermocouple  itself  is  used 
as  a  source  of  temporarily  constant  current.  The  instrument  thus 
avoids  one  of  the  disadvantages  of  the  potentiometer,  namely,  the  battery, 


142  POTENTIOMETERS   FOR  THERMOELEMENT   WORK 

and  therein  lies  perhaps  its  main  advantage.  The  operation  is  shown  in 
Fig.  3.  By  pressing  the  key  K  the  circuit  is  shortened  by  cutting  out  the 
resistance  Rz,  which  tends  to  increase  the  galvanometer  deflection.  At 
the  same  time  the  galvanometer  is  shunted  by  the  resistance  R*,  which 
tends  to  decrease  the  deflection.  These  two  effects  will  offset  each  other, 
and  there  will  be  no  change,  when  the  resistance  of  the  rest  of  the  circuit 
has  a  particular  value.  Hence  by  adjusting  the  resistance  R\  until  the 
depression  of  the  key  K  makes  no  change  in  the  deflection,  the  instru- 
ment becomes  a  perfect  direct  reader,  since  its  circuit  resistance  is  correct. 
The  adjustment  is  not  nearly  as  difficult  as  it  might  at  first  sound,  for 
the  galvanometer  when  the  key  is  down  is  much  more  sensitive  to  resist- 
ance changes.  Hence  one  or  at  most  two  trials  are  nearly  always  suffi- 
cient to  get  the  adjustment.  On  account  of  the  greater  sensitiveness 
shown  by  the  galvanometer  when  the  key  is  down,  the  error  of  reading 
at  that  time  produces  a  negligible  effect.  Hence  the  accidental  error  of 
only  one  reading  comes  in.  Once  set,  the  instrument  can  be  used  as  a 
direct  reader  for  some  time.  The  pyrovolter  can  also  be  used  to  give  this 
advantage  by  means  of  a  special  addition.  The  instrument  is  to  be  more 
fully  described. 

All  these  instruments  (Reichsanstalt  instrument,  pyrovolter,  Harrison 
and  Foote  instrument)  avoid  merely  the  trouble  from  uncertain  resist- 
ance. They  are  dependent  on  the  calibration  of  an  ammeter.  For  higher 
precision  than  that  will  give  there  has  been  with  the  thermoelement  as 
yet  no  auxiliary  which  did  not  involve  the  use  of  the  standard  cell.  The 
attempts  to  avoid  its  use  are  to  be  laid  mainly  to  the  feeling  that  it  is  an 
unnecessary  complication.  But  although  an  additional  complication, 
it  is  not  one  to  be  seriously  shunned  if  precision  is  desired. 

Portable  Potentiometer. — For  the  next  grade  of  precision  after  the  am- 
meter readers,  a  simple  and  portable  potentiometer  is  made  by  the  Leeds 
&  Northrup  Co.  Here  a  coiled  slide  wire  furnishes  the  MN  of  Fig.  1. 
Settings  upon  this  can  be  made  to  100  microvolts  and  read  to  about  10 
microvolts,  or  less  than  a  quarter  of  a  degree  with  ordinary  base-metal 
thermocouples.  A  robust,  portable  galvanometer  forms  part  of  the 
instrument  and  gives  about  the  same  precision  as  the  slide  wire. 

Precision  Potentiometer. — The  next  step  in  precision  is  far  more  con- 
veniently secured  by  abandoning  the  slide  wire,  since  the  contact  on  this 
wire  is  a  source  of  "parasitic"  electromotive  forces  that  are  annoying 
even  when  they  do  not  seriously  impair  the  precision  of  the  observations. 
It  is  true  that  slide-wire  potentiometers  reading  to  1  microvolt  have  been 
used,  and  the  question  as  to  just  what  precision  can  be  advantageously 
obtained  with  them  is  evidently  to  some  extent  a  matter  of  opinion. 
But  they  certainly  cannot  go  much  farther  than  the  simple  poten- 
tiometer just  described,  while  by  passing  over  to  another  type  of 
potentiometer  we  not  only  get  microvolt  precision  with  the  greatest  ease 


WALTER   P.   WHITE  143 

and  certainty,  but  can  with  almost  as  great  ease,  though  at  the  cost  of 
certain  imperative  experimental  features,  read  to  0.1  microvolt  with  only 
a  little  more  care  and  precaution  than  is  needed  for  far  less  precision 
with  any  of  the  other  instruments  just  described.  Reading  to  0.1  micro- 
volt means  that  two  thermocouples  in  series  will  give  the  sensitiveness 
and  more  than  the  precision  of  a  Beckmann  thermometer,  while  a  multiple 
thermoelement  only  3  mm.  in  diameter  will  reach  a  precision  of  ten  times 
that,  and  equal  to  that  of  a  standard  calorimetric  platinum  resistance 
thermometer.  This  precision  of  0.1  microvolt  demands  the  use  of  copper 
switches  at  certain  points  in  the  circuit  and  also  requires  a  galvanometer 
of  considerably  higher  sensitiveness  than  those  known  as  portable. 

A  complete  description  of  such  apparatus  does  not  seem  necessary  in  a 
discussion  more  especially  devoted  to  pyrometry.  Such  instruments, 
however,  are  used  for  the  calibration  and  Comparison  of  standards,  and 
open  up  a  field  for  the  thermoelement  more  extensive  and  varied  and  not 
unlikely  to  become  more  widely  cultivated  than  the  whole  subject  of 
thermoelectric  pyrometry.  It  seems  worth  while,  therefore,  to  state  the 
principle  of  the  modern  thermoelectric  potentiometer  and  to  explain 
what  are  its  essential  requirements.  In  general,  the  advantage  of  resist- 
ance electric  methods,  such  as  that  of  the  Wheatstone  bridge,  consists  in 
the  fact  that  variation  in  the  battery  or  other  source  of  electromotive 
force  is  largely  compensated.  The  advantage  of  potential  methods,  on 
the  other  hand,  lies  in  their  independence  of  contact  resistances. 

In  the  high-precision  potentiometer,  it  is  necessary  to  minimize 
difficulties  coming  from  the  variation  in  the  bull-cell  current,  while  at 
the  same  time  preserving  the  advantage  of  negligible  contact  resistance. 
The  first  has  been  satisfactorily  accomplished  by  the  use  of  lead  and 
nickel  storage  cells  (the  latter  are  nearly  if  not  quite  as  satisfactory  as 
the  lead  cell  and  are  much  less  likely  to  deteriorate) ;  but  the  contact 
resistance  introduces  a  difficulty.  In  Fig.  1  there  are  two  movable 
contacts  M  and  N,  which  come  into  the  galvanometer  circuit  where  no 
appreciable  current  flows  and  where  therefore  resistance  is  not  important. 
If  M  and  N  are  each  nine-step  or  ten-step  dials,  there  is  a  variation  from  1 
to  100  in  the  electromotive  force  which  can  be  read  by  the  dials — but  that 
is  the  maximum  variation  that  can  be  obtained  in  this  simple  way.  If 
the  resistance  M  N  is  varied  by  putting  a  variable  resistance  in  the  battery 
circuit  between  M  and  N,  the  resistance  of  the  necessary  contacts  now 
affects  the  electromotive  force  in  that  part  of  the  circuit.  If  MN  is  small, 
as  it  is  in  reading  the  thermoelement,  it  is  practically  impossible  to  get  a 
satisfactory  potentiometer  in  this  way.  It  is  to  avoid  this  difficulty  and 
get  a  large  range  of  variation  with  only  two  contacts  that  the  slide  wire 
has  been  used. 

Where  the  slide  wire  has  been  considered  inadmissible,  a  number  of 
different  schemes  have  been  used  to  get  more  than  two  dials  without 


144  POTENTIOMETERS    FOR   THERMOELEMENT   WORK 

contact-resistance  error.  The  older  Wolff  potentiometer  simply  in- 
creased the  resistance  of-  the  whole  instrument  to  a  very  high  value; 
this  interfered  with  the  sensitiveness  of  the  galvanometer  and  even  then 
did  not  give  sufficient  precision  for  the  best  thermoelectric  work.  Two 
schemes,  however,  have  been  successful.  One,  that  of  the  Diesselhorst- 
Wolff  potentiometer,  a  combination  of  devices  suggested  by  several 
writers,  in  which  M  and  N  are  on  two  sides  of  a  divided  circuit,  so  that 
even  when  they  are  at  the  same  potential  considerable. resistance  lies  in 
the  path  from  one  to  the  other.  Shunts  applied  to  this  resistance  can 
change  the  difference  of  potential  between  M  and  N  without  introducing 
contact-resistance  error  into  the  main  line.  It  is  still  necessary,  however, 
that  the  contact  resistance  be  kept  small;  hence  the  instrument,  although 
admirably  successful,  demands  almost  daily  attention  for  its  contacts. 

This  difficulty  is  avoided  in  the  White  potentiometer,  where  two  sim- 
ple potentiometers,  like  Fig.  1,  are  in  series  in  the  same  galvanometer 
circuit.  This  has  two  separate  bull  cells,  but  one  of  these  requires  very 
infrequent  adjustment,  while  the  contact  requirements  are  such  that  the 
contacts  have,  under  good  conditions,  literally  been  left  without  attention 
for  more  than  a  year  without  any  detriment.  Where  corrosive  gases  are 
present  this  treatment  might  not  be  possible,  but  this  type  of  potenti- 
ometer evidently  has  a  very  great  special  advantage  under  such  conditions. 
The  principle  of  the  instrument  is  evidently  simple  and  needs  no  further 
explanation.  But  two  features  of  some  importance  seem  to  deserve  a 
more  detailed  description. 

Partial-deflection  Method  of  Reading. — The  development  of  the 
thermoelement  potentiometer  has  been  somewhat  modified  by  a  more  or 
less  accidental  circumstance  arising  out  of  the  fact  that  the  Geophysical 
Laboratory  has  taken  a  part  in  that  development.  When  that  laboratory 
was  first  organized,  the  Director,  Dr.  A.  L.  Day,  in  reading  varying 
temperatures  for  "melting-point  curves"  and  the  like,  did  not  follow  the 
plan,  frequently  and  perhaps  generally  used,  of  using  the  galvanometer  as 
a  null  instrument  and  observing  the  irregularly  varying  times  at  which  its 
deflection  passed  through  zero  as  integral  changes  were  made  in  the 
resistance  setting.  Instead,  the  readings  were  made  at  even  minutes, 
which  necessitated  taking  the  last  figures  of  the  result  from  the  galva- 
nometer deflection.  This  practise  soon  led  to  an  attempt  to  keep  the 
galvanometer  sensitiveness  constant  and  hence  to  the  development  of  the 
"partial  deflection"  methods,  that  is,  partial  direct-reading  methods, 
where  the  potentiometer  matches  most  of  the  unknown  electromotive 
force,  but  a  small  residual  effect  is  directly  read.  When  special  thermo- 
element potentiometer  designs  began  to  be  made,  this  feature  of  constant 
galvanometer  sensitiveness,  that  is,  constant  galvanometer-circuit 
resistance,  was  incorporated  in  them.  The  scheme  was  accepted  by 
Diesselhorst  and  embodied  in  the  Diesselhorst- Wolff  potentiometer  and 


WALTER   P.    WHITE  145 

is  a  feature  of  the  White  potentiometer,  where  it  is  secured  by  the  simple 
device  of  putting  supplementary  coils  of  low  precision  in  series  with  the 
contact  points  of  some  of  the  switch  dials,  so  that  as  the  resistance  directly 
between  M  and  N  is  altered,  the  resistance  in  the  galvanometer  circuit 
nevertheless  remains  unchanged.  This  constancy  of  resistance  is  merely 
a  convenience  for  rapid  reading.  It  does  not  need  to  be  as  accurate  as  it 
would  be  if  the  galvanometer  were  reading  the  whole  quantity,  and 
practically  no  possibility  of  error  is  ever  connected  with  it.  This  feature, 
although  it  characterizes  the  thermoelement  potentiometer,  could  also 
be  applied  to  the  Wheatstone  bridge,  but  has  not  been,  at  least  not  gener- 
ally. The  result  is  that  frequently  resistance  thermometer  measurements 
have  been  regularly  made  by  two  observers  and  with  uneven  'time  in- 
tervals, while  observations  of  equal  precision  could  be  made  with  as 
great  ease,  at  any  desired  times,  by  a  single  observer  with  a  thermoele- 
ment and  a  suitable  potentiometer. 

Double  Potentiometer. — Another  feature  of  this  potentiometer  enables 
the  single  observer  to  read  several  temperatures  in  very  rapid  succession. 
This  is  done  by  having  the  dials  in  duplicate  but  attached  to  the  same 
coils,  and  with  an  arrangement  for  throwing  one  or  the  other  set  of  dials 
into  operation  at  will  along  with  a  different  thermoelement.  Hence 
where  the  temperatures,  for  instance,  of  two  different  bodies  are  being 
followed,  the  observer  can  go  back  and  forth  from  one  to  the  other  without 
having  to  reset  all  his  switches  each  time.  The  gain  is  merely  one  in  time, 
but  that  is  very  often  important.  Such  an  arrangement  could  be  applied 
to  the  resistance  thermometer  or  to  other  apparatus,  but  is  especially 
advantageous  in  the  potentiometer  for  several  reasons.  (1)  The  inde- 
pendence of  contact  resistance,  shown  by  this  potentiometer  system  gen- 
erally, renders  exchanges  of  instrument  easy  to  make  with  rapidity  and 
without  error.  (2)  Single  thermocouples  can  be  very  easily  constructed 
in  practically  exact  duplicates  of  each  other,  so  that  in  any  case  where 
numerous  temperatures  are  to  be  read  the  thermoelement  has  a  decided 
advantage  quite  aside  from  its  sensitiveness  and  small  dimensions.  (3) 
The  thermoelement  furnishes  its  own  power  and  can  be  read  directly  by 
the  galvanometer.  In  almost  every  system  involving  a  number  of 
thermoelements  it  is  easy  to  arrange  a  number  of  them  in  such  a  way  that 
the  reading  of  each  shall  be  a  small  quantity.  Hence  they  can  be  read 
directly  from  the  galvanometer  while  others  in  the  same  set-up  are 
combined  with  the  potentiometer.  The  exchanging  arrangement  in  the 
White  potentiometer  makes  provision  for  this  combination  so  that  three 
different  thermoelements,  at  least,  and  usually  a  much  larger  number,  can 
be  used  without  changing  switches. 

The  instrumental  arrangements  necessary  as  safeguards  with  the 
potentiometer  have  been  fully  described.  The  present  paper  is  not  a 
description  of  potentiometer  manipulation,  but  merely  a  discussion  of  the 


146  POTENTIOMETERS  FOR  THERMOELEMENT   WORK 

advantages  of  different  systems;  hence  it  will  be  sufficient  to  state  what 
these  safeguards  are.  One  is  the  equipotential  shield,  which  guards  the 
system  against  leakage  from  the  relatively  high  voltage  of  commercial 
circuits.  The  second  is  a  switch  constructed  of  copper  so  as  to  be  itself 
free  from  thermoelectric  forces  (an  ordinary  knife  switch  will  usually 
answer  perfectly),  used  to  eliminate  parasitic  thermoelectromotive 
forces  within  the  galvanometer  and  other  parts  of  the  potentiometer 
system.  These  two  safeguards3  are  the  same  in  principle  as  those  neces- 
sary with  other  systems  of  equal  sensitiveness. 

SUMMARY 

Thermocouple  pyrometers  are  read  in  three  ways.  (1)  By  direct 
readers  where  the  current,  and  therefore  the  deflection,  is  proportional  to 
the  electromotive  force  of  the  couple.  (2)  By  potentiometers,  where  the 
galvanometer  merely  helps  to  balance  the  electromotive  force  of  the 
couple  against  that  of  a  standard  cell  by  means  of  known  resistances 
and  a  constant  battery  current.  (3)  By  intermediate  instruments,  such 
as  the  pyrovolter,  employing  the  potentiometer  principle  with  a  constant 
battery,  but  avoiding  the  standard  cell,  and  measuring  current  with  a 
calibrated  galvanometer.  Similar  in  result,  but  different  in  principle,  is 
the  new  Harrison-Foote  instrument,  where  the  circuit  resistance  can  be 
very  quickly  adjusted  to  the  correct  value.  All  these  special  instruments 
avoid  the  main  difficulty  of  a  direct  reader,  namely,  the  error  from  un- 
certain or  variable  resistance.  It  is  necessary  to  use  the  regular  poten- 
tiometer in  order  to  avoid  also  the  uncertainty  (perhaps  1  per  mille)  of 
the  calibration  of  the  direct  reader.  With  a  slide  wire,  a  simple  and  porta- 
ble potentiometer  is  made  good  to  about  10  microvolts,  or  }£°  with  most 
thermocouples.  The  slide  wire  also  permits  readings  to  1  microvolt, 
though  not  altogether  satisfactorily.  Two  special  designs  of  potenti- 
ometer, the  Diesselhorst- Wolff  and  the  White,  enable  readings  to  be  made 
to  0.1  microvolt  or  better,  and  the  White  potentiometer  is  very  little 
affected  by  corrosive  gases.  Both  of  these  are  deflection  potentiometers, 
enabling  part  of  the  readings  to  be  taken  direct  from  the  galvanometer 
with  a  gain  in  speed  and  without  sensible  error.  If  the  potentiometer  is 
arranged  as  a  double  potentiometer,  speed  can  be  gained  in  reading 
different  instruments  simultaneously.  The  precision  of  these  potenti- 
ometers exceeds  that  needed  in  ordinary  pyrometry,  but  is  useful  in  funda- 
mental standardization  work,  in  calorimetry,  and  in  numerous  other 
applications  of  the  thermoelement. 

s  Walter  P.  White:  Thermoelement  Installation,  especially  for  Calorimetry. 
Leakage  Prevention  by  Shielding,  especially  in  Potentiometer  Systems.  Jril.  Am. 
Chem.  Soc.  (1914)  36,  1859,  2011. 


DISCUSSION  147 

DISCUSSION 

T.  R.  HARRISON,  Washington,  D.  C.  (written  discussion*). — Ad- 
vantage will  be  taken  of  this  occasion  to  mention  briefly  a  form  of  double 
potentiometer  which  has  been  used  at  the  Bureau  of  Standards  for 
about  2^  yr.  in  calibrating  thermocouples.  The  calibrations  referred 
to  are  those  made  by  comparison  to  standard  couples. 

In  order  to  insure  equality  of  temperature  between  the  couples,  the 
junctions  are  fused  together;  consideration  of  fundamental  principles 
will  show  that  this  introduces  no  error.  Two  separate  potentiometers 
are  used,  one  connected  to  each  thermocouple,  and  each  potentiometer 
is  provided  with  a  reflecting  galvanometer.  The  two  spots  of  light  are 
reflected  onto  a  single  scale,  the  galvanometers  being  set  in  such  a 
position  that  the  spots  coincide  on  the  scale  at  a  point  marked  zero 
when  the  circuits,  are  open  or  when  the  potentiometers  are  balanced. 
By  setting  one  potentiometer  to  a  desired  value  and  adjusting  the  other 
so  that  both  spots  pass  across  the  scale  together  as  the  temperature  rises 
or  falls,  simultaneous  readings  are  obtained. 

By  making  observations  first  with  a  rising  temperature  and  then 
with  a  falling  temperature,  the  rates  of  rise  and  fall  being  approximately 
equal,  and  taking  the  means  of  the  results  found,  several  minor  errors 
such  as  those  due  to  differences  in  the  time  lags  of  the  two  systems,  etc., 
are  eliminated  or  greatly  reduced.  The  differences  between  the  values 
observed  with  rising  and  falling  temperatures  are  usually  less  than  1° 
with  rare-metal  couples.  By  this  method  a  calibration  may  be  made 
rapidly  and  with  accuracy,  since  the  junctions  are  fused  together  and 
means  are  provided  for  taking  simultaneous  readings  on  the  couples 
while  the  temperature  is  changing. 

On  account  of  the  fact  that  the  junctions  of  the  couples  are  in  elec- 
trical contact  and  the  readings  must  be  made  simultaneously,  it  is 
necessary  to  use  potentiometers  having  entirely  independent  circuits. 

LEASON  H.  ADAMS,!  Washington,  D.  C.  (written  discussion!). — For 
precision  work  with  thermocouples  in  the  laboratory,  all  are  agreed 
on  the  necessity  of  using  a  potentiometer  in  connection  with  a  reflect- 
ing galvanometer;  but  in  the  choice  of  an  instrument  for  the  factory, 
opinions  seem  to  differ.  Doctor  White  has  discussed  the  advantages  and 
disadvantages  of  three  classes  of  portable  instruments :  the  direct- 
reading  millivoltmeter,  the  portable  potentiometer,  and  the  class  to 
which  belong  the  pyrovolter  and  the  heat-meter.  My  experience  with 
pyrometer  installations  in  the  factory  has  led  me  to  prefer  the  portable 


*  Received  Sept.  25,  1919. 

t  Physical  Chemist,  Geophysical  Laboratory. 

t  Received  Sept.  25,  1919. 


148  POTENTIOMETERS   FOR   THERMOELEMENT   WORK 

potentiometer  and  to  consider  that  in  accuracy,  convenience,  and  re- 
liability it  is  far  superior  to  the  other  instruments.  The  fact  that  this 
potentiometer  requires  a  battery  and  a  standard  cell  has  not  proved  to  be 
a  drawback,  since  they  are  built  into  the  case  of  the  instrument  and 
require  very  little  attention.  Moreover,  no  difficulty  has  been  experi- 
enced in  teaching  unskilled  laborers  to  make  accurate  readings  with  the 
the  portable  potentiometer,  a  few  minutes'  instruction  being  sufficient 
in  all  cases.  The  .portable  potentiometer  has  also  proved  to  be  very 
convenient  for  use  in  the  laboratory  when  the  extreme  accuracy  of  the 
precision  potentiometer  is  not  required. 

W.  E.  FORSYTHE,  Nela  Park,  Cleveland,  O. — In  measuring  the  tem- 
perature of  a  lamp,  we  control  -the  current  with  the  potentiometer;  we 
want  to  measure  that  as  accurately  as  possible  and  still  have  some  speed. 
We  have  adopted  the  principle  of  the  deflection  potentiometer.  We 
connected  a  millivoltmeter  between  the  binding  post  of  a  regular  Leeds 
&  Northrup  potentiometer,  which  is  there  to  test  out  the  slide  wire,  and 
a  movable  plug  in  the  Br  dial  of  the  potentiometer.  As  this  deflection 
instrument  is  connected,  only  a  very  small  part  of  the  current  is  measured 
by  it.  Thus,  by  using  the  ordinary  deflection  millivoltmeter,  we  arranged 
the  connections  so  that  currents  can  be  read  to  better  than  1  part  in 
4000  by  simply  reading  a  deflecting  instrument.  We  have  never 
attempted  to  make  this  deflecting  instrument  direct  reading,  but  have 
always  had  the  readings  recorded  as  they  were  made.  At  the  end  of  the 
series,  the  deflections  were  averaged  and  the  corresponding  current 
determined  by  the  potentiometer. 


SELF-CHECKING    GALVANOMETER    PYROMETER  149 


Self-checking  Galvanometer  Pyrometer 

BY    H.    F.    PORTER,*    E.    E.,    TRENTON,    N.    J. 
(Chicago  Meeting,  September,  1919) 

MUCH  has  been  written  relative  to  the  errors  involved  in  the  use  of  a 
galvanometer  for  measuring  thermocouple  electromotive  forces.  In 
general,  it  may  be  said  that  accuracy  with  a  galvanometer  is  secured  only 
at  the  sacrifice  of  durability,  unless  manual  adjustment  is  made  in  the 
operation  of  the  instrument  to  overcome  the  errors  of  resistance  and 
resistance  changes.  The  pyrovolter  and  the  potentiometer  both  require 
manual  adjustment  for  every  reading,  readings  being  taken  on  a  "null" 
method.  .1 

To  the  end  that  some  of  these  balances  may  be  eliminated  and  only 
an  occasional  balance  be  made  as  a  check  reading,  the  "continuously 
deflecting  pyrovolter"  was  developed.  Essentially,  the  operation  of  the 
continuously  deflecting  pyrovolter  is  to  determine  the  resistance  of  the 
thermocouple  circuit,  though  its  value  is  not  noted  or  indicated,  then 
throwing  in  series  with  the  thermocouple  sufficient  resistance  to  bring 
the  sum  up  to  some  standard  value  for  which  the  indications  of  the  galva- 
nometer will  be  correct.  This  result  may,  however,  be  accomplished  in 
a  somewhat  simpler  manner,  for  which  general  method  patents  are  now 
in  the  course  of  application. 

If  we  devise  a  galvanometer  circuit  so  that  by  means  of  a  simple 
adjustment  the  resistance  of  the  entire  circuit  will  be  rendered  equal  to 
some  constant  predetermined  value,  we  may  rely  on  the  galvanometer  to 
give  correct  e.m.f.  indications  (assuming  that  the  errors  of  temperature 
resistance  coefficient  and  cold  junction  have  been  properly  allowed  for), 
and  will  indicate  temperature  continuously  where  the  e.m.f.  measured 
is  developed  by  a  thermocouple. 

In  Figs.  1  and  2,  there  is  in  series  with  the  galvanometer  of  resistance 
g  the  thermocouple  of  resistance  X  and  the  rheostat  of  total  resistance  S. 
To  operate  the  instrument,  that  is,  to  make  a  check  to  compensate  for 
couple  resistance,  a  button  is  depressed  which  connects  points  M  and  N,  as 
shown  in  Fig.  1,  normally  separated,  and  disconnects  points  P  and  Q, 
normally  engaged  in  contact.  The  slider  on  the  rheostat  is  adjusted 
until  the  resistance  a  is  zero  and  the  deflection  D  of  the  meter  is  at  its 
maximum.  This  deflection  is  noted,  and,  still  depressing  the  button, 
the. slider  is  adjusted  until  a  deflection  equal  to  D/2  is  obtained.  The 
button  is  now  released,  closing  the  connection  between  points  P  and  Q 

*  Secretary,  Pyrolectric  Instrument  Co. 


150 


SELF-CHECKING    GALVANOMETER   PYROMETER 


and  breaking  the  contact  of  M  and  N,  as  shown  in  Fig.  2.  The  resistance 
R3  of  the  entire  circuit  after  this  adjustment  is  complete  equals  S,  a 
constant  value  equal  to  the  total  resistance  of  the  rheostat  shown.  This 
is  easily  proved  as  follows: 

Call  the  resistance  of  the  entire  circuit  in  Fig.  1,  Ri}  that  is,  when  a  is 
zero 

Ri  =  g  +  X  (l) 

For  deflection  D/2,  the  circuit  resistance  is  R2 

R2  =  g  +  X  +  a  (2) 

Since  adding  resistance  a  to  the  circuit  halved  the  current  flowing, 
and  hence  the  deflection,  the  circuit  resistance  must  have  been  doubled 
when  the  adjustment  was  complete  or  2Ri  =  R2.  By  substitution  from 


FIG.  1. 


FIG.  2. 


formulas  (1)  and  (2),  2(g  +  X)  =  g  +  X  +  a;  whence  g  +  X  =  a. 
When  the  button  is  released  and  connections  are  made  between  P  and  Q, 
simultaneously  breaking  the  junction  between  points  M  and  N,  the  circuit 
resistance  Rs  becomes  Rs  =  b  +  g  +  X]  and  since  g  +  X  =  a,  R3  = 
6+  a. 

By  definition  of  a  and  b,  they  are  the  component  parts  of  the  rheostat 
and  a  +  b  =  S;so  that  R3  =  a  +  b  =  S  which  is  the  result  it  was  desired 
to  prove. 

The  following  characteristics  of  this  circuit  may  be  of  interest.  Maxi- 
mum allowable  resistance  in  thermocouple  circuit  external  to  instrument 
is  M  —  (s  —  g).  The  e.m.f.  range  of  the  instrument  is  given  by 
E  =  S  I,  where  I  is  the  current  required  to  deflect  the  galvanometer  to 
full  scale. 

The  relative  error  of  setting  is  small  in  direct  proportion  as  (6  -\-  X)  is 


DISCUSSION  151 

small  in  comparison  with  g.  Also  the  error  of  setting  for  D/2  is  inversely 
increased  with  reduction  in  the  value  of  D. 

Many  variations  of  this  type  of  circuit  are  possible,  though  their 
method  is  essentially  the  same  as  that  outlined.  It  must  be  remembered 
that  check  readings  can  only  be  taken  when  the  conditions  are  such  that 
the  e.m.f.  of  the  couple  is  practically  constant,  but  there  is  no  need  for 
such  checks  except  when  it  is  supposed  that  the  circuit  resistance  may 
have  changed,  changes  being  due  to  temperature  resistance  coefficient 
of  leads,  couple,  of  galvanometer  coil,  depth  of  immersion  of  couple, 
corrosion  of  some  part  of  the  circuit  reducing  its  cross-sectional  area  of 
conductor  or  adding  contact  resistance,  or  the  shortening  of  the  couple 
or  couple  leads.  If  the  rheostat  S  is  made  of  manganin  or  some  alloy  of 
practically  negligible  temperature  resistance  coefficient,  the  instrument  is 
free  from  errors  due  to  change  in  the  resistance  of  the  galvanometer  coil, 
and  hence  the  instrument  is  free  from  errors  of  indication  due  to  tempera- 
ture changes  of  the  instrument  coil. 

The  main  objection  to  an  instrument  of  this  type  is  that  it  can  only 
be  used  conveniently  with  one  couple  at  a  fairly  constant  depth  of  im- 
mersion and  temperature.  It  is  impractical  to  employ  it  with  several 
couples  and  a  selective  switch  or  other  similar  device,  unless  all  the  couple 
circuits  are  of  strictly  the  same  resistance — a  circumstance  that  is  difficult 
to  secure  and  almost  impossible  to  maintain.  The  chief  advantages  are 
the  elimination  of  any  form  of  cell,  dry  or  standard,  from  the  instrument 
circuit  and  continuous  deflection,  with  only  occasional  manual  adjustment. 

The  circuits  offer  unusual  possibilities  for  the  instruction  of  students 
in  electrical  measurements.  Knowing  the  current  sensitivity  of  the 
galvanometer  at  full  scale  and  the  value  of  the  rheostat,  it  is  possible  to 
determine  the  galvanometer  resistance,  battery  resistance,  and  voltage, 
and  to  correct  a  millivoltmeter  from  all  errors  due  to  line  drop  or  extrac- 
tion of  current  from  a  high  resistance  shunt. 

DISCUSSION 

PAUL  D.  FOOTE  AND  T.  R.  HARRISON,  Washington,  D.  C.  (written 
discussion*). — There  are  several  methods  for  measuring  the  internal 
resistance  of  a  battery,  the  line  resistance  in  a  circuit  containing  an  e.m.f., 
or  the  true  value  of  this  e.m.f.  The  writers,  however,  were  the  first  to  em- 
ploy a  simple  principle  whereby  the  total  resistance  of  the  circuit  is  ad- 
justed to  a  preassigned  value  for  which  the  scale  of  the  millivoltmeter  is 
graduated,  the  only  e.m.f.  employed  being  that  of  the  source  measured. 
One  of  the  simplest  forms  of  these  instruments  is  described  on  pages 
84-85.  In  papers  now  in  press  we  have  described  some  twenty  modifica- 
tions of  this  simple  design,  all  operating  upon  the  same  fundamentally 
new  principle.  The  instrument  devised  by  Mr.  Porter  is  of  this  general 

*  Received  Sept.  25,  1919. 


152  SELF-CHECKING    GALVANOMETER    PYROMETER 

type  but,  unlike  the  designs  we  have  recommended,  is  open  to  serious 
objections. 

If  the  e.m.f .  of  the  source  being  measured  is  equivalent  to  the  full- 
scale  range  of  Mr.  Porter's  instrument,  the  adjustments  can  be  made 
only  if  by  chance  the  external  resistance  x  equals  s  —  g.  Suppose 
that  the  full-scale  deflection  corresponds  to  e  millivolts;  the  current 
required  to  produce  this  deflection  is  i  =  e/s.  In  the  first  adjustment, 
however,  the  current  i'  =  e/(x  +  g).  Since  x  +  g  =  a  by  the  second 
adjustment,  and  since  in  general  a  <  s  =  a  +•&,  the  current  i'  is  larger 
than  i  in  the  ratio  s  (x  +  g) .  To  keep  the  pointer  on  the  scale  through- 
out adjustment,  therefore,  the  e.m.f.  to  be  measured  must  net  exceed 
the  fraction  (re  +  g)/s  of  the  full-scale  deflection.  When  x  is  reduced 
until  it  equals  %s  —  g  (i.e.,  when  a  =  ^s)  we  have  x  +  g  =  %s  so 
that  (x  +  g)/s  equals  %r  hence  only  one-half  of  the  scale  e.m.f.  can 
be  used  for  adjusting  the  resistances.  With  couple  and  leads  of  zero 
resistance,  the  maximum  e.m.f.  for  which  adjustment  can  be  made  is 
g/s  times  the  full  scale  e.m.f.  which  may  or  may  not  be  less  than  one-half 
of  the  scale  range  of  the  instrument,  depending  on  the  characteristics  of 
the  individual  instrument. 

Since  the  ratio  of  maximum  deflection  to  actual  e.m.f.  is  (x  +  g)/s, 
the  ratio  of  the  half  deflection  to  actual  e.m.f.  is  2  (x  +  g)/s,  and  an  error 
in  adjustment  to  the  half  deflection  will  appear  in  the  final  reading  mul- 
tiplied by  this  factor,  which  ranges  from  2,  where  x  =  s  —  g,  to  2g/s 
as  a  minimum. 

A  more  satisfactory  arrangement,  which  utilizes  the  full  scale,  is 
illustrated  by  the  accompanying  illustration.  The  normal  operating 


position  is  with  the  key  k  closed.  The  current  flowing  in  this  case  is 
i  =  e/(x  +  g).  With  the  key  open  the  resistance  x  is  adjusted  until 
the  deflection  is  halved;  viz.,  i/2  =  e/(x  -\-  g  +  s).  Hence  x  +  g  =  s, 
for  which  resistance  the  scale  of  the  instrument  is  graduated.  With 
this  arrangement,  adjustment  can  always  be  made  with  full-scale  e.m.f. 
Obviously  the  ratio  of  deflections  may  be  other  than  ^.  In  some  in- 
stances it  may  be  of  advantage  to  use  a  ratio  0.9,  in  which  case  a 
double  scale  could  be  employed,  one  scale  graduated  in  intervals  0.9  as 
great  as  the  other  scale.  Another  disadvantage  of  Mr.  Porter's  instru- 
ment, as  described,  is  that  an  e.m.f.  or  linear  scale  must  be  employed, 


DISCUSSION  153 

since  proper  adjustment  could  not  be  obtained  by  halving  the  deflec- 
tion on  a  temperature  scale.  Furthermore,  the  instrument  must  be  set 
to  read  zero  on  open  circuit,  which  prohibits  the  usual  cold-junction  ad- 
justment. These,  and  other,  objections  are  successfully  met  in  the  in- 
struments described  by  us  in  the  paper  just  referred  to,  and  further- 
more, high  sensitivity  in  adjustment  is  assured. 

H.  F.  PORTER  (author's  reply  to  discussion*). — The  discussion  by 
Messrs.  Foote  and  Harrison  opens  with  a  direct  attack  on  the  ethics 
followed  by  the  writer  in  describing  what  they  claim  is  a  modification 
of  their  own  device.  Freely  admitting  that  the  basic  principles  of  their 
invention  were  taken  from  Dr.  Northrup's  book  and  similar  standard 
works  of  reference,  they  openly  challenge,  at  the  time  of  their  first  public 
announcement  of  their  work,  the  right  of  another  to  describe  a  device 
similar  in  its  results  to  their  own.  The  writer's  invention,  though  pos- 
sibly it  may  not  have  antedated  that  of  Foote  and  Harrison  in  its  first 
conception,  is  in  no  sense  whatsoever  an  imitation  of  their  device.  It 
was  independently  conceived  and  worked  out  long  before  the  writer  had 
any  knowledge  of  their  circuits.  Further,  it  may  be  remarked  that  the 
purpose  of  this  article  was  not  to  show  in  how  many  ways  the  "Heat- 
meter  "  circuits  might  be  imitated.  Such  efforts  as  those  put  forth  in  the 
technical  press  of  a  year  or  so  ago,  when  attempts  were  made  to  ade- 
quately imitate  the  Pyrovolter,  may  well  be  left  to  others. 

As  to  the  mathematical  remarks:  A  meter  is  designed  to  allow  for 
certain  definite  maximum  variation  in  thermocouple  resistance.  It  is 
not  a  matter  of  the  resistance  X  by  chance  being  equal  to  or  less  than  any 
particular  value.  It  requires  but  a  very  superficial  examination  to 
disclose  the  fact  that  the  Heatmeter  is  capable  of  operation  only  in  case 
by  chance  the  resistance  of  the  thermocouple  circuit  is  not  over  20  ohms, 
unless  an  extremely  fragile  type  of  meter  is  employed.  Messrs.  Foote 
and  Harrison  claim  extreme  accuracy  of  setting  and  adjustment  for 
their  meter.  It  is  true  they  have  it,  but  at  the  expense  of  durability.  By 
their  own  admission,  nine-tenths  of  the  current  is  shunted  from  the  meter 
(and  shunting  a  meter  with  so  comparatively  low  a  resistance  shunt  which 
has  a  very  appreciable  contact  resistance  in  the  shunting  circuit  is  a 
rather  dangerous  procedure  from  the  viewpoint  of  accuracy),  thus  re- 
quiring, for  even  base-metal  ranges,  a  fragile  and  moderately  high- 
resistance  type  of  movement.  The  whole  advantage  of  the  writer's 
scheme  lies  in  its  practical  applicability  to  rugged,  durable,  practical, 
reliable,  low-resistance,  double-pivot  type  instruments,  such  as  have  been 
very  successfully  employed  in  the  Pyrovolter.  They  criticize  the  diffi- 
culties encountered  in  halving  the  deflection,  which,  as  the  writer  ex- 
plained at  the  meeting,  have  been  very  simply  overcome.  The  circuits 
were  described  in  their  present  form  for  the  sake  of  simplicity  in  showing 
the  principle. 

*  Received  Dec.  6,  1919. 


154  USEFULNESS    OF   BASE-METAL   THERMOCOUPLES 


Some  Factors  Affecting  the  Usefulness  of  Base -metal 
Thermocouples 

BY   O.    L.    KOWALKE,*   MADISON,    WIS. 
(Chicago  Meeting,  September,  1919) 

DURING  the  last  few  years  the  use  of  base-metal  thermocouples  has 
increased  very  considerably  in  various  industries,  due  to  the  necessity  for 
more  precise  control  of  temperatures.  The  base-metal  couple  has  the 
advantages  of  being  robust  to  a  surprising  degree,  cheap  as  compared  with 
platinum  couples,  sufficiently  accurate  for  most  operations,  rapid  in 
indicating  changes  in  temperature,  easily  renewed  or  repaired,  and  of 
generating  a  much  higher  electromotive  force  than  the  noble-metal 
couples.  There  are,  however,  some  factors,  such  as  indicating  and  record- 
ing instruments,  the  insulation  on  the  elements,  the  constancy  and  homo- 
geneity of  the  wires,  and  the  resistance  to  oxidation  in  the  furnace,  that 
limit  the  usefulness  of  the  temperature- measuring  device.  It  is  the 
purpose  of  this  paper  to  discuss  in  what  manner  these  factors  affect  the 
usefulness  of  the  couples. 

MEASURING  INSTRUMENTS 

There  are  at  present  two  sorts  of  measuring  instruments  in  general  use, 
millivoltmeters  and  potentiometers.  The  deflection  of  the  millivoltmeter 
is  proportional  to  the  amount  of  current  flowing  through  the  movable 
coil,  and  the  amount  of  current  flowing  in  the  entire  circuit  is  dependent 
on  the  electromotive  force  generated  and  the  total  resistance  of  the  circuit. 
It  is  obvious  that  the  resistance  of  the  couple  will  increase  when  heated 
and,  for  a  given  temperature,  the  increase  is  roughly  proportional  to  the 
length  of  couple  heated.  In  view  of  these  changes  in  resistance,  would  a 
low-  or  high-resistance  millivoltmeter  give  the  more  accurate  readings? 

Let  E  =  electromotive  generated  by  the  couple,  IR  =  volts  drop 
through  the  millivoltmeter,  Ir  =  volts  drop  through  couple  and  the 
leads  to  the  millivoltmeter;  then 

E  =  IR  +  Ir. 

Assume  that  the  resistance  of  the  millivoltmeter  R  is  2  ohms,  and  that 
the  resistance  of  the  couple  and  leads  r  is  0.01  ohm  when  2  in.  of  the  couple 
is  heated  to  1000°  C.,  and  further  that  when  all  of  the  effective  length  of 
the  couple  is  heated  to  1000°  C.  the  resistance  of  couple  and  leads  r  is  0.10 
ohm.  The  total  resistance  of  the  circuit  may  thus  increase  from  2.01  ohms 
to  2.10  ohms  or  4.4  per  cent. ;  therefore  the  current  flowing  is  decreased  pro- 

*  Professor  of  Chemical  Engineering,  University  of  Wisconsin. 


O.    L.    KOWALKE  155 

• 

portionately  and  consequently  the  deflection  of  the  millivoltmeter  is  also 
decreased.  Now  assume  that  the  resistance  of  the  millivoltmeter  R  is  30 
ohms  and  that  the  same  changes  in  resistance  in  the  couple  and  leads  r  take 
place  as  above.  Thus  the  increase  in  total  resistance  of  the  circuit  is  from 
30.01  ohms  to  30.10  ohms  or  0.3  per  cent.;  the  current  decrease  and  de- 
flection of  the  millivoltmeter  will  be  a  like  amount.  Thus,  it  is  clear  that 
a  high-resistance  millivoltmeter  will  give  readings  that  are  less  affected 
by  changes  of  resistance  due  to  depth  of  immersion  than  a  low-resistance 
instrument. 

In  the  potentiometer  system,  the  electromotive  force  from  a  standard 
cell  is  made  to  oppose  the  electromotive  force  generated  by  the  couple. 
A  galvanometer  is  placed  in  the  circuit  of  the  thermocouple  in  such  a 
manner  that  no  deflection  is  obtained  when  the  electromotive  force  from 
the  standard  cell  or  its  auxiliary  just  balances  that  from  the  thermocouple. 
Thus,  no  current  flows  in  the  thermocouple  circuit  when  the  measurement 
is  made,  hence  the  length  of  the  couple  and  its  resistance  due  to  depth  of 
immersion  are  immaterial  with  the  potentiometer. 

The  reliability  of  the  millivoltmeter  depends  on  the  permanence  of 
the  magnet,  the  correct  adjustment  for  freedom  of  movement  of  the  coil 
carrying  the  needle,  and  good  electrical  contacts  in  all  the  wires  of  the 
circuit.  Unless  a  calibration  is  made,  there  is  no  way  of  knowing  how 
much  in  error  a  given  deflection  may  be.  The  reliability  of  the  potenti- 
ometer depends  on  the  permanence  of  the  standard  cell.  When  the  poten- 
tiometer works,  it  is  usually  right;  when  it  is  not  right,  it  won't  work. 
The  millivoltmeter  is  cheaper  than  the  potentiometer;  both  are  about 
equally  robust.  The  indicating  millivoltmeter  can  be  read  without 
manipulation;  the  indicating  potentiometer  must  be  manipulated  for 
balance.  Both  types  are  made  recording,  but  the  potentiometer  can  be 
attached  to  more  couples  than  the  millivoltmeter. 

INSULATION  OF  THE  WIRES 

Materials  such  as  asbestos  twine  covered  with  water  glass  and  also 
with  water  glass  mixed  with  fine  carborundum,  woven-asbestos  tubing, 
porcelain  tubing,  and  fireclay  bushings  have  been  used  for  the  electrical 
insulations  of  the  wires  of  thermocouples.  Obviously,  a  material  used 
for  this  purpose  should  be  capable  of  enduring  high  temperatures  without 
breaking  down,  withstand  a  certain  amount  of  rough  handling,  and  not 
combine  with  the  thermocouple  when  hot. 

Asbestos  twine  when  wound  closely  on  the  wires  of  the  couple  and 
covered  with  a  paint  containing  sodium  silicate  makes  a  fair  insulator. 
The  asbestos  breaks  down,  however,  when  heated  to  1000°  C.  or  more  for 
extended  periods  of  time  and  thus  the  wires  are  left  bare,  and  liable  to 
short  circuit.  In  Fig.  1  are  shown  some  pictures  of  couples  wound  with 
asbestos  twine  and  painted  with  fine  carborundum.  After  using  these 
couples  near  1000°  C.  for  some  time,  it  was  noticed  that  the  iron  wire  B 


156 


USEFULNESS    OF  BASE-METAL   THERMOCOUPLES 


had  grown  to  nearly  twice  its  original  diameter  and  could  be  broken  easily 
in  one's  fingers.  The  iron,  asbestos,  and  carborundum  had  combined 
into  a  friable  mass;  there  was  no  free  iron  left.  The  fracture  has  a  green- 
ish purple  color.  Iron  at  high  temperatures  combines  readily  with  car- 
borundum and  thus  the  couple  is  destroyed.  The  woven-asbestos  tubing 
breaks  down  readily  and  is  not  so  permanent  as  the  twine  wrapped  on  the 
wire  and  painted  with  sodium  silicate. 

.  In  Fig.  2  is  shown  a  couple  with  wires  insulated  from  one  another  by 
fireclay  bushings  1  in.  (25  mm.)  long,  about  %  in.  (9.5  mm.)   outside 


FIG.  1.  —  COUPLES  WOUND  WITH  ASBESTOS  TWINE  AND  PAINTED  WITH  FINE 

CARBORUNDUM.    . 


diameter  and  ^£5  m-  (4-7  mm.)  bore.  These  bushings  are  used  by  several 
manufacturers  of  thermocouples  and  have  been  found  very  satisfactory  in 
this  laboratory.  In  the  illustration,  a  slight  fluxing  of  the  oxides  on  the 
wire  with  the  bushing  is  noticeable  on  the  constantan  wire  which  had 
been  raised  to  1100°  C.  On  nickel-chromium  wires,  such  fluxing  action 
has  not  been  observed.  These  bushings  have  shown  good  strength  under 
hard  use;  they  are  easily  replaced  when  broken  and  they  are  cheap. 


Since  the  voltage  generated  by  a  couple  for  a  given  temperature  is  the 
summation  of  all  the  electromotive  forces  due  to  the  contact  of  two  dis- 


O.    L.    KOWALKE 


157 


similar  metal  parts,  the  wires"  of  the  couple  should  be  as  homogeneous 
as  possible  if  the  electromotive  force  indicated  is  that  generated  at  the  hot 


FIG.  2. — COUPLES  IN  WHICH  WIRES  ARE  INSULATED  FROM  ONE  ANOTHER  BY  FIRECLAY 

BUSHINGS. 


900 

800 

• 

T3 

0 

L. 

o>  7r)n 

/ 

P  

i 

X 

iA 

^ 

/ 

^ 

k 

c 

V 
O 

£600 

L 
0< 
V 

D 
500 

400 
300 

^ 

// 

y 

7 

©  First  calibration,  4-  inches  heated 
*  Second  calibration,  t5  inches  heated 
•fy  Last  calibration,  IS  inches  heated 

/ 

Y 

2 

/ 

0              15              20              25              30             35             40             45              50             55        '     60 
Millivolts 

FIG.  3. — CALIBRATION  OF  COUPLE  20. 

junction  of  the  two  wires.  If  the  wires  are  not  homogeneous,  there  will 
be  set  up  at  each  junction  of  dissimilar  metals,  a  voltage  that  is  a  function 
of  the  temperature  at  that  point.  Should  the  depth  of  immersion  of  such 
a  couple  be  varied,  the  resultant  voltage  will  change,  even  though  the 


158 


USEFULNESS    OF   BASE-METAL   THERMOCOUPLES 


temperature  remains  constant.  ]t  was  found,  after  a  series  of  tests1 
made  in  this  laboratory,  that  there  are  several  couples  obtainable  that  are 
sufficiently  homogeneous  for  those  installations  where  an  accuracy  of 
about  25°  to  50°  C.  will  serve  the  purpose. 

Couples  purchased  from  a  number  of  prominent  makers  were  cut  to 
lengths  of  about  18  in.  (45  cm.).  To  each  couple  about  3  ft.  (0.9m.)  of 
flexible  lamp  cord  was  soldered,  and  the  wires,  at  soldered  joints  as  well 
as  all  other  required  points,  were  insulated  from  each  other.  The 


1200 


1100 


1000 


0  First  calibration.,  4 inches  heated 
&  Second  call  bration,  15 inches  heated 
<!>  Last  calibration  IS  inches  heated 


ZOO 


0  5  10  15  20  Z5 

Millivolts 

FIG.  4. — CALIBRATION  OF  COUPLE  18. 


couples  were  then  calibrated,  one  at  a  time,  over  the  range  given  by  the 
manufacturer  against  a  standardized  platinum  couple.  First,  all  couples 
were  calibrated  with  a  length  of  4  in.  (10  cm.)  heated.  Second,  all 
couples  were  calibrated  with  a  length  of  15  in.  (38  cm.)  heated  to  deter- 
mine the  effect  and  presence  of  heterogeneity  in  the  wires.  Third,  all 
couples  were  subjected  to  a  heat  treatment  for  periods  of  20  to  24  hr. 
each  at  temperatures  of  400°,  600°,  and  800°  C.  After  the  treatment 
at  each  of  these  temperatures  each  couple  was  calibrated  singly. 

1  Kowalke:  Trans.  Am.  Electrochem.  Soc.  (1913)  24,  377. 


O.    L.    KOWALKE 


159 


Electrically  heated  tube  furnaces  were  used  for  all  the  tests.  One 
furnace  was  10  in.  (25  cm.)  long  and  the  other  was  20  in.  long,  each  having 
a  tube  about  1  in.  bore.  These  were  used  for  the  calibrations  at  4  in. 
and  15  in.  depth  of  immersion,  respectively.  So  that  all  the  couples  could 
be  heated  together  for  the  treatments  at  400°,  600°,  and  800°  C.,  a  furnace 
24  in.  long  and  having  a  tube  2^  in.  bore  was  used. 

Asbestos  disks  were  placed  on  the  base-metal  and  the  standard 
couples  to  keep  them  centered  in  the  furnace.  The  hot  junctions  of 
the  two  couples  were  in  contact.  The  temperature  was  raised  to  about 
300°  C.  before  readings  were  taken  and  then  the  temperature  was  in- 
creased by  intervals  of  100°  C.  At  each  point  the  temperature  was 
maintained  stationary  for  a  period  of  2  min.  to  insure  equilibrium.  The 
cold  junctions  of  both  couples  during  calibration  were  kept  at  0°  C.  by 
means  of  an  ice  bath.  All  measurements  were  made  on  Leeds  &  Northrup 
Type  K  potentiometers,  a  separate  potentiometer  being  used  for  each 
couple. 

The  data  on  two  of  the  couples  only  is  presented  here  to  illustrate 
how  near  to  and  how  far  from  being  homogeneous  and  constant  the 
couples  were  found  to  be.  The  composition  of  the  wires  of  the  most 
satisfactory  couple,  Couple  No.  20,  is  as  follows:  Positive  terminal,  Fe, 
99.8  per  cent.;  Negative  terminal,  Cu,  52.3  per  cent.,  Ni,  48.0  per  cent. 

TABLE  1. — Data  on  Couples 


First  Calibration, 
4  In.  Heated 

Second  Calibration, 
15  In.  Heated 

Last  Calibration  after  Third 
Heat  Treatment  at  800°  C. 

Degrees  C. 

Millivolts 

Degrees  C. 

Millivolts 

Degrees  C. 

Millivolts 

318 

15.70 

245 

11.99 

324 

16.88 

8 

6 
15 

445 
540 
634 

22.51 
27.66 
32.99 

352 
441 
530 

17.97 
22.75 
27.62 

459 
553 
645 

24.12 
29.40 
34.59 

0> 

a 

721 

825 

38.11 

44.85 

653 
700 

34.67 
37.66 

735 
835 

40.09 
46.93 

c5 

896 

49.41 

783 

42.85 

928 

52.70 

997 

55.02 

893 

49.84 

980 

54.77 

984 

55.17 

240 

6.75 

230 

6.32 

222 

6.42 

330 

9.09 

325 

8.34 

330 

8.61 

435 

10.66 

428 

9.49 

446 

9.96 

00 

i—  c 

525 

11.75 

516 

10.38 

554 

11.18 

5 

645 

13.64 

621 

11.76 

655 

12.61 

£ 

738 

15.49 

723 

13.43 

763 

14.65 

a 
"S 

O 

843 
933 

18.20 
20.28 

805 
895 

15.23 
17.05 

838 
927 

16.34 
18.25 

o 

1026 

22.19 

981 

18.51 

1025 

20.08 

1109 

24.16 

1072 

20.46 

1099 

21.58 

1174               22.77 

160  USEFULNESS   OF  BASE-METAL  THERMOCOUPLES 

The  composition  of  the  wires  of  the  couple  found  most  unsatisfactory, 
Couple  No.  18,  is:  Positive  terminal,  Fe,  99.9  per  cent.;  Negative  terminal, 
Al,  1.14  per  cent.,  Ni,  98.3  per  cent.  These  data  are  further  shown 
graphically  in  Figs.  3  and  4,  and  Table  1. 

It  is  apparent  from  the  graphs  that  couple  No.  18  shows  a  difference 
of  120°  C.  at  1000°  C.  between  the  immersions  to  4  in.  and  15  in.,  respec- 
tively, and  that  the  variation  of  electromotive  force  with  temperature  is 
not  uniform  throughout  the  range.  The  lack  of  agreement  of  the  cali- 
brations with  4  in.  and  with  15  in.  of  the  couple  immersed  is  apparently 
due  to  a  lack  of  proper  annealing  of  the  wires  for  the  agreement  of  the 
second  and  the  last  calibrations  is  as  good  as  that  found  on  several  other 
couples.  It  is  further  apparent  from  the  graphs  for  couple  No.  20  that 
the  variation  of  electromotive  force  with  temperature  is  regular  through- 
out the  range  and  that  the  constancy  of  the  couple  is  satisfactory.  There 
is  a  change  of  only  25°  C.  in  the  indications  from  first  to  last  calibration. 

RESISTANCE  TO  OXIDATION 

The  combination  of  iron  with  constantan  for  couples  is  one  that  gives 
practically  a  "straight  line"  for  the  relation  of  electromotive  force  to 
temperature;  it  also  gives  a  higher  electromotive  force  at  a  given  tem- 
perature than  most  other  combinations  of  wires;  and  it  is  further  possible 
to  get  these  two  wires  remarkably  homogeneous.  The  great  disadvan- 
tage of  iron  is  its  property  of  oxidizing  rapidly  at  temperatures  above 
700°  C.  If  iron  were  protected  against  oxidation  by  some  means  that 
would  not  affect  the  electromotive  force  the  usefulness  of  the  iron -con- 
stantan combination  would  be  greatly  extended.  It  has  been  shown  by 
W.  E.  Ruder2  that  "calorizing"  iron,  "which  consists  in  producing  a 
rich  aluminum  alloy  upon  the  surface  of  the  metal"  practically  prevents 
oxidation  below  1000°  C. 

One  of  the  calorized  wires  used  in  these  tests  was  donated  by  the 
Research  Laboratory  of  the  General  Electric  Co.,  the  other  calorized 
wire  was  purchased  from  the  Brown  Instrument  Co.  The  uncalorized 
iron  and  the  constantan  wires  were  purchased  from  the  Leeds  &  Northrup 
Co.  The  iron  wires,  both  calorized  and  uncalorized,  were  approximately 
0.14  in.  (3.5  mm.)  diameter.  The  wire  from  the  General  Electric  Co. 
was  straight  and  had  a  rather  thin  coating  of  calorizing;  the  wire  from 
the  Brown  Instrument  Co.  came  bent  double  on  itself  and  the  calorized 
coat  was  so  heavy  that  some  of  the  alloy  chipped  off  when  the  wire  was 
straightened. 

Three  sets  of  couples  were  made:  Couple  No.  1QB,  constantan  vs. 
calorized  iron  (General  Electric  Co.),  Couple  No.  32B,  constantan  vs. 
calorized  iron  (Brown  Instrument  Co.),  Couple  No.  815,  constantan 
vs.  uncalorized  iron  (Leeds  &  Northrup  Co.).  Before  making  the  couples 
the  wires  were  heated  with  the  electric  current  to  a  bright  red  heat  for 

*  Trans.  Am.  Electrochem.  Soc.  (1915)  27,  253. 


O.    L.    KOWALKE 


161 


several  minutes  to  remove  any  strains.  The  couples  made  were  each 
12  in.  long.  For  the  hot  junction,  the  wires  were  fused  in  the  electric 
arc,  then  fireclay  bushings  were  strung  on  the  constantan  wires,  and 
finally  flexible  copper  lamp  cord  was  soldered  to  each  element. 

For  calibrating  and  for  heat  treatments  the  same  types  of  furnaces 
and  methods  were  used  as  have  been  previously  described.  The  electro- 
motive force  measurements  were  made  on  Leeds  &  Northrup  portable 
potentiometers.  The  couples  were  calibrated  as  annealed,  then  they 
were  heated  for  24  hr.  at  200°  C.  and  given  a  second  calibration,  then 
heated  again  for  24  hr.  at  900°  C.  and  calibrated  a  third  time.  After 


IUUU 

900 
800 

v  700 
"D 
E 
? 

£  600 
u 
</> 

V 

J?  MJO 

n 

400 
300 
200 

S 

s 

* 

/ 

> 

/ 

/ 

/ 

/ 

/ 

V 

/         ®  First  calibration 
A  Second  calibration  after  hec 
-^-  Third  calibration  after  hea 
»   Calibration  of  couple  No.  10 

ihng  at  ff 
ting  at  9i 

•>0*C.for2 
W°C.fbr?< 

4  hours 
1  hours 

</ 

- 

0              15              20              Z5             30             35             40              45              50             55            60 

Millivolts 
FIG.  5. — CALIBRATION  OF  COUPLE  WB. 

the  third  calibration,  about.  1%  in-  (3.8  cm.)  was  cut  from  the  hot  junc- 
tion end  of  each  couple  and  the  remainder  fused  together  again.  Thus: 
Couple  10(7  was  made  from  Couple  105,  Couple  32C  was  made  from 
Couple  325,  Couple  81C  was  made  from  Couple  815.  The  remaining 
couples  were  calibrated  at  only  three  points  to  see  whether  any  changes 
in  calibration  had  taken  place  due  to  changes  in  the  wire  or  method 
of  fusion.  The  results  are  shown  in  Table  2  and  Figs.  5,  6,  and  7. 

As  shown  in  Fig.  2,  the  constantan  wires  E  and  D  and  the  uncalorized 
wire  B  have  oxidized  badly.  The  calorized  wires  A  from  the  Brown 
Instrument  Co.  (couple  325)  and  C  from  the  General  Electric  Co. 
(couple  105)  have  not  oxidized  to  any  extent  upon  being  given  three 
calibrations  and  two  heat  treatments.  The  wire  C  with  the  thin  coating 


162 


USEFULNESS   OP  BASE-METAL   THERMOCOUPLES 


1000 


900 


800 


o>700 


«J  600 


500 


400 


300 


o  First  calibration 

&  Second  calibration  after  heating  at  800  °C  For  24  hours 
-ty  Third  calibration  after  heating  at  90O  °C  for  24  hours 
a  Calibration  of  couple  No.SIC 


30 


•is 


35  40 

Millivolts 

FIG.  6. — CALIBRATION  OF  COUPLE  81 B. 


1000 
900 

600 

ta 

73 

O 

,o>700 

c 
• 
O 

v  600 

• 

L. 

o> 

V 

0 

500 

400 
300 
200 

)^ 
1 

# 

.. 

&/  . 

Jy 

'/ 

A 

^/ 

/ 

£. 

^ 

/ 

X 

g 

A 

/  / 

/ 

^ 

SS    ©   First  calibration 
/          A  Second  calibration  after  heating  at  800  °C  For  24 
-<j>-  Third  calibration  aFttr  heating  at  900°C  for  24 
m  Calibration  of  couple  No.  32  C. 

hours 
hours 

/ 

Y 

*/ 

0              15               ZO              25             30             35             40             45              50              55            60 

Millivolts 

FIG.  7. — CALIBRATION  OF  COUPLE  32#. 


O.    L.    KOWALKE 


163 


TABLE   2. — Comparisons  of  Calibrations  of   Colorized  and   Uncalorized 
Iron-Constantan  Couples 


First  Calibration 

Second  Calibration  after 
Heating  at  800°  C.  for  24  Hr. 

Third  Calibration  after 
Heating  at  900°  C.  for  2  Hr. 

Degrees  C. 

Millivolts 

Degrees  C. 

Millivolts 

Degrees  C. 

Millivolts 

05 

0 

288 
400 

14.2 
20.9 

301 

458 

14.8 
23.7 

358 
453 

17.4 
23.2 

i 

530 
700 

28.1 
42.1 

568 
815 

30.0 
45.2 

553 

738 

29.1 
40.3 

a 

o 
U 

795 
880 
950 

44.0 
49.5 
53.8 

910 

980 

51.1 
55.5 

850 
929 
965 

47.5 
52.4 
54.6 

375 

17.9 

374 

19.8 

371 

19.0 

«5 

508 

26.3 

466 

25.0 

500 

26.6 

00 

605 

32.3 

572 

31.1 

609 

32.8 

1 

0> 

705 
811 

38.6 
45.5 

680 
769 

37.5 
43.0 

742 
823 

41.3 
46.0 

OH 

874 

49.3 

834 

47.4 

905 

51.5 

a 

973 

55.5 

912 
973 

52.4 
55.9 

977 

55.5 

279 

13.9 

333 

16.8 

320 

16.0 

oq 

529 

29.3 

439                 22.8 

435 

22.5 

CO 

625 

35.5 

561                 29.9 

537 

28.0 

o 

711 

41.2 

650                 35  .  0 

627 

33.2 

0 

818 

47.2 

780                 43  .  3 

711 

37.6 

a 

887 

51.7               847         i         47.4 

812 

44.9 

o 
O 

965 

56.2               910 
990 

51.4 
56.2 

900 
1015 

50.4 
57.3 

TABLE  3. — Calibrations  of  Couples  IOC,  32C,  81C. 


No. 

IOC 

No.  32C 

No.  81C 

Degrees  C. 

Millivolts 

Degrees  C. 

Millivolts 

Degrees  C. 

Millivolts 

406 
720 
927 

20.4 
39.1 
52.0 

408 
735 
949 

21.8 
38.5 
52.9 

485 
702 
948 

24.9 
37.2 
53.6 

was  in  better  shape  than  the  wire  A  with  the  thick  coating,  which  flaked 
off  and  developed  oxidized  spots.  Comparison  of  Figs.  5  and  6  shows 
that  calorized  iron  gives  the  same  electromotive  force  against  constantan 
as  the  uncalorized  iron  and  that  both  couples  are  about  equally  constant. 
Couple  No.  32B  did  not  give  such  good  results  as  Nos.  10J?  and  81B. 
The  first  calibration  of  No.  325  shows  a  higher  electromotive  force  for  a 
given  temperature  than  the  other  couples,  but  after  the  heat  treatment 


164  USEFULNESS    OF  BASE-METAL    THERMOCOUPLES 

at  800°  C.,  couple  No.  32Z?  gave  a  calibration  that  checked  very  well  with 
them.  This  discrepancy  is  probably  due  to  insufficient  annealing  at 
the  beginning. 

DISCUSSION 

T.  R.  HARRISON,  Washington,  D.  C.  (written  discussion*). — Mr. 
Kowalke  shows  that  a  high-resistance  millivoltmeter  is  subject  to  smaller 
errors,  due  to  change  in  resistance  of  the  thermocouple  to  which  it  is 
attached,  than  is  a  low-resistance  instrument.  He  uses  as  examples 
instruments  having  resistances  of  30  and  2  ohms,  respectively.  At  the 
Bureau  of  Standards,  resistances  above  300  ohms  would  be  considered 
high  and  30  ohms  rather  low. 

In  the  tests  on  calorized-iron  thermocouples,  a  difference  between 
the  first  and  third  calibrations  of  the  couples  serves  to  show  changes 
due  to  the  intermediate  heat  treatment  and  only  under  certain  conditions 
would  changes  in  calibrations  due  to  changes  in  the  wire  be  detected  by 
a  recalibration  after  1^  in.  had  been  cut  from  the  hot-junction  end  of  the 
couple  and  the  ends  re  welded. 

The  calibration  of  a  couple  depends  only  on  the  thermoelectric  prop- 
erties of  that  part  of  the  couple  which  lies  within  the  region  of  non- 
uniform  temperature,  i.e.,  the  temperature  gradient;  hence,  so  long  as  the 
temperature  gradient  falls  along  wires  of  similar  thermoelectric  properties 
no  change  in  the  electromotive  force  is  produced  by  altering  the  metals  of 
those  parts  of  the  circuit  that  are  at  uniform  temperature.  Usually  at 
the  cold-junction  end  of  the  circuit,  copper  leads,  brass  binding  posts, 
manganin  resistance  coils,  and  various  other  materials  form  part  of  the 
circuit,  but  so  long  as  all  are  at  a  uniform  temperature  (or  if  for  each 
junction  between  unlike  metals  there  is  a  similar  opposing  junction 
at  the  same  temperature)  no  resultant  thermoelectromotive  force  will 
be  produced  thereby.  Likewise,  if  a  length  of  several  inches  at  the  hot- 
junction  end  of  the  couple  is  at  uniform  temperature,  it  matters  not  if  this 
section  is  unlike  other  parts  of  the  couple,  so  long  as  there  is  good  metallic 
connection  and  no  source  of  e.m.f.  other  than  thermoelectric  is  present. 
Cutting  off  part  of  the  hot  end  of  the  couple  within  such  a  region  of  uni- 
form temperature  would  not  alter  the  e.m.f.  of  the  couple. 

Changes  in  a  couple  originally  .homogeneous  may  be  detected  by 
making  one  calibration  with  the  temperature  gradient  along  a  section 
of  the  couple  that  has  been  subjected  to  furnace  conditions  and  another 
with  the  gradient  along  a  section  that  has  not  been  exposed  to  deteriorat- 
ing conditions.  The  latter  should  be  the  same  as  the  original  calibration. 
If  the  alteration  had  taken  place  while  the  couple  was  in  use  at  a  given 
depth  of  immersion,  a  calibration  in  this  position  would  be  intermediate 
between  calibrations  made  as  above,  since  only  part  of  the  wires  within 
the  temperature  gradient  would  have  undergone  the  n  aximum  change. 

*  Received  Sept.  25,  1919. 


MEASURING    TEMPERATURES    WITH    THERMOCOUPLES  165 


Tables  and  Curves  for  Use  in  Measuring  Temperatures  with 
Thermocouples 

BY    LEASON   H.    ADAMS,*   B.   S.,   WASHINGTON,    D.    C. 
(Chicago  Meeting,  September,' 19 19) 

THE  thermocouple  as  a  device  for  the  measurement  of  temperature 
is  rivaled  only  by  the  platinum-resistance  thermometer.  Both  instru- 
ments are  capable  of  the  highest  precision,  but  the  thermocouple,  on 
account  of  its  cheapness,  ease  of  construction,  and  small  cross-section, 
is  finding  a  continually  widening  field  of  usefulness  for  industrial  control 
as  well  as  for  laboratory  measurements.  Formerly,  the  thermocouple 
was  subject  to  two  disadvantages:  errors  due  to  lack  of  homogeneity  of 
the  metal  and  the  labor  involved  in  the  interpolation  between  fixed 
points  on  the  temperature  scale.  Former  publications  from  the  Geo- 
physical Laboratory  have  described  the  methods1  for  the  selection  and  test- 
ing of  thermocouple  wire  and  have  presented  standard  calibration  curves2 
for  platinum-platinrhodium  and  copper-constantan  couples,  so  that  the 
most  important  objections  to  the  thermocouple  as  a  precision  thermometer 
have  been  removed. 

The  calibration  tables  published  in  1914  covered  the  range  0-1755° 
for  the  platinum-platinrhodium  couple  and  0-350°  for  copper-constantan. 
It  has  seemed  desirable  to  extend  the  table  for  copper-constantan  to 
—  200°,  and  also  to  include  a  table  for  the  Hoskins  thermocouple. 
Accordingly,  in  this  paper,  the  new  tables  are  presented,  together 
with  a  brief  explanation  of  their  use;  and,  finally,  certain  diagrams  and 
a  paragraph  on  "cold-junction  corrections"  are  given. 

Standard  Calibration  Tables. — In  Fig.  1,  which  illustrates  how  the 
electromotive  force  (e.m.f.)  of  each  of  the  three  couples  varies  with  the 
temperature,  the  temperatures  of  one  junction  are  plotted  as  abscissas 
and  the  corresponding  thermo-e.m.f.'s  as  ordinates.  The  second  junction 
of  the  couple  is  supposed  to  be  at  0°;  the  curves,  therefore,  pass  through 
the  origin,  and  the  e.m.f.  changes  sign  at  this  point.  The  extent  of  the 
solid  part  of  each  line  indicates  the  useful  range  (so  far  as  known)  of  the 

*  Physical  Chemist,  Geophysical  Laboratory,  Carnegie  Institution  of  Washington. 

1  W.  P.  White:  Phys.  Rev.  (1906)  23,  449;  Amer..  Jnl.  Sri.  [4]  (1909)  28,  474;  Jnl. 
Am.  Chem.  Soc.  (1914)  36,  2292;  P.  D.  Foote,  T.  R.  Harrison  and  C.  O.  Fairchild: 
Met.  &  Chem.  Engng.  (1918)  18,  343,  403. 

2  R.  B.  Sosman:  Amer.  Jnl.  Sti.  [4]  (1910)  30,  7;  L.  H.  Adams  and  J.  Johnston: 
Ibid.  [4]  (1912)  32,  534;  L.  H.  Adams:  Jnl.  Wash.  Acad.  (1913)  3,  469;  Jnl.  Am.  Chem. 
Soc.  (1914)36,65. 


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-200            0            200          400          GOO          800          1000         1200  .       1400        1600 
Temperature,  Degrees  Centigrade 

3.  i.  —  VARIATION    OF    THERMOELECTROMOTIVE    FORCE    WITH   TEMPERATURI 

ARE  BROKEN  IN  REGIONS  OF  TEMPERATURES  IN  WHICH  E.M.F.  IS  NOT  ACCURATE! 
N  OR   IN  WHICH  THE  COUPLES  ARE  NOT  PARTICULARLY  SUITABLE    FOR  TEMPER/ 
MEASUREMENT. 

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Temperature,  Degrees  Centigrade 

FIG.  2. — SENSITIVITY  OF  THREE  KINDS  OF  COUPLES  AT  VARIOUS  TEMPERATURES;  dE/dT 

IN  MICROVOLTS  PER  DEGREE,  IS  PLOTTED  AS  A  FUNCTION  OF*TEMPERATURE. 


LEASON   H.    ADAMS 


167 


particular  couple,  while  the  dotted  parts  of  the  lines  represent  thee.m.f.'s 
in  the  region  of  temperature  where  the  couples  deteriorate  rapidly  or 
where  the  temperature-e.m.f.  relation  is  imperfectly  known.  The  sensi- 
tivity of  the  couples  at  various  temperatures  is  exhibited  by  the  curves  in 
Fig.  2.  Here,  again,  temperature  is  plotted  as  abscissas,  while  the 
ordinates  represent  the  sensitivity  of  the  respective  couples  dE/dT;  or, 
what  is  practically  the  same  thing,  the  number  of  microvolts  for  1° 
change  in  temperature.3 

TABLE  1. — Fixed  Points  for  Use  in  Thermometry 


Substance 

Transforma- 
tion 

Tempera- 
ture, 
Degrees   C. 

Substance 

Transforma- 
tion 

Tempera- 
ture, 
Degrees  C. 

boiling  point 

-182  98 

Sulfur  

boiling  point 

444  .  55 

—  112  0 

Antimony  

melting  point 

630.0 

—  78  5 

melting  point 

658.7 

—  38  88 

Silver          

960.2 

Water      

0.00 

Gold  

melting  point 

1062.6 

Water 

100.00 

melting  point 

1082.8 

Naphthalene  

boiling  point 

217.95 

Lithium  metasilicate  . 

melting  point 

1201. 

Tin     

231.9 

Diopside  

melting  point 

1391.5 

305.9 

Nickel  

melting  point 

1452.6 

320  9 

melting  point 

1549.5 

Zinc  

melting  point 

419.4 

Platinum  

melting  point 

1755. 

The  most  important  fixed  points4  for  use  in  thermometry  are  given  in 
Table  1 .  The  temperatures  given  are  in  degrees  centigrade  on  the  thermo- 
dynamic  scale.  The  boiling  points  are  for  760  mm.  pressure;  for  other 
pressures  p,  the  corresponding  temperatures  T  may  be  obtained  with 
sufficient  accuracy  for  small  differences  of  pressure,  by  use  of  the  correc- 
tion factor  A  in  the  formula  T  =  T0  +  A  (p-760) .  The  values  of  A  are 
as  follows: 


SUBSTANCE 


CORRECTION 
FACTOR,  A 


Oxygen 0.013 

Carbon  dioxide 0.016 

Water..  0.037 


SCBSTANCE  CORRECTION 

FACTOR,  A 

Naphthalene 0.057 

Benzophenone 0 . 063 

Sulfur..  0.092 


Platinum-platinrhodium  Couple.5 — Table  2  is  the  same  as  the  one 
published  previously,6  and  is  reproduced  here  without  change. 


3  dE/dT  is  sometimes  called  the  thermoelectric  power. 

4  In  this  connection  see  A.  L.  Day  and  R.  B.  Sosman:  Amer.  Jnl.  Sci.  [4]  (1910) 
29,  93;   (1912)  33,  517;  F.  Henning:  Ann.  Phys.  (1913)  43,  294;  Wilhelm:  U.  S. 
Bureau  of  Standards  Sci.  Paper  No.  294. 

"One  wire  is  of  pure  platinum,  the  other  is  90  per  cent,  platinum,  10  per  cent, 
rhodium. 

•L.H.Adams:  Jnl.  Am  Chem.  Soc.  (1914)  36,  65. 


168        MEASURING  TEMPERATURES  WITH  THERMOCOUPLES 

TABLE  2. —  Values  for  Plati 


^T 

1000 

2000 

3000 

4000 

5000 

o 

0 

147.1       265.4 

374.3 

478.1 

578.3 

17.8 

12.  6        11.  S 

10.  6        10.  S 

9.8 

100 

17.8 

159.7       276.6 

384.9       488.3 

588.1 

16.7 

IS.  4 

11.1 

10.6 

10.1 

9.8 

200 

34.5 

172.1 

287.7 

395.4 

498.4 

597.9 

16.8 

IS.g 

//.O 

70.5 

10.1 

9.8 

300 

50.3 

184.3 

298.7 

405.9 

508.5 

607.7 

16.1 

12.0 

11.0 

10.4      w.i 

9.7 

400 

65.4 

196.3 

309.7 

416.3       518.6 

617.4 

14-6 

11.8 

10.9 

10.4 

10.0 

9.7 

600 

80.0 

208.1 

320.6 

426.7 

528  .6       627  .  1 

14-1 

11.0 

10.  S 

10.4        10.0 

9.7 

600 

94.1 

219.7 

331.5 

437.1 

538.6 

636.8 

13.7 

11.5 

10.  S 

•  10.3 

10.0 

9.7 

700 

107.8 

231.2 

342.3 

447.4 

548.6 

646.5 

13.4 

11.5 

JO.  7 

10.  S 

9.9 

9.6 

800 

121.2 

242.7 

353.0 

457.7 

558.5 

656.1 

13.1 

11.4 

10.7 

10.2 

9.9 

9.6 

900 

134.3 

254.1 

363.7 

467.9 

568.4 

665.7 

1S.S        11.3 

10.  <? 

10.  S 

9.9 

9.6 

1000       147.1       265.4 

374.3 

478.1 

578.3 

675.3 

E 
tin 

13,000 

14,000 

15,000 

16,000 

17,000 

18,000 

0 

1289.3 

1372.4 

1454.8 

1537.5 

162a. 

1704.3 

8.4 

8.3 

S.S 

8.3 

8.3 

8.3 

100 

1297.7 

1380.7 

1463.0 

1545.8 

1629.2 

1712.6 

8.3 

8.3 

S.S 

8.3 

8.4 

8.4 

200 

1306.0 

1389.0 

1471.2 

1554.1 

1637.6 

1721.0 

8.3 

8.3 

S.S 

S.S 

.  8.3 

8.3 

300 

1314.3 

1397.3 

T479.4 

1562.4 

1645.9 

1729.3 

8. 

8.3 

8.3 

8.4 

8.4 

8.4 

400 

1322.6 

1405.6 

1487.7 

1570.8 

1654.3 

1737.7 

8.3 

8.2 

8.3 

8.3 

8.3 

8.3 

500 

1330.9 

1413-.8 

1490.0 

1579.1 

1662.6 

1746.0 

8.S 

S.S 

8.3 

8.4 

8.3 

8.3 

600 

1339.2 

1422.0 

1504.3 

1587.5 

1670.9 

1754.3 

8.3 

S.S 

8.3 

8.3 

8.4 

700 

1347.5 

1430.2 

1512.6 

1595.8 

1679.3 

8.3 

S.S 

8.3 

8.4 

8.3 

800 

1355.8 

1438.4 

1520.9 

1604.2 

1687. 

8.3 

S.S 

8.3 

8.3 

8.4 

900 

1364.1 

1446.6      • 

1529.2 

1612.5 

1696.0 

•      8.3 

S.S 

8.3 

8.4 

8.3 

1000 

1372.4 

1454.8 

1537.5 

1620.9 

1704.3 

The  values  of  e.m.f.  as  given  lie  very  close  to  the  mean  of  the  three 
couples  E,  F,  and  G  used  as  standards  by  Day  and  Sosman  in  fixing  the 
high-temperature  scale.  In  this,  as  in  the  two  succeeding  tables,  tem- 
peratures (on  the  thermodynamic  scale)  and  temperature  differences  are 
given  for  each  100  microvolts.  The  last  digit  in  the  temperature  values 
in  each  table  is  given  for  purposes  of  interpolation  and  for  estimating 
small  temperature  differences. 

Copper-constantan  Couple. — Table  3  gives  the  temperature  and  tem- 
perature difference  for  the  copper-constantan  thermocouple. 

The  curve  as  given  represents  about  the  mean  e.m.f.'s  of  con- 
stantan7  wire  from  various  makers.  The  part  of  the  table  lying 
between  0  and  350°  is  the  same  as  that  previously  published,8  and 

7  Constantan  (known  also  by  various  trade  names  such  as  "Advance,"  "Ideal," 
"  IAIA,"  and  so  forth)  contains  about  60  per  cent,  copper  and  40  per  ».  ickel. 

8L.  H.  Adams:  Zoo.  cit. 


LEASON    H.    ADAMS 

num-Platinrhodium  Couple* 


169 


6000                     7000                  8000                  9000                 10,000 

11,000 

12,000 

675.3                   769.5 

861.1            ;     950.4              1037.3 

1122.2 

1205.9 

9.5-                      3.3 

9.0  ;                  8.8 

8.6 

8.4 

8.S 

684.8                   778.8 

870.1 

959.2 

1045.9 

1130.6 

1214.2 

9.6 

9.S 

9.0 

8.8 

8.6 

8.4 

8.4 

694.3 

788.0 

879.1 

968.0 

1054.4 

1139.0 

1222.6 

9.6 

S.2 

5.0 

8.7 

8.6 

8.4 

8.3 

703.8 

797.2 

888.1 

976.7 

1062.9 

1147.4 

1230.9 

9.5 

0.« 

9.0 

8.7 

8.6 

8.4 

8.4 

713.3 

806.4 

897.1 

985.4 

1071.4 

1155.8 

1239.3 

9.4 

9.S 

S.O 

5.7 

8.6 

8.4 

8.3 

722.7 

815.6 

906.1 

994.1 

1079.9 

1164.2 

1247.6 

9.4 

9.1 

5.9 

8.7 

8.6 

8.S 

8.3 

732.1 

824.7 

915.0 

1002.8 

1088.4 

1172.5 

1255.9 

9.4 

9.1 

8.9 

8.7 

8.6 

8.4 

8. 

741.5 

833.8 

923.9 

1011.5 

1096.9 

1180.9 

1264.3 

9.4 

9.1 

S.S                    S.0 

8.6 

8.S 

8.S 

750.9 

842.9 

932.8 

1020.1 

1105.4 

1189.2 

1272.6 

9.S 

S.J 

8.8 

8.6 

8.4 

8.4 

8.4 

760.2 

852.0 

941.6              1028.7 

1113.8 

1197.6 

1281.0 

S.S 

9.1 

5.8 

8.6 

8.4 

8.3 

8.3 

769.5 

861.1 

950.4              1037.3 

1122.2 

1205.9 

1289.3 

*  Standard  calibration  curve  for  Pt-Pt-Rh  (10  per  cent.  Rh)  thermocouple,  giving 
the  temperature  and  temperature  differences  for  every  100  microvolts.  Fixed 
junction  is  at  0°.  For  use  in  conjunction  with  a  deviation  curve  determined  by  cali- 
bration of  the  particular  couple  at  some  of  the  following  fixed  points: 


Degrees 
C. 

Micro- 
volts 
>»» 

Degrees 

Micro- 
volts 

ito 

100  00 

643 

Silver  melting  point  

960.2 

9,111 

Naphthalene,  boiling  point  
Tin,  melting  point  

217.95 
231.9 

1,585 
1,706 

Gold,  melting  point  
Copper,  melting  point  

1,062.6 
1,082.8 

10.296 
10,534 

Benzophenone,  boiling  point.  .  .  . 

305.9 

2,365 

LiiSiOj,  melting  point  

1,201. 

11,941 

320.9 

2  503 

1,391.5 

14,230 

419  4 

3,430 

1  452  6 

14  973 

Sulfur,  boiling  point  

444.55 

3,672 

Palladium,  melting  point  

1,549.5 

16,144 

Antimony,  melting  point  

630.0 

5,530 

1,755. 

18,608 

658.7 

5,827 

was  calculated  from  the  equation  :#  =  74.672T7  -  13892  (1  -  g- 
The  values  above  350°  were  calculated  from  the  same  formula  and  are 
now  added  to  the  table,  for  the  convenience  of  those  who  wish  to  use  the 
copper-constantan  couple  for  rough  measurements  at  the  higher  tem- 
peratures. It  should  be  remembered,  however,  that  this  couple,  unless 
made  of  heavy  wire,  deteriorates  rapidly  at  temperatures  above  300°. 
The  remainder  of  the  table9  was  calculated  from  the  equation:  E  = 
92.2077  -  29770  (1  -  e-°-00]*T). 

The  fixed  points  used  were  the  boiling  points  of  oxygen  and  carbon 
dioxide,  and  the  mercury  melting  point.  Between  —  183°  and  —  220°, 
the  figures  in  the  table  were  obtained  by  extrapolation,  but  since  there  is 
no  reason  for  suspecting  a  sudden  change  in  the  slope  of  the  curve  in 
this  region,  the  extrapolation  is  not  violent.  The  copper-constantan 


9  For  *        jratures  below  0°,  interpolations  between  widely  separated  fixed  points 
cannot  bt  carried  out  with  the  same  confidence  as  for  higher  temperatures. 


170  MEASURING   TEMPERATURES   WITH   THERMOCOUPLES 

TABLE  3.— Temperatures  and  Temperature 


E 

M» 

-5000 

-4000       -3000 

-2000 

-1000 

-0 

0 

-  169  .  14 

-124.46 

-  87.86 

-55.81 

-  26  .  82 

0 

5.20 

4.01 

S.42 

3.05 

2.79 

2.60 

100 

-174.34 

-128.47 

-  91.28 

-58.86 

-29.61 

-  2.60 

5.40 

4.  09 

3.46 

3.08 

2.81 

2.62 

200 

-179.74 

-132.56 

-  94.74 

-61.94 

-32.42 

-5.22 

5.64 

4.18 

S.5/ 

3.11 

2.84 

£.03 

300 

-185.38 

-136.74 

-  98.25 

-65.05 

-  35  .  26 

-  7.85 

5.89 

4.28 

3.67 

3.15 

2.86 

2.65 

400 

-191.27 

-141.02 

-101.82 

-68.20 

-38.12 

-10.50 

6.17 

4.39 

3.  OS 

3.19  \       2.89 

#.07 

500 

-  197  .  44 

-145.41 

-105.45 

-71.39 

-41.01 

-13.17 

6.61 

4.50 

3.68 

S.22 

2.90 

2.69 

600 

-203.95 

-149.91 

-109.13 

-74.61 

-43.91 

-15.86 

6.97 

4.61 

3.74 

S.«<? 

2.93 

2.71 

700 

-210.92 

-154.52 

-112.87 

-77.87 

-46.84 

-18.57 

7.65 

4.73 

3.  80 

3.29 

2.96 

«.  73 

800 

-218.47 

-159.25 

-116.67 

-81.16 

-49.80 

-21.30 

4.87 

3.S0 

3.33 

2.99 

2.76 

900 

-164.12 

-120.53 

-84.49 

-52.79 

-24.05 

6.  OS 

3.93 

3.  37 

3.02 

0.77 

1000 

-169.14 

-124.46 

-87.86 

-55.81 

-26.82 

^                      7000                   8000 

9000 

10,000 

11,000 

12,000 

0 

155.95 

175.50 

194.62 

213.36 

231.74 

249.82 

1.97 

1.93 

1.89 

/.S5 

1.82 

1.79 

100 

157.92 

177.43 

196.51 

215.21 

233.56 

251.61 

1.97 

1.93 

1.89 

1.85 

1.82 

1.79 

200 

159.89 

179.36 

198.40 

217.06 

235.38 

253.40 

1.97 

1.92 

/.SS 

r.M 

1  .82 

1  .78 

300 

161.86 

181.28 

200.28              218.91 

237  .  20 

255.18 

1.96 

1.92 

1.88                   1.84 

1.81 

1.78 

400 

163.82 

183  .  20 

202.16              220.75              239.01 

256.96 

1.96 

1.91 

1.88 

1.54   '                1.81 

1.78 

500 

165.78 

185.11 

204.04 

222.59              240.82 

258  .  74 

1.95 

1.91 

/.S7 

1.84 

1.81 

1.78 

600 

167.73 

187.02 

205.91 

224.43 

242.63 

260.52 

1.96 

1.91 

/.»7 

1.83 

/.SO 

1.77 

700 

169.68 

188.93 

207.78 

226.26 

244.43 

262.29 

1.94 

1.50 

1.86 

J.SS 

/.SO 

/.77 

800 

171.62 

190.83 

209.64 

228.09 

246.23 

264.06 

l.»4 

1.90 

/.s<? 

1.83 

1.80 

/.77 

900 

173.56 

192.73 

211.50 

229.92 

248.03              265.83 

1.94 

1.89 

1.86 

/.&? 

/.7S 

/.77 

1000 

175.50 

194  .  62 

213.36 

231.74 

249  .  82 

267.60 

*  Standard  calibration  curve  for  copper-constantan  thermocouple  giving  the  tem- 
perature and  temperature  differences  for  every  100  microvolts.  Fixed  junction  is 
at  0°.  For  use  in  conjunction  with  a  deviation  curve  determined  by  calibration  of 
the  particular  couple  at  some  of  the  following  fixed  points: 


Degrees 

Micro- 
volts 

M» 

Degrees 
C. 

Micro- 
volts 

/IB 

Oxygen,  boiling  point.  ....... 

-182.98 

5,258 

Naphthalene,  boiling  point.  .  . 

217.95 

10,248 

Carbon  dioxide,  boiling  point 

-   78.5 

2,719 

Tin,  melting  point  

231.9 

1  1  ,009 

—   38  88 

1,426 

305.9 

15,203 

Water,  boiling  point  

100.00 

4,276 

Cadmium,  melting  point  

320.9 

16,083 

LEASON   H.    ADAMS 

Differences  for  Copper-constantan  Thermocouple* 


171 


0 

1000 

2000 

3000 

4000 

5000 

6000 

0 

25.27 

49.20 

72.08 

94.07 

115.31 

135.91 

2.59 

9.41 

2.  33 

2.23 

2.16 

2.09 

2.03 

2.59 

27.72 

51.53 

74.31 

96.23 

117.40 

137.94 

2.57 

2.4i 

r           2.32 

2.23 

2.15 

2.  08 

2.02 

5.16 

30.15 

53.85 

76.54 

98.38 

119.48 

139.96 

2.56 

*.4 

>                 2.  31 

2.22 

2.14 

2.08 

2.02 

7.72 

32.57 

56.16 

78.76 

100.52 

121.56 

141.98 

2.55 

2.4J 

2.30 

2.21 

2.14 

2.07 

2.0; 

10.27 

34.98 

58.46 

80.97 

102.66 

123.63 

143.99 

2.53 

2.4( 

>                 2.30 

2.20 

2.13 

2.06 

2.01 

12.80 

37.38 

60.76 

83.17 

104.79 

125.69 

146.00 

•„>  .  .-;,' 

2.  Si 

>    •             2.28 

2.20 

2.12 

2.06 

2.00 

15.32 

39.77 

63.04 

85.37 

106.91 

127.75 

148.00 

2.51 

2.  31 

t                 2.27 

2.19 

2.11 

2.05 

2.00 

17.83 

42.15 

65.31 

87.56 

109.02 

129.80 

150.00 

2.49 

2.S< 

r           2.27 

2./S 

2.10 

2.04 

1.99 

20.32 

!  44.51 

67.58 

89.74 

111.12 

131.84 

151.99 

2.4S 

2.  Si 

F                2.25 

2.17 

2.10 

2.04 

1.99 

22.80 

46.86 

,  69.83 

91.91 

113.22 

133.88 

153.97 

2.47 

2.3; 

f                 2.05 

2.16 

2.09 

2.03 

1.98 

25.27 

49.20 

72.08 

94.07 

115.31 

135.91 

155.95 

13,000 

14,000 

15,000 

16,000 

17,000 

18,000 

19,000 

267  .  60 

285.13 

302.42 

319.49 

336.36 

353.08 

369.61 

1  .76 

1.74 

1.72 

1.70 

1.68 

1.66 

•  1.64 

269  .  36 

286.87 

304.14 

321.19 

338.04 

354.74 

371.25 

1.76 

1.74 

1.71    \ 

1.69 

1.68 

1.66 

1.64 

271.12 

288.61 

305.85 

322  .  88 

339.72 

356.40 

372.89 

1.76 

1.74 

1.71 

1  .69 

1.68 

1  .66 

1.64 

272.88 

290.35 

307.56 

324  .  57 

341.40 

358.06 

374  .  53 

7.76 

1.73 

1.71 

1.69 

1.67 

1.66 

1.64 

274.64 

292.08 

309.27 

326.26 

343.07 

359.72 

376.17 

1.76 

1.73 

J.7/ 

1.69 

1.67 

1  .65 

1.63 

276.40 

293.81 

310.98 

327.95 

344  .  74 

361.37 

377  .  80 

1.75 

1.73 

1.71 

1.69 

1.67 

1.65 

1.63 

278.15 

295.54 

312.69 

329.64 

346.41 

363.02 

379.43 

1.75 

1  .72 

1.70 

1.68 

1.67 

1.65 

1.63 

279.90 

297  .  26 

314.39 

331.32 

348.08 

364.67 

381.06 

1.75 

1.72 

1.70  \ 

1.68 

1.67 

1.65 

1.63 

281.65 

298.98 

316.09 

333.00 

349.75 

366.32 

382.69 

1.7-4 

1.72 

1.70 

1.68 

1.67 

1.65 

1.63 

2  }3  .  39 

300.70 

317.79 

334.68 

351.42 

367.97 

384  .  32 

J.74 

1.72 

1.70 

1.68 

1.66 

1.64 

1.63 

235.13 

302.42 

319.49 

336.36 

353.08 

369.61 

385.95 

thermocouple  is  probably  suitable  for  measuring  temperatures  as  low  as 
20°  absolute  (—  253°  C.),  provided,  of  course,  that  an  adequate  calibration 
can  be  obtained.10 

Hoskins  Couple.11 — Table  4  as  given  represents  very  closely  the  obser- 
vations for  a  sample  of  wire  drawn  down  to  No.  20  B.  &  S.  (0.032  in.  or 
0.81  mm.)  and  then  annealed  by  heating  electrically  to  600°  for  a  few 
seconds. 


10  See  K.  Onnes:  Verslag.  Akad.  Wetenschappen  (1914)  23,  703. 

11  Of  the  two  wires  of  this  couple,  one  called  "chromel"  contains  10  per  cent 
chromium  and  90  per  cent,  nickel,  while  "alumel,"  the  other  wire,  contains  2  per  cent, 
aluminum  and  98  per  cent,  nickel. 


172        MEASURING  TEMPERATURES  WITH  THERMOCOUPLES 

TABLE  4. — Temperatures  and   Temperature  Differences  for  Chromel- 
alumel  Thermocouple* 


E 

mo 

-  o 

0 

10 

20 

30 

40 

0 

0.0 

0.0 

250.1 

490.5 

733.8 

991.3 

IS.  7 

IS.  6 

13.0 

11.9 

15.5 

15.5 

0.5 

-    12.7 

i2.6 

263.1  ' 

502.4 

i     746.3 

1004.8 

IS.  1 

12.4 

15.  fl 

15.0 

15.5 

IS.  6 

1.0 

-   25.8 

25.0 

276.0 

514.4 

758.8 

1018.4 

IS.  4 

12.3 

12.7 

12.0 

15.5 

15.  8 

1.5 

-  39.2 

37.3 

288.7 

526.4 

771.3 

1032.2 

IS.  8 

12.8 

15.5 

12.0 

15.  6 

13.  S 

2.0 

-   53.0 

49.5 

301.2 

538.4 

783.9 

1046.0 

14.4 

12.  S 

7*.  S 

15.0 

15.  6 

15.  9 

2.5 

-   67.4 

61.7 

313.5 

550.4 

796.5 

1059.9 

15.0 

12.1 

15.1 

15.0 

12.7 

14-1 

3.0 

-   82.4 

73.8 

325.6 

562.4 

809.2 

1074.0 

16.8 

12.1 

11.9 

15.1 

IS.  7 

14.  I 

3.5 

-  98.2 

85.9 

337.5 

574.5 

821.9 

1088.1 

16.8 

12.2 

11.6 

15.1 

IS.  7 

14.5 

4.0 

-115.0 

98.1 

349.1 

586  .  6 

834.6 

1102.3 

18.1 

12.2 

11.6 

12.1 

15.  S 

14.4 

4.5 

-133.1 

110.3 

360.6 

598.7 

847.4 

1116.7 

20  1 

12.4 

11.  S 

15.1 

12.8 

14.5 

5.0 

-153.2 

122.7 

372.1 

610.8 

860.2 

1131.2 

22.8 

15.  s 

11.7 

12.8 

15.  S 

14-5 

5.5 

-176.0 

135.2 

383.8 

623.0 

873.  1 

1145.7 

28.  5 

15.  ff  ' 

//.S 

15.5 

15.  fl 

14.  6 

6.0 

-204.5 

147.8 

395.6 

635.2 

886.0 

1160-.3 

12.8 

11.8 

12.2 

13.0 

14-  7 

6.5 

160.4 

407.4 

647.4 

899.0 

1175.0 

15.7 

11.8 

15.5 

13.0 

14.8 

7.0 

173.1 

419.2 

659.7 

912.0 

1189.8 

IS.  7 

11.8 

15.  S 

15.1 

14-* 

7.5 

185.8 

431.0 

672.0 

925.1 

1204.6 

12.8 

11.9 

15.5 

15.1 

15.0 

8.0 

198.6 

442.9 

684.3 

938.2 

(1220.0) 

12.8 

11.9 

15.  S 

13.8 

15.0 

8.5 

211.4 

454.8 

696.6 

951.4 

(1235.0) 

12.9 

£/.« 

IS.  4 

15.5 

16.0 

9.0 

224.3 

466.7 

709.0 

964.6 

(1251.0) 

15.0 

11.  s 

15.4 

15.5 

10.0 

9.5 

237.2 

478.6 

721.4 

977.9 

(1267.0) 

IS.  9 

11.  S 

15.  .4 

15.4 

lff.0 

10.0 

250.1 

490.5 

733.8 

991.3 

(1283.0) 

*  Standard  calibration  curve  for  chromel-alumel  (Hoskins)  thermocouple  giving 
the  temperature  and  temperature  differences  for  every  0.5  millivolt.  Fixed  junction 
is  at  0°.  ,  For  use  in  conjunction  with  a  deviation  curve  determined  by  calibration 
of  the  particular  couple  at  some  of  the  following  fixed  points: 


Degrt 

OS 

Milli- 
volts 
mo 

Degrees 

Milli- 
voltB 

me 

444 

18  07 

1062  6 

42  60 

Antimony,  melting  point  
Aluminum,  melting  point.  .  .  . 
Silver,  melting  point  ........ 

630 
658 
960 

0 

? 

25.79 
26.96 
38.83 

Copper  fin  air),  melting  point 
Copper  (pure),  melting  point.  . 

1065.0 
1082.8 

42.68 
43.31 

LEASON    H.    ADAMS 


173 


The  curve  of  e.m.f.  at  temperatures  above  100°  C.  is  so  nearly  a 
straight  line  that  a  table  representing  the  calibration  data  was  readily 
constructed  by  an  adjustment  of  differences.  The  remainder  of  the  table 
was  calculated  from  the  equation:  E  =  55.8071  -  3465  (1  -  e~* •**•"*). 
Although  the  values  extend  beyond  1200°,  only  couples  with  wires  several 
millimeters  in  diameter  will  last  very  long  at  temperatures  above  1000°; 
and  even  at  somewhat  lower  temperatures  the  e.m.f.  of  the  couples 
decreases  markedly  with  use.  The  only  remedy  is  frequent  recalibra- 
tioD.  It  is  evident  from  the  table  (and  also  by  inspection  of  Fig.  2),  that 
at  low  temperatures  the  Hoskins  couple  is  slightly  more  sensitive  than 
the  copper-constantan,  while  at  ordinary  temperatures  dE/dT  is  almost 
exactly  the  same  for  the  two  couples. 


30 


3>  20 

£ 

! 

I  15 


£  10 


2000        4000 


6000        8000        10,000      12,000      14,000      16,000      18,000 
E,  E.M.F.  in  Microvolts 


Fia.  3. — TYPICAL   DEVIATION   CURVE   FOR  COPPER-CONSTANTAN    THERMOCOUPLE, 

AS  DETERMINED  BY  CALIBRATION  OF  THE  PARTICULAR  ELEMENT  AT  THREE  POINTS. 
THE  OBSERVED  E.M.F.  MINUS  THE  E.M.F.  ACCORDING  TO  THE  STANDARD  TABLE  IS 
PLOTTED  AGAINST  THE  OBSERVED  E.M.F. 

Deviation  Curves. — Standard  tables  such  as  these  have  no  absolute 
significance;  they  are  merely  arbitrary  reference  curves  that,  although 
representing  fairly  well  the  temperature  e.m.f.  functions  for  certain 
couples,  are  intended  for  use  with  an  appropriate  deviation  <iurve.  An 
explanation  of  this  method  of  procedure  has  already  been  given  by 
Sosman12  and  by  Adams.13  The  correction  curve  is  determined  for 
each  element  by  calibration  at  several  of  the  fixed  points— preferably  three 
or  more — given  in  Table  1;  whence  it  is  simply  constructed  by  plotting 


12  R.  B.  Sosman:  Amer.  Jnl.  Sci.,  loc.  cit. 

13  L.  H.  Adams:  Jnl.  Amer.  Chem.  Soc.,  loc.  cit. 


174  MSUREAING   TEMPERATbxtES  WITH   THERMOCOUPLES 


as  ordinate  (AE  =  E  observed  minus  E  standard)  against  E^s.  as 
abscissa,  and  joining  up  the  various  points.  Then  in  order  to  obtain 
the  temperatures  corresponding  to  the  electromotive  force  indicated 
by  the  element,  the  appropriate  value  of  AE  (as  obtained  from  its 
deviation  curve  by  inspection)  is  subtracted  algebraically  from  the  ob- 
served value  of  E  before  the  latter  is  converted  into  degrees  by  means 
of  the  table.  It  is  obvious  that  the  required  accuracy  is  secured  by 
plotting  the  deviation  curve  on  a  small  scale;  a  sheet  of  coordinate  paper 
20  by  20  cm.  is  ample.  There  need  be  no  apprehension  of  error  in  the 
use  of  this  method  even  with  deviations  of  as  much  as  several  hundred 
microvolts;  especially  if  sufficient  calibration  points  are  taken  within  the 
specific  temperature  range,  and  if  the  deviation  curve  so  obtained  does 
not  depart  too  far  from  a  straight  line. 

To  illustrate  by  an  actual  case:  a  copper-constantan  couple  gave  at 
100°,  217.95°,  and  305.9°  e.m.f.  readings  of-  4280,  10,262,  and  10,529 
microvolts  respectively.  The  values  of  AE  are  therefore  4,  14,  and  26 
microvolts;  and  when  plotted  against  the  corresponding  values  of  E0bs., 
yield  the  deviation  curve  shown  in  Fig.  3.  Now,  suppose  that  at  a 
certain  temperature  the  couple  reads  9112  microvolts.  The  problem 
is  to  find  the  temperature  corresponding  to  this  e.m.f.  Referring  to  the 
figure,  we  find  that  the  deviation  at  9112  microvolts  is  12.  Subtracting 
12  from  9112  gives  9100  microvolts  as  the  "  standard"  e.m.f.,  and  from 
Table  3  it  is  seen  at  once  that  the  standard  e.m.f.  of  9100  corresponds 
to  196.51°,  which  is  the  required  temperature.  Thermocouples  are  often 
connected  to  instruments  provided  with  scales  graduated  in  degrees. 
For  such  cases,  these  standard  tables  are  of  little  use  except  in  so  far  as 
they  may  prove  useful  in  the  construction  of  the  scales. 

Fixed-junction  Correction.  —  Thermocouples  have  (effectively)  two 
junctions.  The  "business  end"  of  the  couple  is  usually  called  the  hot 
junction,  and  the  other  end  the  cold  junction.  Thk,  terminology  becomes 
confusing  when  a  couple  is  used  for  the  measurement  of  low  temperatures, 
.for  then  the  cold  junction,  although  still  in  an  ice  bath,  for  example,  is 
relatively  warm  compared  with  the  hot  junction,  which  may  be  immersed 
in  liquid  air.  It  would  seem  more  reasonable  to  call  the  two  ends  of  the 
thermocouple  the  fixed  junction  and  the  variable  junction.  It  irray  be 
claimed  that  the  fixed  junction  may  vary  and  that  the  variable  junction 
may  be  temporarily  fixed,  but  it  would  seem  that  after  all  it  is  an  im- 
portant privilege  of  the  variable  junction  to  vary,  and  it  is  the  duty  of  the 
fixed  junction  to  stay  more  or  less  fixed,  and  that  there  is  no  serious  danger 
of  confusing  the  two  ends  of  a  couple  if  they  are  so  named. 

The  calibration  tables  are  made  up  on  the  assumption  that  the  fixed 
junction  is  maintained  at  0°  C.,  which  in  the  long  run  is  the  mo^  con- 
venient and  satisfactory  procedure.  The  standard  method  now  is  to 
use  a  vacuum-jacketed  flask  filled  with  ice  into  which  is  inserted  the 


LEASON    H«,  ADAMS 


175 


thermocouple  junction  protected  by  a  glass  tube  closed  at  one  end  and 
partly  rilled  with  kerosene.  If  it  is  not  feasible  to  have  the  fixed  junction 
at  0°,  a  fixed-junction  correction  must  be  applied.  This  correction,  in 
general,  is  not  equal  to  the  temperature  of  the  fixed  junction  and  depends 
on  both  the  temperature  T0  of  the  fixed  junction  and  the  temperature  T 


10 


20  30  40  50  60  70  80 

Temperature  of  Fixed-junction,  Degrees  Centigrade 


00 


FIG.  4. — CURVES  FOR  OBTAINING  THE  FIXED-JUNCTION  CORRECTION  OF  PLATINUM- 
PLATINRHODIUM  THERMOCOUPLES.  FOR  A  GIVEN  OBSERVED  TEMPERATURE  (e.g.,  400°) 

CORRECTION  TO  BE  ADDED  TO,  OBSERVED  TEMPERATURE  IS  GIVEN  AS  FUNCTION  OF  TEM- 
PERATURE OF  FIXED  JUNCTION. 

of  the  variable  junctioft.     It  may  be  applied  by  any  one  of  the  following 
three  methods.14 

The  e.m.f.15  corresponding  to  T0  may  be  added16  directly  to  the  e.m.f. 
ET-TO  and  the  resultant  e.m.f.  ET,  converted  into  degrees  by  means  of 
the  proper  table  (Tables  2,  3,  or  4).  Thus  if  a  platinum-platinrhodium 
couple^gives  a  reading  of  6000/jy  (microvolts),  T0  being  50°,  the  value  of 
ETo,  according  to  Table  2,  is  298>iv,  which  added  to  6000  gives  6298  as 


14  C.  Offerhaus  and  E.  H.  Fischer:  Electrochem.  &  Met.  Ind.  (1908)  6,  362; 
P.D.  Foote:  U.S.  Bureau  of  Standards  Bull.  9  (1913)  553. 

16  Which  may  be  determined  from  Table  2,  3,  or  4,  in  conjunction,  when  necessary, 
with  the  proper  deviation  curve,  which  for  the  sake  of  simplicity,  we  may  assume  does 
not  devirAe  from  the  standard  curve. 

16  With  a  direct-reading  instrument  this  may  be  accomplished  mechanically  by 
changing  the  zero  of  the  instrument  so  that  when  short-circuited  it  indicates  the  fixed- 
junction  temperature. 


176 


the  value  of  ET,  which  by  referring  to  the  table  corresponds  to  T  =  703.6°. 
This  method  of  correction  is  mathematically  exact. 

By  another  method,  which  may  be  more  convenient  for  direct-reading 


10  20  30  40  50  60  70  80  90  100 

Temperature  of  Fixed-junction,  Degrees  Centigrade 

FIG.  5. — SIMILAR  TO  FIG.  4,  BUT  FOB  A  COPPBR-CONSTANTAN  THERMOCOUPLE. 


instruments,  the  correction  is  obtained  by  multiplying  the  fixed-junction 
temperature  by  the  factor  /  =  (dE/dT}0/  (dE/dT);  i.e.,  the  ratio  of  the 
slope  of  the  ET  curve  at  T  to  the  mean  slope  from  0  to  T0.  In  other 


100 


II » 


(iO 


40 


10  20  30  40  50  60  70  80  90          100 

Temperature  of  Fixed-junction,  Degrees  Centigrade 

FIG.  6. — SIMILAR  TO  FIG.  4,  BUT  FOR  A  CHROMEL-ALUMEL  (HOSKINS)  COUPLE. 


words,  the  true  temperature  T  may  be  obtained  from  the  expression : 
T  =  T'+  fT0,  T'  being  the  uncorrected  temperature.  The  slopes  of  the 
ET  curves  may  be  obtained  by  taking  the  reciprocals  of  the  numbers 
appearing  in  the  difference  columns  of  Tables  2, 3,  and  4.  Taking  the  same 


LEASON    H.    ADAMS  177 

example  as  given  above  under  (1),  the  apparent  temperature  T'  is  675.3° 
(Table  2).     Assuming  that  T  will  be  about  700°,  (dE/dT)  = 


10.53/iy  per  degree  and  (dE/dT)0  =  _  16__  =     5.96;   /   is,    therefore, 

5.96/10.53=  0.566.  Then  the  correction  is  0.566  X  50  =  28.3°,  which 
added  to  675.3°  gives  703.6°  as  the  true  temperature.  This  method 
yields  results  that  ordinarily  are  correct  to  within  a  few  tenths  of  a 
degree. 

The  third  method  for  fixed-junction  correction  is  a  graphical  one.  By 
means  of  the  curves  shown  in  Figs.  4,  5,  and  6,  the  correction  may  be 
determined  by  inspection.  In  these  diagrams  the  corrections  to  be 
added  to  the  uncorrected  temperatures  are  plotted  as  ordinates  and  the 
fixed-junction  temperatures  as  abscissas.  In  each  of  the  three  figures, 
curves  are  drawn  for  several  temperatures  (uncorrected)  of  the  variable 
junction.  As  an  example  of  the  use  of  these  curves,  suppose  that  the 
apparent  reading  of  a  copper-constantan  couple  is  250°  and  that  the 
fixed-junction  temperature  is  30°.  Interpolating  between  the  curves 
for  200°  and  300°  in  Fig.  5  shows  that  the  correction  is  21°.  The  true 
temperature  is  therefore  250  +  21  =  271°. 

Summary  and  Concluding  Remarks.  —  Three  kinds  of  thermocouples 
are  extensively  used  for  measuring  temperatures. 

1.  The  platinum-platinrhodium  couple  is  the  standard  for  laboratory 
measurements  between  300°  and  1700°,  and  when  properly  protected  it. 
has  been  successfully  employed  for  commercial  work.     It  is  usually 
free  from  noticeable  inhomogeneities  and  withstands  long  exposure  to 
high  temperatures  without  serious  deterioration;  but  it  is  subject  to  the 
disadvantages  of  relatively  low  sensitivity  and  of  high  initial  cost. 

2.  Copper-constantan  forms  the  most  satisfactory  combination  for 
use  over  the  range  from  300°  to  minus  200°  and  below.     It  is  inexpensive, 
several  times  more  sensitive  than  platinum-platinrhodium,  and  both 
metals  are  readily  obtainable  in  a  fairly  homogeneous  state  and  in  the 
form  of  wires  of  convenient  sizes.     Upon  exposure  in  air  to  temperatures 
above  300°,  both  the  copper  and  the  constantan  gradually  oxidize  and  the 
e.m.f.  of  the  couple  falls  off;  finally,  the  wires  become  brittle  and  fall  to 
pieces. 

3.  The  Hoskins  couple  (chromel-alumel)  is  an  important  one  for 
industrial  installations.     Of  all  the  base-metal  couples  it  is  the  most 
resistant  to  oxidation,  but  at  temperatures  above  a  red  heat  it  deterio- 
rates more  and  more  rapidly  as  the  temperature  is  increased,  so  that  its 
limit  of  usefulness  does  not  extend  beyond  1000°,  except  for  very  heavy 
wires  which  may  last  a  short  time  at  temperatures  as  high  as  1200° 
or  1300°. 

The  only  reliable  method  for  interpreting  the  e.m.f.  of  a  thermocouple 

12 


178        MEASURING  TEMPERATURES  WITH  THERMOCOUPLES 

in  terms  of  temperature  requires  a  calibration  at  certain  fixed  points  on 
the  temperature  scale.  For  interpolation  between  these  fixed  points,  much 
time  and  labor  can  be  saved  by  the  use  of  a  standard  table  in  conjunction 
with  a  deviation  curve  determined  for  the  particular  couple  by  calibra- 
tion at  several  (preferably  three  or  more)  temperatures.  In  this  paper 
such  tables  are  presented  for  platinum-platinrhodium  from  0°  to  1750°,  for 
copper-constantan  between  minus  200°  and  plus  400°,  and  chromel-alumel 
(Hoskins)  from  minus  200°  to  plus  1200°.  These  tables  are  merely  arbi- 
trary reference  curves  that  give  temperatures  -corresponding  to  a  series  of 
true  e.m.f.'s,  i.e.,  e.m.f.'s  as  read  by  a  potentiometer  or  other  compensating 
device  using  a  galvanometer  as  a  null-instrument,17  and  are  to  be  used 
only  in  conjunction  with  an  appropriate  deviation  curve. 

17  Such  instruments  are  much  more  reliable  than  the  ordinary  mill  i voltmeter  with 
either  a  temperature  or  millivolt  scale. 


A   REFERENCE   STANDARD    FOR  BASE-METAL   THERMOCOUPLES         170 


A  Reference  Standard  for  Base -metal  Thermocouples 

BY    N.    E.    BONN,*  B.  SC.   IN  E.  E.,    PHILADELPHIA,    PA. 
(Chicago  Meeting,  September,  1919) 

IT  is  well  known  that  most  of  the  materials  entering  into  the  manu- 
facture of  thermocouples  are  subject  to  variations  in  their  thermoelectric 
characteristics,  the  chief  causes  of  which  are:  differences  in  chemical 
composition;  the  previous  history,  which  includes  mechanical  working, 
aging,  oxidation,  and  contamination;  and  mode  of  use,  such  as  depth  of 
immersion,  etc. 

In  the  case  of  a  very  extensively  used  thermoelement,  the  iron-con- 
stantan  couple,  neither  iron  nor  constantan  is  absolutely  uniform,  and 
a  number  of  checks  are  made  during  the  process  of  manufacture.  As 
both  are  subject  to  diversion  from  a  standard  value,  neither  can  be  used 
for  checking  the  other.  It  is,  therefore,  necessary  to  find  a  third  metal 
against  which  iron  and  constantan  may  be  checked.  It  was  to  find  such 
a  metal,  that  the  present  investigation  was  undertaken. 

To  be  a  reliable  checking  standard,  the  metal  must  possess  a  fairly 
high  melting  point,  it  must  be  obtainable  in  sufficient  purity,  and  it 
must  be  uniform  and  constant  with  respect  to  its  thermoelectric  qualities. 
The  choice  is,  as  a  result,  limited  to  a  few  metals,  such  as  copper,  gold, 
and  silver. 

Copper  has  the  advantage  of  low  cost,  which  makes  it  possible  to  use 
each  wire  only  once  and  thus  avoid  any  variations  due  to  continued  use, 
although  previous  investigators  have  found1  that  continued  use  does  not 
affect  the  calibration  of  a  copper-constantan  thermocouple.  Copper 
also  has  the  advantage  of  a  large  thermal  electromotive  force  against 
constantan  and  would  be  desirable  as  a  checking  standard  if  it  could  be 
shown  that  it  possessed  the  proper  thermoelectric  characteristics.  The 
investigation,  accordingly,  resolved  itself  into  three  phases:  the  uni- 
formity of  commercial  electrolytic  copper,  the  effect  of  aging  and  method 
of  annealing,  and  the  calibration  of  the  copper-constantan  thermocouple. 

Before  describing  the  experiment,  it  may  be  well  to  mention  that  all 
possible  precautions  were  taken  to  reduce  the  errors  of  observation  to  a 
minimum.  The  apparatus  used  consisted  of  a  hand-regulated  vertical 
electric  furnace,  a  Leeds  &  Northrup  Type  K  potentiometer,  a  moving- 
coil  galvanometer  of  high  microvolt  sensitivity,  and  a  multiple  switch, 
which  made  it  possible  to  check  simultaneously  several  pieces  of  copper 
against  one  piece  of  constantan.  Readings  were  taken  only  after  the 
furnace  was  steady  for  at  least  10  min.  and  only  those  readings  recorded 
that  did  not  change  after  three  consecutive  trials.  The  cold  junction 

*  Research  Engineer,  Leeds  &  Northrup  Co. 
1  U.  S.  Bureau  of  Mines  Bull.  145  (1918). 


180         A   REFERENCE   STANDARD     FOR  BASE-METAL   THERMOCOUPLES 

was  maintained  at  0°  C.  by  means  of  a  bath  especially  constructed  for  the 
purpose. 

In  order  to  avoid  using  the  same  constantan  wire  all  the  time,  several 
wires  were  cut  from  the  same  coil  and  checked  against  the  same  piece  of 
copper  wire  at  a  temperature  of  about  1400°  F.  (760°  C.).  As  no  appreci- 
able difference  in  the  resulting  electromotive  force  could  be  found,  these 
wires  were  arbitrarily  chosen  as  "standard  constantan"  and  later  used  in 
the  course  of  the  investigation  as  such. 

The  question  of  uniformity  of  commercial  copper  wire  involved  the 
testing  of  many  samples  coming  from  widely  different  sources.  Some 
were  obtained  from  the  storerooms  of  the  Leeds  &  Northrup  Co.,  others 
came  from  various  makers  of  copper  wire,  still  others  were  drawn  into 
wire  from  different  pieces  of  copper  scrap.  Wires  from  fifteen  sources, 
all  previously  annealed,  were  tested  under  exactly  the  same  conditions 
at  1300°  F.  (704°  C.)  against  the  "standard  constantan "  mentioned.  No 
differences  in  the  electromotive  forces  greater  than  that  equivalent  to 
0.5°  F.  (0.28°  C.)  were  observed. 

The  study  of  the  effect  of  aging  presented  some  difficulty,  as  wire 
known  to  be  very  old  could  not  be  obtained.  Artificial  aging  at  higher 
than  room  temperatures  caused  no  changes,  and  it  seems  to  be  safe  to 
conclude  that  the  same  is  true  for  gradual  aging  at  room  temperatures. 
In  any  event,  wire  that  was  known  to  have  been  received  from  the  maker 
over  a  year  before  testing  gave  the  same  e.m.f .  as  new  wire.  The  ques- 
tion, however,  is  being  left  open  and  the  affect  of  aging  will  be  studied 
at  greater  length  when  wire  that  has  been  put  away  for  the  purpose  is 
tested  some  time  in  the  future. 

In  order  to  determine  whether  the  method  of  cooling  after  annealing 
had  any  effect  on  the  thermal  e.m.f.  of  copper,  three  samples  were  heated 
to  1500°  F.  (816°  C.)  and  allowed  to  cool  in  various  ways.  One  was 
plunged  directly  into  cold  water,  another  was  permitted  to  cool  in  air  at 
room  temperature,  and  the  third  was  cooled  over  night  in  the  electric 
furnace.  There  resulted  no  appreciable  variation  in  the  thermal  e.m.f. 
of  the  three  wires. 

The  foregoing  investigation  seems  to  fully  warrant  the  conclusion 
that  copper  is  an  ideal  standard  for  checking  constantan  and  other  base- 
metal  thermoelements,  as  it  can  be  easily  obtained  in  the  electrolytic 
form,  which  appears  to  have  the  same  thermoelectric  properties  regardless 
of  its  origin. 

It  is  stated  by  a  number  of  authorities  that  copper  is  not  suitable  for 
pyrometric  purposes  at  even  moderately  high  temperatures.  Dr.  Burg- 
ess would  limit  its  use  to  600°  C.,  while  Dr.  Griffith  maintains  that  300°  C. 
is  the  limit  for  copper.  This  in  no  way  contradicts  our  conclusions, 
since  the  limits  suggested  refer  only  to  continuous  use,  due  to  oxidation 
and  short  life,  but  for  checking  purposes,  where  a  wire  may  be  used  only 
once  or  a  very  few  times,  there  is  no  apparent  reason  why  copper  could 
not  be  used  up  to  900°  Centigrade. 


ALLOYS    SUITABLE    FOR    THERMOCOUPLES  181 


Alloys  Suitable  for  Thermocouples  and  Base- metal 
Thermoelectric  Practice 

BY  J.   M.   LOHR,*   PH.  D.,   DETROIT,   MICH. 
(Chicago  Meeting,  September,  1919) 

THE  characteristics  and  uses  of  thermocouples  of  platinum  and  the 
platinum  alloys  being  so  well  known,  this  paper  will  be  confined  to  base- 
metal  couples.  During  the  past  decade,  there  has  developed  a  strong 
demand  for  thermocouples  made  of  metals  cheaper  than  the  platinum 
alloys,  and  suitable  for  industrial  heat  measurements.  Much  research  has 
been  done  but  only  a  very  limited  number  of  alloys  have  been  found  to 
possess  the  necessary  characteristics  for  general  technical  use. 

In  the  development  of  thermocouple  alloys  it  is  necessary  to  consider 
the  following  points:  The  electromotive  force  and  temperature  relations; 
permanency  or  constancy;  reproducibility;  manufacturing  difficulties; 
durability;  and  accuracy  in  use. 

The  electromotive  force  developed  by  the  thermocouple  should  in- 
crease with  the  rise  of  temperature  according  to  some  definite  law.  It 
is  desirable  that  the  electromotive  force  should  vary  directly  with  the 
temperature  difference  of  its  junctions,  in  other  words,  giving  a  curve 
linear  or  nearly  so.  It  is  also  desirable  that  the  resulting  electromotive 
force  be  as  large  as  possible  for  any  given  range  of  temperature,  thereby 
insuring  greater  accuracy  of  readings. 

The  materials  composing  the  thermocouple  must  be  perfectly  homo- 
geneous and  must  remain  so  under  continued  use.  When  parasitic  cur- 
rents develop  in  a  couple  or  a  couple  deteriorates,  its  usefulness  is  at 
once  seriously  impaired.  Everyone  who  has  had  any  experience  in  the 
use  of  thermocouples  knows  the  extreme  vigilance  and  continual  checking 
and  rechecking  required  to  guard  against  deterioration  and  parasitic 
currents.  Parasitic  currents  are  probably  the  most  troublesome  phase 
of  thermocouple  work  to  deal  with.  Their  real  cause  is  not  clear.  They 
seem  to  be  due  to  several  causes,  resulting  both  from  the  manufacture  of 
the  wire  and  the  uses  of  the  couple  under  different  conditions.  It  is 
believed  that  possible  segregation  and  cavities  of  occluded  gas  in  the 
casting,  as  well  as  the  crystal  structure  due  to  improper  annealing,  on 
the  one  hand,  and  the  effect  of  gases  and  contaminating  materials  on  the 
outside  surface  of  the  couple  when  in  use  may  be  contributing  causes. 

*  Superintendent  of  Foundry,  Hoskins  Manufacturing  Co. 


182  ALLOYS   SUITABLE   FOR   THERMOCOUPLES 

When  the  couples  of  an  installation  burn  out,  it  is  necessary  that  new 
elements  or  couples  of  the  same  quality,  characteristics,  and  niilli voltage 
range  be  available  for  replacement.  This  necessitates,  on  the  part  of 
the  manufacturer,  the  ability  to  reproduce  the  alloys  in  quantity  at  will. 

Investigations  may  develop  alloys  with  all  the  necessary  qualities  and 
characteristics  for  a  satisfactory  thermocouple,  yet  the  difficulties  of 
manufacturing  them  on  a  production  basis  may  present  almost  insur- 
mountable difficulties.  Doubtless,  few  users  of  thermocouples  realize 
fully  the  infinite  pains  and  care  necessary  in  the  production  of  highly 
accurate  thermocouple  wire.  Of  all  the  classes  and  types  of  alloy  work, 
this  probably  meets  with  the  widest  range  of  difficulties.  The  selection 
of  the  grades  of  raw  materials  going  into  the  alloys,  the  accuracy  of  the 
composition,  the  methods  of  melting,  deoxidizing,  casting,  rolling,  anneal- 
ing, and  final  calibration  must  be  studied  and  followed  to  the  minutest 
detail,  and  any  slight  variation  in  any  of  these  steps  will  very  likely  cause 
serious  trouble. 

The  user  is  interested  primarily  in  the  life  of  the  couple  he  has  installed 
and  its  continued  accuracy  during  this  life.  In  technical  operations, 
it  is  highly  important  that  a  couple  should  have  a  reasonably  long  life, 
thereby  obviating  the  necessity  and  expense  of  replacements.  To  be 
dependable,  the  results  must  be  absolutely  accurate  and  reliable. 

Originally,  out  of  the  great  number  of  alloys  upon  which  experiments 
were  made  for  base-metal  couples,  one  composed  of  commercial  nickel 
and  commercial  iron  seemed  to  offer  the  best  possibilities,  but  certain 
characteristics  of  each  metal  prevented  these  alloys  from  giving  the 
accuracy  necessary.  For  instance,  as  is  well  known,  nickel  under- 
goes a  molecular  transformation  between  446°  F.  (230°  C.)  and  734°  F. 
(390°  C.)  which  makes  it  unsuited  for  thermoelectric  work  over  this  range.' 
However,  it  may  be  used  from  752°  F.  (400°  C.)  to  1652°  F.  (900°  C.). 
Iron  is  subject  to  the  development  of  heavy  parasitic  currents.  To  replace 
the  iron  element,  nickel-chromium,  known  commercially  under  the  trade 
name  "chromel,"  was  developed.  This  alloy  (10  per  cent.  Cr)  generates 
the  highest  negative  electromotive  force  of  any  alloy  suitable  for  a  ther- 
mocouple. It  proved  to  be  so  much  superior  to  iron  that  a  correspond- 
ing element  to  replace  nickel  had  to  be  developed.  A  nickel-silicon  alloy 
was  tried  but  later  discarded  on  account  of  its  becoming  brittle  with  use. 
Pure  nickel-aluminum  stood  up  well  at  high  temperatures,  but  it  too  had 
to  be  discarded  on  account  of  its  becoming  brittle  with  use  at  low  tempera- 
tures; however,  nickel-aluminum  with  some  modifications  was  found 
to  give  a  perfectly  satisfactory  element.  This  alloy  is  known  commer- 
cially as  "alumel."  For  lower  ranges  of  temperature,  nickel-copper  was 
developed,  to  be  used  with  nickel-chromium,  but  in  the  development  of 
this  alloy  it  was  found  that  increasing  the  percentage  of  copper  decreased 
the  life  of  the  alloy. 


J.    M.    LOHR  183 

BASE-METAL  COUPLES  IN  GENERAL  USE 

At  the  present  time,  there  are  in  general  use  the  following  base-metal 
couples:  (1)  Nickel-chromium  (chromel) — nickel-aluminum  (alumel)  for 
temperatures  up  to  2500°  F.  (1370°  C.).  (2)  Iron — constantan  for  tem- 
peratures up  to  possibly  1800°  F.  (982°  C.).  (3)  Nickel— nickel-iron- 
chromium,  for  temperatures  up  to  probably  1500°  F.  (815°  C.),  but 
which  cannot  be  produced  with  sufficient  accuracy  to  warrant  extensive 
use.  (4)  Nickel-iron-chromium  (chromel  X) — nickel-copper  (copel)  for 
temperatures  up  to  1000°  F.  (537°  C.). 

The  possibility  of  duplicating  the  exact  millivoltage  values  in  the 
manufacture  of  chromel,  alumel,  and  copel  as  well  as  iron  and  con- 
stantan, has  led  to  a  departure  from  the  original  methods  of  manufacturing 
thermocouples.  Coils  of  the  proper  class  of  wire,  having  a  definite 
standard  millivoltage,  are  now  supplied,  and  couples  can  be  made  as 
needed,  without  further  calibration.  This  method  of  supplying  thermo- 
couples has  found  a  ready  acceptance  with  the  users  of  such  apparatus. 

Of  the  various  base-metal  thermocouples,  the  chromel-alumel  couples 
have  undoubtedly  received  the  widest  use  and  are  generally  considered 
the  most  accurate  and  durable.  A  large  motor  corporation  that  has  in- 
stalled these  alloys  in  all  of  its  plants  states  that  it  has  obtained  exceptional 
results  both  in  accuracy  and  durability  and  finds  no  difficulty  in  main- 
taining a  system  accuracy  of  better  than  15°  F.  (8°  C.),  with  95  per  cent, 
of  their  couples  well  within  10°  F.  (5°  C.).  This  is  undoubtedly  a  high 
degree  of  accuracy  and  probably  as  high  as  it  is  possible  to  obtain  com- 
mercially with  any  material.  A  large  plate-glass  manufacturer  states 
that  in  annealing  glass  at  about  2000°  F.  (1093°  C.)  chromel-alumel  couples 
give  more  accurate  results  than  the  platinum-platinum-rhodium  couple. 
As  to  the  value  of  chromel-alumel  thermocouples  for  laboratory  work, 
it  might  be  of  interest  to  mention  that  a  prominent  educational  institution 
states  that  these  couples,  used  unprotected,  as  is  often  necessary  in  re- 
search work,  are  less  liable  to  alteration  than  the  best  platinum,  over  the 
same  range,  up  to  1472°  F.  (800°  C.). 

THREE  MAIN  FACTORS  IN  THERMOELECTRIC  PRACTICE 

General  thermoelectric  practice  involves  mainly  three  things :  Proper 
protection  of  the  hot  end  of  the  couple;  proper  protection  of  the  cold 
end  of  the  couple;  and  proper  care  of  the  thermocouple  itself. 

Owing  to  the  great  susceptibility  to,  and  danger  of  contamination  from 
oxidizing  and  reducing  gases,  particularly  at  high  temperatures,  all  ther- 
mocouples should  be  protected  in  some  manner.  The  writer  will  not 
go  into  this  phase  of  the  subject,  as  this  is  to  be  discussed  in  another 
paper  in  this  symposium. 


184  ALLOYS    SUITABLE    FOR   THERMOCOUPLES 

Methods  of  Controlling  Temperature  at  Cold  End. — Inasmuch  as  the 
pyrometer  indicates  not  the  actual  temperature  of  the  hot  end  of  the  cou- 
ple but  rather  the  difference  in  temperature  between  the  cold  and  hot 
ends,  it  is  very  evident  that  the  temperature  at  the  cold  end  should  be 
known  and  kept  as  constant  as  possible.  Various  methods  are  em- 
ployed for  this  purpose.  The  method  chosen  depends,  of  course,  on 
the  kind  of  installation  and  local  conditions,  but  the  main  object  is  to 
locate  the  cold  end  at  such  a  point  that  it  is  free  from  any  disturbing 
source  of  heat. 

The  methods  used  in  commercial  installations  for  controlling  the 
temperature  at  the  cold  end  include : 

1.  Wells,  in  which  the  cold  end  is  located  8  ft.  (2.4  m.)  or  more  under- 
ground and  any  distance  from  the  couple. 

2.  A  Thermos  bottle  placed  at  some  convenient  location  in  the  plant 
and  used  with  the  same  system  of  connections  as  the  well. 

3.  Water  and  steam  jackets  in  which  running  water  or  steam  passes 
around  the  cold  end  maintaining  a  constant  temperature. 

4.  The  thermostat  cold-end  box,  which  depends  on  the  expansion  of  a 
metal  strip  making  an  electrical  contact,  thereby  lighting  an  electric  bulb 
which,  in  turn,  maintains  the  proper  temperature  in  the  box. 

5.  A  device  consisting  of  four  resistances  connected  after  the  manner 
of  the  Wheatstone  bridge,  with  the  thermocouple  connected  in  series  in 
one  of  the  four  resistances.     A  copper  resistance  coil  is  also  connected 
at  the  cold  end  in  series  with  the  couple  and  in  the  same  arm  of  the  bridge 
as  the  couple.     The  resistance  is  adjusted  so  that  when  the  cold  end  is  at 
the  temperature  for  which  the  meter  is  set,  no  current  from  the  battery 
flows  through  the  meter.     When  the  cold  end  is  heated,  the  bridge  is 
thrown  out  of  balance  enough  to  compensate  for  the  cold-end  error. 
This  involves  adjusting  a  resistance  in  series  with  the  battery  to  make  the 
compensation  correct.     Such  adjustment  must  be  done  very  frequently 
and  is  a  disadvantage  to  this  method.     The  adjustment  is  accomplished 
by  substituting  a  coil  of  low-temperature  coefficient  wire  for  the  couple 
and  compensating  coil.     Then  the  resistance  in  series  with  the  battery 
is  adjusted  to  make  the  meter  read  to  a  definite  mark. 

6.  An  automatic  mercury-bulb  compensator,  consisting  of  a  small 
glass  bulb  and  capillary  tube  containing  mercury,  into  which  a  loop  of 
fine  platinum  wire  dips.     This  is  inserted  in  the  thermoelectric  circuit 
near  the  cold  junction.     The  mercury  expands  or  contracts  under  tem- 
perature changes,  cutting  in  or  out  resistance  in  the  circuit.     This  acts 
in  opposition  to  the  change  in  electromotive  force  with  temperature  at 
the  cold  end,  so  that  a  balance  may  be  established.     But  this  gives  a 
percentage  compensation  for  an  addition  error,  or  in  other  words,  com- 
pensates properly  at  one  point,  giving  too  low  a  compensation  at  low 
temperature  and  over  compensates  at  high  temperatures. 


J.    M.    LOHR  185 

7.  A  method  in  use  where  the  cold  ends  can  be  brought  close  to  the 
meter,  consists  of  a  device  composed  of  a  compound  strip  of  two  metals 
having  unequal  coefficients  of  expansion,  attached  to  the  spring  control- 
ling the  pointer,  so  that  the  reading  of  the  meter  is  the  temperature  of  the 
surroundings  when  no  current  is  flowing. 

For  portable  work,  the  cold  end  is,  of  course,  at  the  meter,  in  which 
case  a  thermometer  on  the  meter  is  necessary,  so  that  the  zero  setting 
may  be  accurately  made. 

Of  all  the  systems  in  use  for  controlling  the  temperature  of  the  cold 
end,  the  well  system  probably  finds  the  greatest  general  use  and  is  con- 
sidered by  many  the  most  satisfactory,  although  there  may  be  special 
cases  where  some  one  of  the  other  methods  could  be  used  with  good 
results.  The  chief  disadvantage  of  this  system  is  the  cost  of  "lead 
wires, "  particularly  in  a  large  building  where  the  meter  must  be  placed  a 
long  distance  from  the  thermocouple.  In  such  cases,  it  is  undoubtedly 
much  cheaper  to  use  the  Thermos  bottle,  which,  all  things  considered,  is 
probably  most  satisfactory,  next  to  the  well  system.  In  the  use  of  the 
thermostat  cold-end  box,  the  electrical  connections  may  become  broken ; 
and  with  the  use  of  the  water  and  steam  jacket,  a  leak  may  cause  the  flow 
of  water  or  steam  to  discontinue,  in  either  case  affecting  the  temperature 
of  the  cold  end  to  such  an  extent,  possibly,  that  the  product  of  a  large 
bank  of  furnaces  may  be  damaged  before  the  operator  has  had  time  to 
notice  it.  In  general,  the  well  system  is  more  applicable  to  high-resist- 
ance equipment,  whereas  for  the  low-resistance,  providing  several  couples 
are  to  be  used  with  one  meter,  the  cold  ends  are  probably  best  taken  care 
of  by  using  a  water  jacket  in  connection  with  each  one. 

THERMOCOUPLE   EXTENSIONS 

This  brings  up  the  subject  of  thermocouple  extensions,  or  "leads" 
as  they  are  commonly  called.  Such  extensions  are  principally  used  with 
high-resistance  installations.  They  cannot  be  used  for  cold-end  regula- 
tion with  low-resistance  couples,  because  the  amount  of  wire  required 
would  offer  too  high  resistance.  As  is  well  known,  the  extension  wires 
merely  serve  the  purpose  of  lengthening  out  the  couple,  and  may  be  made 
of  a  different  alloy  from  that  of  the  couple,  as  copper  nickel,  properly 
calibrated,  or  they  may  consist  of  the  same  alloy  as  that  of  the  couple, 
but  in  either  case  of  a  much  smaller  diameter  to  avoid  excessive  cost. 

Various  methods  of  connecting  a  series  of  thermocouples,  by  means  of 
extension  leads  through  the  cold-end  well  or  Thermos  bottle  to  the  meter, 
are  in  use.  The  manufacturers  of  chromel-alumel  thermocouples  are 
using  four  methods  with  very  satisfactory  results. 

One  well  or  Thermos  bottle  may  be  provided  for  each  couple.  This 
method  is  of  advantage  where  the  furnaces  are  situated  over  a  large 


186 


ALLOYS   SUITABLE   FOR  THERMOCOUPLES 


area,  because  it  reduces  the  amount  of  extension  leads  to  a  minimum,  but 
there  is  the  cost  of  providing  a  number  of  wells  or  Thermos  bottles.  With 
this  method,  the  couple  may  be  of  the  usual  length,  with  a  flexible  ex- 
tension of  the  same  alloy  through  the  well  or  bottle,  and  then  continued 
with  copper  through  the  switch  to  the  meter. 

A  second  method  is  to  take  care  of  all  the  thermocouples  with  one  well 
or  Thermos  bottle.  This  method  should  be  used  when  the  furnaces  are 
close  together  and  the  meter  is  located  a  great  distance  from  them  and 


FIG.  1.- 


/  z  j  +  L——'       -weu. 

-WIRING  DIAGRAM  FOR  ONE  COUPLE  EXTENSION  CIRCUIT. 


also  where  it  would  be  impossible  to  drill  individual  wells  to  have  the 
same  temperature.  In  other  respects  this  is  identical  with  the  first 
method. 

A  third  method  employs  a  "common"  cold  end.  Only  one  couple 
extension  circuit  is  run  into  the  well  to  take  care  of  the  cold  end  of  all  the 
couples,  Fig.  1.  All  the  thermocouple  extensions  terminate  at  one 
point,  called  a  " junction  box,"  insuring  the  same  temperature  to  all 
connections  within.  It  will  be  noted  on  the  diagram  that  the  couples 
are  connected  by  extension  leads  to  the  junction  box..  Extension  leads 
of  the  same  alloy  also  connect  the  junction  box  to  the  well,  and  connec- 
tions from  the  junction  box  through  the  multipoint  switch  to  the  meter 
are  made  with  copper. 

A  fourth  method,  which  is  very  similar  to  the  one  just  described,  is  to 
run  the  couple  extensions  to  a  switch,  which  takes  the  place  of  the  junc- 
tion box,  Fig.  2.  Only  one  well  is  required;  this  holds  the  common  cold- 


J.    M.    LOHR 


187 


end  extension.  This  combination  may  be  used  when  the  meter  and 
switch  are  located  close  to  the  couples  and  where  the  furnaces  are  com- 
pactly placed. 


Kt-L 


p 


t°*s 

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2«==ciM 

±HMbh 

««w-i  I     ;     I 


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1 


WfJLL 


FIG.    2.  —  INSTALLATION    OF    ONE     WELL;    COUPLE     EXTENSION    WIRES    RUN    TO 

SWITCH. 


CARE  OF  THERMOCOUPLES 

The  results  from  base-metal  couples,  considering  the  great  number 
in  use,  have  been  highly  satisfactory;  and  given  equal  care  with  that  of 
the  platinum  couple,  will  probably  afford  the  user  equal  satisfaction  on 
the  basis  of  cost.  But  in  many  cases  more  care  should  be  exercised  in 
the  general  handling  and  use  of  base-metal  couples,  if  one  may  judge  from 
the  appearance  of  many  of  those  that  are  returned  to  the  factory  for  re- 
pairs. Covered  with  dirt  and  furnace  material,  they  are  frequently  almost 
unrecognizable.  To  obtain  the  best  results,  every  pyrometer  installa- 
tion should  be  checked  periodically,  preferably  once  a  month,  and  a  sys- 
tematic record  should  be  preserved.  The  methods  to  be  used,  whether 
by  means  of  a  check  couple  or  by  the  melting  points  of  pure  metals,  arc 
too  well  known  to  need  mention  here. 


188  RECENT  IMPROVEMENTS  IN  PYROMETRY 


Recent  Improvements  in  Pyrometry 

BT   R.    P.   BROWN,*    PHILADELPHIA,    PA. 
(Chicago  Meeting,  September,  1919) 

To  gain  some  idea  of  the  progress  recently  made  in  the  measurement 
of  high  temperatures,  we  must  review  the  temperature-measuring  devices 
of  the  past.  As  far  back  as  1782,  Wedge  wood,  a  famous  potter  in  Eng- 
land, attempted  to  measure  his  kiln  temperatures  by  means  of  clay 
trials,  or  test  pieces,  that  indicated  the  expansion  or  contraction  that 
occurred  with  certain  changes  in  temperature.  He  also  produced  cones 
of  clay,  formed  of  various  mixes,  to  form  a  whole  series  for  the  range 
of  temperature  met  with  in  firing  clay  products.  These  cones  are  still 
extensively  used  to  measure  kiln  temperatures  in  the  pottery  industry. 
Cones  are  affected  not  only  by  time  but  by  temperature  and  the  rate 
of  firing;  consequently,  they  are  not  an  accurate  measure  of  temperature. 
In  addition,  it  has  been  common  practice  for  years  to  attempt  to  measure 
temperatures  with  fusible  salts.  Capsules  of  these  are  inserted  in  the 
furnace  and  indicate,  by  melting,  when  a  certain  temperature  has  been 
attained.  While  such  salts  cannot  be  considered  a  precision  form  of 
temperature-measuring  device,  they  have  proved  reasonably  accurate 
in  checking  temperature.  . 

MERCURIAL  THERMOMETERS 

Mercurial'  thermometers  have  been  known  for  years  as  a  standard  de- 
vice for  measuring  moderate  temperatures.  The  mercurial  thermometer 
for  temperatures  up  to  600°  F.  (316°  C.)  was  furnished  with  a  vacuum 
above  the  mercury  column.  For  temperatures  above  the  boiling  points 
of  mercury,  674°  F.  (356°  C.),  it  is  supplied  with  the  mercury  column 
under  pressure.  If  a  mercurial  thermometer  is  graduated  to  1000°  F. 
(538°  C.),  there  is  some  200  Ib.  gas  pressure  applied  above  the  mercury 
column  to  prevent  the  mercury  boiling.  The  thermometer  manufac- 
turers have  shown  very  little  progress,  of  late,  in  producing  a  high-tem- 
perature mercurial  thermometer.  This  is  apparently  due  to  the  fact 
that  glass  tubing  will  soften  at  temperatures  much  above  1000°  F.;  also 
the  pressure  necessary  to  prevent  boiling  becomes  excessive.  But  it 
would  seem  that  with  the  progress  being  made,  these  maximum  tem- 
peratures for  a  mercury  thermometer  can  be  increased  by  the  develop- 


President,  The  Brown  Instrument  Co. 


R.    P.   BROWN  189 

ment  of  glass  with  a  considerably  higher  softening  point  and  of  sufficient 
strength  to  apply  all  the  required  pressure  to  prevent  boiling  of  the 
mercury  at  a  considerably  higher  temperature  than  1000°  F.  (538°  C.). 

EXPANSION  PYROMETERS 

Mechanical  pyrometers  operating  through  the  difference  in  expansion 
of  iron  and  brass,  or  iron  and  graphite,  have  been  manufactured  for  75 
years.  A  steel  tube  containing  a  rod  of  graphite  extends  into  the  furnace 
and  by  multiplying  the  difference  in  expansion  that  takes  place  and  the 
movement  of  a  pointer  across  the  dial  of  the  instrument,  an  indication 
of  temperature  is  secured.  Approximately  0.009  in.  (0.228  mm.)  dif- 
ference in  expansion  occurs  for  every  100°  F.  (56°  C).  rise  in  temperature 
to  800°  F.  (427°  C.).  This  type  of  pyrometer  has  always  had  a  tendency 
to  change  in  its  readings  with  time,  particularly  if  used  for  temperatures 
above  800°  F.,  due  to  constant  heating  and  cooling  of  the  metal  tube. 
This  occasions  readjustments  of  the  pointer  to  compensate  for  this  error. 

Mechanical  pyrometers  are  today  still  extensively  used  for  measuring 
the  temperature  in  bread-baking  and  core-drying  ovens,  where  accurate 
temperature  measurements  are  not  required,  and  where  a  temperature 
indication  within  25°  F.  plus  or  minus  is  satisfactory.  By  annealing 
the  steel  tubes  at  a  considerably  higher  temperature  than  they  will  be 
subjected  to,  and  for  quite  a  length  of  time,  it  has  been  found  possible  to 
partly  eliminate  the  error  that  would  otherwise  result,  due  to  constant 
heating  and  cooling  of  the  expansion  pyrometer.  Furthermore,  annealing 
the  graphite  rods  has  had  a  beneficial  result.  This  type  of  pyrometer 
cannot  be  recommended  where  accurate  temperature  measurements  are 
required  and  for  use  at  temperatures  above  1200°  F.  (648°  C.). 

GAS  THERMOMETERS 

The  gas  or  air  thermometer,  another  form  of  expansive  instrument, 
was  a  device  originally  used  to  determine  the  true  temperature  scale; 
it  is  only  comparatively  recently  that  this  type  of  instrument  has  been 
used  to  any  extent  to  measure  temperatures  commercially.  A  good  form 
for  industrial  use  consists  of  a  copper  bulb  containing  nitrogen  gas  con- 
nected to  the  recording  gage  by  a  small-bore  capillary  tube  which  is  pro- 
tected by  a  heavy  flexible  armored  tube.  The  recording  gage  contains 
the  usual  form  of  Bourdon  spring  used  in  steam  gages.  When  the  bulb 
is  heated,  the  gas  expands  and  the  pressure  applied  along  the  capillary 
tube  expands  the  spring  in  the  instrument  and  causes  the  pointer  to  move 
across  the  scale  or  chart.  For  temperatures  of  800°  F.  (426°  C.),  ap- 
proximately 150  Ib.  (68  kg.)  pressure  is  exerted  on  the  spring  for  the  full 
scale  reading.  In  consequence,  a  heavy  spring  can  be  used  and  the  in- 
strument is  exceedingly  robust  in  construction. 


190  RECENT  IMPROVEMENTS  IN  PYROMETRY 

By  substituting  a  bulb  of  pure  nickel  for  the  copper  bulb  or  by  apply- 
ing a  heavy  deposit  of  nickel  on  the  outside  of  the  copper  bulb,  oxidizing 
may  be  prevented.  By  using  brazing  solders  with  a  melting  point  of 
1600°  F.  or  above,  these  instruments  may  be  used  at  temperatures  as 
high  as  1500°  F.  It  would  seem  that  considerable  progress  can  be  shown 
in  adapting  the  gas  pyrometer  for  measuring  higher  temperatures  than 
it  has  been  used  for  in  the  past.  Heretofore,  1000°  F.  (538°  C.)  has  been 
considered  the  limit  of  temperature  for  this  type  of  instrument. 

RESISTANCE  THERMOMETERS 

The  resistance  thermometer  is  not  a  new  form  of  temperature-meas- 
uring instrument,  having  been  used  for  some  30  years.  Marked  progress 
has  been  shown  during  the  past  few  years  in  adapting  this  instrument  to 
more  satisfactorily  meet  industrial  conditions.  The  principle  on  which 
the  resistance  thermometer  operates  is  the  change  in  the  resistance  of  a 
metal  due  to  change  in  temperature.  A  platinum  or  nickel  coil,  protected 
by  a  suitable  tube,  is  inserted  at  a  point  where  the  temperature  is  to  be 
measured  and,  with  a  constant  current  passing  through  the  coil  of  wire, 
the  resistance  decreases  or  increases  with  changes  in  temperature.  This 
change  in  resistance  can  be  easily  measured,  as  an  adjustable  resist- 
ance is  used  to  balance  the  resistance  of  the  bulb  and  a  galvanometer 
shows  when  the  balance  is  reached.  An  adjustable  resistance  is  furnished 
with  a  sliding  contact  arm  and  temperature  scale. 

The  combining  of  the  instrument  in  a  simple  and  robust  form  for 
installation  in  a  power  plant  or  chemical  works  naturally  adapts  this 
instrument  better  to  meet  industrial  conditions  than  the  laboratory  type 
of  instrument  formerly  used.  Furthermore,  by  using  the  instrument 
with  suitable  resistance,  a  resistance  thermometer  is  now  supplied  for 
use  on  110-  or  220-volt  direct-current  lighting  circuits  where  storage 
batteries  or  dry  cells  are  not  desirable.  Through  experimenting  with 
nickel  alloys,  it  has  been  possible  to  utilize  a  nickel-alloy  bulb  for  tem- 
peratures as  high  as  800°  F.  (422°  C.)  with  very  satisfactory  results. 
For  temperatures  above  300°  F.  (148°  C.)  it  was  formerly  considered 
necessary  to  use  a  platinum  bulb. 

It  is  not  generally  recognized  that  a  resistance  thermometer  is  an 
exceptionally  desirable  instrument  for  measuring  low  temperatures,  and 
there  are  numerous  application  for  sthis  instrument  in  many  industries. 
Twenty-five  or  more  drying  rooms,  cold-storage  rooms,  etc.,  can  be 
easily  connected  up  to  one  central  resistance  thermometer. 

THERMOELECTRIC  PYROMETERS 

It  is  safe  to  say  that  by  far  the  majority  of  all  the  pyrometers  in  use 
today  for  measuring  temperatures  above  1000°  F.  operate  on  the  thermo- 
electric principle.  A  thermoelectric  pyrometer  consists  of  a  thermo- 


R.    P.   BROWN  191 

couple  and  a  measuring  device  and  wires  joining  the  thermocouple 
and  the  measuring  device.  Any  two  pieces  of  wire  of  dissimilar 
materials,  for  instance  one  wire  of  copper  and  one  of  iron  with  one  end 
twisted  or  welded  together,  will  generate  a  small  current  of  electricity 
if  the  junction  is  heated.  The  current  produced  is  very  small.  Wires 
of  precious  metals,  used  for  high-temperature  measurements,  for  example 
one  wire  of  pure  platinum  .and  one  of  90  per  cent,  platinum  and  10  per 
cent,  rhodium,  generate  only  0.01  volt  or  10  millivolts,  at  a  temperature 
of  2000°  F.  Wires  of  base  metals,  for  instance  one  wire  of  pure  iron  and 
one  of  a  copper-nickel  alloy  known  as  constantan,  produce  five  times  the 
millivoltage  of  the  platinum  thermocouple.  Notable  progress  has  been 
made  in  the  development  of  base-metal  thermocouples  for  use  up  to 
2000°  F.  (1093°  C.).  For  higher  temperatures  platinum-rhodium  thermo- 
couples, as  used  for  some  20  or  30  years,  are  still  used. 

Platinum-rhodium  Thermocouples. — Platinum-rhodium  thermocouples 
are  usually  furnished  with  a  wire  0.02  in.  (0.5  mm.)  in  diameter,  but 
this  diameter  is  increased  or  decreased  to  suit  the  requirements.  The 
platinum  thermocouples  are  ordinarily  protected  with  a  tube  of  either 
porcelain  or  silica,  depending  on  the  conditions.  A  silica  or  quartz  tube 
is  preferable  where  the  instrument  must  be  portable  and  subjected  to 
rapid  changes  in  temperature.  Where  installed  permanently,  the 
platinum-rhodium  thermocouple  should  be  protected  by  a  glazed 
porcelain  tube,  suitable  to  withstand  high  temperatures  without  softening. 
It  is  advantageous  to  protect  the  porcelain  tube  with  an  extra  tube  of  a 
refractory  material  called  Durax,  with  tubes  of  firebrick,  graphite,  or 
some  other  suitable  material.  These  extra  tubes  protect  the  porcelain 
tubes  from  sudden  changes  in  temperature  or  from  mechanical  injury 
or  breakage.  The  platinum-rhodium  thermocouples  are  absolutely 
reproducible,  that  is  platinum-rhodium  thermocouples  can  be  bought 
with  a  pyrometer  equipment  today  and  5  years  hence  additional  thermo- 
couples can  be  ordered  that  will  accurately  reproduce  the  values  of  the 
present  thermocouples. 

Base-metal  Thermocouples. — For  temperatures  up  to  1200°  F.,  a 
very  satisfactory  thermocouple  consists  of  one  wire  of  iron  and  one  of 
constantan.  For  temperatures  above  this  and  as  high  as  2000° 
F.  (1093°  C.)  a  nickel-chromium  thermocouple  has  proved  most 
satisfactory — one  wire  consisting  of  90  per  cent,  nickel  and  10  per  cent, 
chromium,  and  the  other  wire  of  98  per  cent,  nickel  and  the  balance 
aluminum,  silicon,  and  manganese.  Experiments  in  calorizing  base- 
metal  thermocouple  wire  to  increase  its  life  at  high  temperatures  have  not 
been  productive  of  very  satisfactory  results  and  experiments  do  not 
indicate  that  calorizing  is  desirable  but  it  is  possible  to  increase  the 
life  of  the  base-metal  thermocouples  materially  by  suitable  protecting 
tubes. 


192  RECENT  IMPROVEMENTS  IN  PYROMETRY 

Base-metal  thermocouple  wires  can  be  secured  in  all  diameters  run- 
ning from  0.01  in.  (0.25  mm.)  up  to  0.25  in.  (6.3  mm.).  Certain  tests 
require  thermocouple  wires  of  exceedingly  small  diameter  to  secure 
sensitiveness  and  quick  reading.  For  permanent  service  the  heavier 
wires  will  increase  the  life,  particularly  where  the  thermocouple  is  sub- 
jected to  constant  service  at  temperatures  as  high  as  1600°  or  1800°  F. 
(870°  or  982°  C.).  Without  doubt  a  heavy  wire  thermocouple  increases 
the  lag  in  the  reading,  but  this  is  not  noticeable  in  large  heat-treating 
furnaces. 

If  several  base-metal  thermocouples,  for  example,  one  wire  of  iron  and 
one  of  constantan,  are  made  up  and  later  additional  coils  of  the  same  wire 
are  procured  to  reproduce  these  thermocouples,  the  various  thermocouples 
may  vary  as  much  as  50°  F.  (28°  C.)  at  a  temperature  of  1400°  F.  (760°  C.). 
In  the  case  of  thermocouples  of  nickel-chromium  wire  the  variation  may 
be  as  much  as  30°  F.  (16°  C.)  plus  or  minus,  depending  on  the  particular 
coils  from  which  the  wire  was  cut.  In  order  to  overcome  this  variation, 
all  the  available  wire  should  be  taken  and  a  certain  part  that  will  repro- 
duce the  standard  values  within  5°  F.  plus  or  minus  secured.  The  bal- 
ance of  the  wire  can  be  used  as  shunted  thermocouples. 

The  shunted  thermocouple  is  the  most  satisfactory  where  a  customer  is 
using  thermocouples  of  the  same  length  and  where  the  insertion  inside  the 
furnace  is  between  2  and  12  in.  (5  and  30  cm.)  in  each  instance.  In 
shunting  the  thermocouples,  the  millivoltage  is  reduced  approximately 
2  millivolts;  if  the  millivoltage  falls,  this  thermocouple  can  be  restandard- 
ized  at  any  time  by  readjusting  the  shunt.  With  the  unshunted  thermo- 
couple, it  is  necessary  to  cut  down  the  length  of  the  thermocouple,  or  it 
must  be  junked.  Individual  conditions  determine  which  type  of  thermo- 
couple should  be  used;  without  doubt  there  are  places  where  the  shunted 
couple  is  preferable.  The  unshunted  thermocouple  should  be  used  where 
various  lengths  of  thermocouples  are  required  and  where  thermocouples 
will  in  certain  instances  be  inserted  over  12  in.  inside  the  furnace. 

The  progress  shown  recently  in  the  manufacture  of  base-metal  thermo- 
couples has  been  largely  in  the  ability  to  standardize  thermocouples  by  a 
shunt  for  certain  requirements  or  to  furnish  wire  of  unusual  accuracy  for 
unshunted  thermocouples. 

Protecting  Tubes  for  Base-metal  Thermocouples. — Marked  progress 
has  been  made  in  the  past  few  years  in  the  development  of  suitable  pro- 
tecting tubes  for  increasing  the  life  of  a  base-metal  thermocouple.  It  is 
true  that  a  base-metal  thermocouple  of  nickel-chromium  wire  can  be 
used  in  an  electric  furnace  at  temperatures  of  1400°  to  1600°  F.  satis- 
factorily for  long  periods  of  time  without  deterioration;  but  where  it  is 
necessary  to  install  the  thermocouple  in  a  furnace  where  gases  are 
prevalent  or  where  chemicals  or  acids  exist,  a  suitable  protecting  tube 
must  be  used. 


R.    P.   BROWN  193 

Where  the  base-metal  thermocouple  will  not  be  subjected  to  tempera- 
tures above  1200°  F.  (648°  C.),  an  ordinary  wrought-iron  protecting  tube 
gives  very  satisfactory  results.  If  the  temperatures  will  run  as  high 
as  1500°  F.  (816°  C.),  the  life  of  a  wrought-iron  protecting  tube  can  be 
materially  increased  by  calorizing  the  pipe.  This  is  a  process,  developed 
by  the  General  Electric  Co.,  which  impregnates  the  pipe  with  an  alumi- 
num oxide.  Our  experiments  show  that  calorizing  wrought-iron  pipe 
will  increase  the  life  two  or  three  times  at  temperatures  up  to  1500°  F. 
(816°  C.). 

Tubes  of  nickel-chromium  alloy,  either  80  per  cent,  nickel  or  20 
per  cent,  chromium,  or  a  nickel-chromium-iron  alloy  with  approximately 
25  per  cent,  iron  added  will  give  most  satisfactory  service  in  heat-treating 
furnaces  at  temperatures  up  to  1700°  F.  (927°  C.).  In  the  manufacture 
of  nickel-chromium  tubes,  it  has  been  difficult  to  prevent  sand  holes  or 
leaky  tubes  and  every  tube  should  be  carefully  tested  under  pressure 
against  leaks  to  insure  a  tight  tube.  If  a  nichrome  tube  leaks,  it  would 
be  far  better  to  use  an  ordinary  wrought-iron  tube. 

One  of  the  most  difficult  problems  was  to  secure  a  satisfactory  tube 
for  use  in  galvanizing  baths — molten  zinc,  lead,  and  aluminum — and  it 
has  only  been  within  the  last  few  months  that  a  satisfactory  tube  for 
this  service  has  been  developed.  This  tube  is  known  as  "resisteat. "  At 
a  certain  plant  in  Philadelphia,  it  seemed  impossible  to  make  a  tube 
last  over  about  2  weeks,  but  a  resisteat  tube  was  in  use  for  3  months; 
then  the  thermocouple  had  to  be  removed  for  repairs,  not  because  the 
resisteat  tube  was  destroyed,  but  because  the  wrought-iron  pipe  above 
the  resisteat  tube  had  been  broken  off.  This  resisteat  tube  originally 
had  a  wall  thickness  of  %$  in-  (7.9  mm.),  and  when  cut  apart  after  use  for 
3  months  had  a  wall  thickness  of  full  ;Ke  m-  (4-7  mm.)  and  was  perfectly 
tight. 

Base-metal  thermocouples  can  be  used  in  ceramic  kilns  and  in  severe 
conditions  where  temperatures  will  occasionally  attain  2200°  F.  (1204°  C.) 
by  protecting  a  nickel-chromium  thermocouple  with  a  gas-tight  porcelain 
tube  and,  in  turn,  protecting  this  with  an  extra  refractory  tube.  It  is 
true  that  if  the  thermocouple  was  subjected  to  such  severe  temperatures 
constantly,  satisfactory  life  would  not  be  secured,  but  in  the  ceramic 
kiln  the  maximum  temperature  is  attained  for  not  more  than  24  hr. 
in  probably  every  2  weeks,  and  for  this  service  the  thermocouple  de- 
scribed gives  very  satisfactory  results. 

Thermocouples  of  iron  and  constantan  wire,  which  are  very  success- 
fully used  for  moderate  temperatures,  are  frequently  supplied  in  a  pro- 
tecting tube  packed  with  carbon  or  other  powdered  substances,  which 
will  exclude  gases.  This  increases  the  life  of  an  iron-constantan  thermo- 
couple but  this  construction  is  very  undesirable  for  a  nickel-chromium 
thermocouple,  as  greater  life  for  the  latter  is  secured  by  circulation  of  air 

13 


194  RECENT   IMPROVEMENTS   IN   PYROMETRY 

than  by  excluding  air  entirely.  In  other  words,  an  iron-constantan 
thermocouple  is  most  satisfactory  when  used  in  a  reducing  atmosphere, 
whereas  a  nickel-chromium  thermocouple  is  best  under  oxidizing  con- 
ditions. The  subject  of  protecting  tubes  for  base-metal  thermocouples 
has  been  considered  at  considerable  length,  because  the  user  of  a  pyro- 
meter generally  finds  that  the  only  real  difficulty- is  to  maintain  the 
thermocouple  in  good  condition. 

Insulation  of  Wires  of  Base-metal  Thermocouples. — The  original 
insulation  used  on  a  base-metal  thermocouple  was  asbestos  string  or 
tubing  painted  with  a  solution  of  carborundum,  fire  sand,  and  sodium 
silicate  (water  glass),  mixed  to  a  paste.  This  insulation  rapidly  disinte- 
grates and  should  not  be  used  for  temperatures  above  1000°  F.  (540°  C.). 
The  most  suitable  form  of  insulator  is  a  porcelain  bead  or  tube,  which 
is  not  affected  by  temperatures  up  to  the  limit  of  a  base-metal  thermo- 
couple; this  is  the  form  that  has  been  generally  adopted. 

Cold-junction  Compensation. — It  is  one  of  the  properties  of  a  thermo- 
couple that  the  voltage  it  generates  is  dependent  on  the  temperature 
of  the  hot  junction,  which  is  placed  in  the  furnace,  and  the  cold  junction, 
which  is  the  point  at  which  the  alloy  wires  of  the  thermocouple  join 
the  copper  leads  to  the  instrument.  It  is,  therefore,  particularly  im- 
portant that  the  cold  junction  of  the  thermocouple  be  maintained  at  a 
uniform  temperature,  for  if  a  base-metal  thermocouple  is  in  use  and  its 
cold  junction  is  heated  10°,  the  decreased  voltage  generated  by  the  thermo- 
couple will  cause  the  instrument  to  read  approximately  10°  low.  If  the 
cold-junction  temperature  decreases,  the  pyrometer  will  read  high  to 
approximately  the  same  extent. 

Until  recently,  no  methods  were  adopted  to  take  care  of  this  source 
of  error  at  the  cold  junction  of  the  thermocouple.  In  recent  years,  how- 
ever, it  has  been  customary  to  run  compensating  leads  of  the  same  ma- 
terial as  the  thermocouple  to  a  distant  point  where  the  temperature  is 
uniform,  instead  of  having  the  cold  junction  just  beside  the  furnace  wall, 
where  it  might  vary  several  hundred  degrees.  These  compensating 
leads,  in  duplex  form,  can  be  run  into  a  pipe  driven  in  the  ground  10  or 
15  ft.  (3  or  4.5  m.)  where  the  temperature  will  remain  constant  within 
5°  winter  or  summer,  see  Fig.  1 .  From  my  experience,  I  consider  this 
is  the  best  method  to  secure  a  constant  cold-junction  temperature,  as  it 
only  takes  a  few  hours  to  drive  into  the  ground  a  piece  of  pipe  pointed  at 
the  lower  end  and  when  once  installed  the  cold-junction  question  is 
settled.  It  has  been  common  practice,  in  past  years,  to  maintain  the 
cold  junction  at  as  nearly  a  constant  temperature  as  possible  by  running 
water  around  the  cold  junction;  this  maintains  the  temperature  at  that 
of  running  water,  but  unfortunately  this  may  vary  at  least  20  or  30°  from 
winter  to  summer.  This  method  has  been  very  largely  abandoned  of 
late. 


R.    P.   BROWN 


195 


Where  it  is  impossible  to  place  the  cold  junction  in  the  ground  on 
account  of  the  furnaces  being  on  an  upper  floor  of  the  building,  or  for 
other  reasons,  a  compensating  box  can  be  used  in  the  form  of  a  calo- 


End  of  2"lnside  Iron  Pipe 


Cold  Junction  of  rhe 

Compensating  leads  and  ihe  Copper  leads 


,End  of  Outside^  Iron  Pipe  Welded  and  pointed 
to  be  Driven  in  Ground 


FIG.  1. — COLD  JUNCTION  INSTALLED  IN  GROUND. 

rimeter,  which  will  maintain  the  temperature  constant  within  2°  at  all 
times.     The  common  form  of  compensating  box  shown  in  Fig.  2  consists 


FIG.  2. — COLD-JUNCTION  COMPENSATING  BOX. 

of  a  lamp  and  thermostat  which  opens  and  closes  a  circuit  to  the  lamp  as 
the  thermostatic  metal  expands  and  contracts,  and  control  within  2°  F. 
is  quite  possible. 


RECENT   IMPROVEMENTS   IN    PYROMETRY 


Millivoltmeter  and  potentiometer  pyrometers  can  be  also  supplied 
with  automatic  means  in  the  meter  to  compensate  for  changes  in  tem- 
perature of  the  cold  junction,  provided  the  compensating  leads  are 
brought  to  the  instrument.  With  this  type  of  meter,  either  hand  adjust- 
ment can  be  made  at  the  meter  for  the  temperature  surrounding  the 
meter  and  cold  junction,  or  the  instrument  can  be  designed  to  compensate 
automatically.  No  matter  what  the  type  of  pyrometer,  the  instrument 
should  be  adjusted  properly  for  the  actual  temperature  of  the  cold 
junction  of  the  thermocouple.  Improved  instruments  are  equipped  with 
a  zero  adjuster  to  permit  of  adjusting  the  pointer  for  the  actual  cold 
junction  temperature. 

Millivoltmeter  Method  of  Measuring  Thermocouple  Voltage. — There 
are  two  distinct  methods  of  measuring  the  voltage  produced  by  a  thermo- 
couple, that  is,  the  millivoltmeter  and  the  potentiometer  methods. 


FIG.  3. — MILLIVOLTMETER  FOR  MEASURING  THERMOELECTRIC  VOLTAGE. 

The  milli voltmeter,  shown  in  Fig.  3,  consists  of  a  permanent  horse- 
shoe magnet  with  its  pole  pieces,  in  the  field  of  which  a  copper  wound 
coil  swings  in  jeweled  bearings.  Millivoltmeters  have  been  in  extensive 
commercial  use  abroad,  and  to  some  extent  in  this  country  for  20  or  30 
years,  but  the  instruments  were  of  such  delicate  construction  as  to  be 
really  unsuitable  for  general  commercial  use.  The  instruments  were 
supplied  with  a  moving  coil  hung  between  fine-wire  suspensions,  which 
are  easily  broken  through  jars  or  handling  in  transit.  On  account  of 
this  delicate  construction,  some  10  years  ago,  a  standard  form  of  switch- 
board millivoltmeter  was  adopted  extensively  in  this  country.  This  is 
the  same  instrument  commonly  used  with  a  shunt,  as  an  ammeter. 
The  instrument  had  a  resistance  from  2  to  5  ohms  and  each  individual 
instrument  had  to  be  calibrated  for  a  thermocouple  of  a  certain  length 
for  use  with  leads  or  wiring  of  a  definite  length.  Slight  changes  in  resis- 
tance due  to  changes  in  the  length  of  the  thermocouple  or  the  length  of 
the  wiring  materially  affected  the  indications  of  the  instruments  as  the 
internal  resistance  of  the  millivoltmeter  was  so  low.  Serious  errors 


R.    P.   BROWN 


197 


occurred  also,  due  to  atmospheric  changes  in  temperature  along 
the  wiring,  which  naturally  affected  the  resistance  of  the  circuit.  Actual 
tests  show  that  with  a  low-resistance  millivoltmeter  of  5  ohms  resistance, 
an  atmospheric  change  in  temperature  from  50  to  100°  F.  along  50  ft.  of 
wiring  from  the  thermocouple  to  the  instrument  will  make  the  milli- 
voltmeter read  18°  low  at  1200°  F.  It  was  naturally  impossible  to  pro- 
cure great  accuracy  with  such  an  instrument. 

In  the  last  5  or  6  years,  great  progress  has  been  made  in  the  develop- 
ment of  a  high-resistance  milli voltmeter  that  for  all  practical  purposes 
overcomes  this  trouble  entirely.  The  moving  element  is  shown  in  Fig. 
4.  High-resistance  pyrometers  are  produced  today  with  a  copper- 
wound  moving  element  having  a  resistance  of  15  ohms  per  millivolt.  A 


FIG.  4. — MOVING  ELEMENT  OF  BROWN  HIGH-RESISTANCE  MILLIVOLTMETER. 

platinum  thermocouple  produces  approximately  20  millivolts  at  a  tem- 
perature of  3000°  F.,  the  usual  maximum  of  the  scale,  and  such  an  instru- 
ment in  consequence  has  an  internal  resistance  of  300  ohms.  This  same 
type  of  high-resistance  millivoltmeter  used  with  a  nickel-chromium  base- 
metal  thermocouple  producing  approximately  40  millivolts  at  2000°  F. 
will  have  an  internal  resistance  of  600  ohms. 

By  the  use  of  a  special  form  of  aluminum-alloy-wound  movable 
element,  wound  with  wire  0.003  in.  in  diameter  the  number  of  ampere- 
turns  can  be  increased  25  per  cent,  or  more,  and  the  weight  of  the 
coil  reduced.  Consequently,  using  the  same  springs,  magnets,  etc.,  we 
obtain  a  considerable  increase  in  sensitivity  over  the  copper-wound 
movable  element.  But,  reducing  the  weight  permits  the  use  of  lighter 
springs.  Therefore  the  internal  resistance  can  be  still  further  increased 


198  RECENT  IMPROVEMENTS  IN  PYROMETRY 

so  that  it  is  possible  to  increase  the  instrument  resistance  to  30  ohms  per 
millivolt..  A  millivolt  meter  graduated  to  3000°  for  a  platinum  thermo- 
couple will  have  600  ohms  internal  resistance,  or  for  a  nickel-chromium 
base-metal  thermocouple  graduated  to  2000°  will  have  1200  ohms  resis- 
tance. Such  an  internal  resistance  eliminates  entirely  all  errors  due  to 
line  resistance,  length  of  thermocouple,  or  atmospheric  changes  in  tem- 
perature along  the  leads. 

It  has  been  stated  that  an  atmospheric  change  in  temperature  of  50°  F. 
along  50  ft.  of  wiring  would  make  a  low-resistance  pyrometer  read  18°  low 
at  1200°  F.  This  low-resistance  pyrometer  had  a  resistance  of  5  ohms. 
Comparing  this  resistance  with  600  ohms,  for  the  average  high-resistance 
instrument,  gives  a  ratio  of  1  to  120.  If  the  previous  error  was  18°  F., 
it  would  be  reduced  to  0.1°. 

The  standard  type  of  millivoltmeter  measures  temperatures  directly 
without  any  hand  manipulation  whatsoever.  The  scale  is  graduated 
directly  in  temperature  degrees  and  the  operator  can  read  the  temperature 
at  any  time  on  the  scale.  Without  doubt,  the  millivoltmeter  is  par- 
ticularly advantageous  on  account  of  its  simplicity. 

An  improved  form  of  millivoltmeter  pyrometer  has  been  recently 
developed  by  Messrs.  Paul  D.  Foote  and  Thomas  R.  Harrison,  of  Wash- 
ington, D.  C.  This  instrument  affords  a  ready  means  of  adjusting  a 
millivoltmeter  for  any  change  in  resistance  up  to  15  ohms  of  the  circuit 
of  thermocouple,  leads,  and  instrument.  Heretofore,  instruments  of  this 
kind,  which  have  been  developed  in  the  last  few  years,  have  required  a 
dry  cell  to  balance  the  voltage  of  the  thermocouple  but  with  the  new 
instrument  designed  by  Messrs.  Harrison  and  Foote  no  dry  cell  or 
other  source  of  current  than  the  thermocouple  is  required.  By  simply 
pressing  a  button  and  turning  a  knob,  the  instrument  can  be  instantly 
adjusted  for  any  resistance  of  the  circuit  up  to  15  ohms.  This  is  an 
unusually  desirable  instrument  where  material  variations  in  the  length 
of  leads  will  occur  or  where  there  are  a  number  of  thermocouples  con- 
nected up  to  one  central  instrument  with  leads  of  unusual  length.  This 
instrument  of  Messrs.  Harrison  and  Foote  is  a  marked  improvement 
over  the  Brown  Heatmeter  developed  and  patented  by  the  writer  some  2 
years  ago,  and  will  be  known  as  the  Brown  Improved  Heatmeter. 

Potentiometer  Method  of  Measuring  Thermocouple  Voltages. — In  this 
method  of  temperature  measurement  the  electromotive  force  produced  by 
a  thermocouple  is  measured  by  opposing  to  it  a  known  variable  electro- 
motive force,  usually  a  dry  cell  contained  in  the  instrument,  so  that  when 
a  balance  is  reached,  no  current  flows.  A  galvanometer  is  used  to  indi- 
cate the  point  when  a  balance  is  reached  and  the  galvanometer  then 
indicates  zero,  the  voltage  of  the  thermocouple  being  equal  to  the  impressed 
dry-cell  voltage.  After  the  thermocouple  voltage  has  been  balanced 
against  the  voltage  of  the  dry  cell,  the  actual  measurement  is  that  of  the 


B.   P.   BROWN  199 

dry-cell  circuit,  hence  this  measurement  is  entirely  independent  of  the 
resistance  of  the  circuit,  including  the  thermocouple,  lead  wires,  and 
galvanometer.  As  a  consequence,  the  instrument  is  independent  of  the 
resistance  of  the  circuit  of  the  thermocouple  and  leads,  and  compensating 
leads  can  be  run  to  the  instrument  500  ft.  (152  m.)  distant,  if  desired. 
Changes  in  resistance  of  the  various  parts  of  the  circuit  due  to  changes  in 
length  or  atmospheric  changes  will  have  no  effect  on  the  indication. 

The  advantage  of  the  potentiometer  method  of  measuring  tempera- 
ture is  its  extreme  precision  and  its  independence  of  resistance  changes 
throughout  the  thermocouple  circuit.  It  has  the  disadvantage,  as  com- 
pared with  the  millivoltmeter  method,  that  it  is  not  direct  reading  and 
that  some  outside  source  of  current,  a  dry  cell  or  storage  battery  for 
example,  is  necessary  as  a  source  of  current  to  oppose  the  thermocouple, 
and  this  cell  must  be  replaced  or  recharged  from  time  to  time. 

There  are  portable  potentiometers  on  the  market  that  automatically 
compensate  for  the  changes  in  temperature  of  the  cold  junction  of  the 
thermocouple,  provided  the  compensating  leads  are  brought  to  the  in- 
strument and  they  measure  the  millivoltage  of  the  thermocouple  with 
extreme  precision.  The  writer  has  been  recently  granted  a  patent  on  a 
portable  potentiometer  in  which  a  resistance  is  wound  spirally  on  an 
insulated  drum  or  cylinder.  The  scale  is  drawn  in  a  spiral  with  the  index 
traveling  across  the  scale  concentric  with  its  travel  along  the  spiral 
resistance.  This  type  of  portable  potentiometer  has  a  scale  8  ft.  long. 
The  standard  scale  is  graduated  up  to  50  millivolts  in  0.02  millivolt,  or 
50  graduations  to  a  millivolt.  Where  used  to  measure  temperature  of  a 
nickel-chromium  thermocouple  range  of  2000°  F.,  each  graduation  is 
equal  to  2°  F.  (1.1+°  C.)  and  a  reading  can  easily  be  secured  to  one-fifth 
of  this.  There  is  no  question  but  that  with  such  an  instrument  extreme 
precision  is  attainable  in  measuring  temperatures  with  a  thermocouple. 

Recording  Thermoelectric  Pyrometers. — There  are  procurable  today 
pyrometers  that  accurately  plot  a  record  of  the  temperature  as  indicated 
by  a  thermocouple.  These  instruments  are  supplied  with  either  circular 
charts  for  recording  over  a  period  of  24  hr.  or  with  strip  charts,  lasting 
as  long  as  2  months  without  replacement.  Recording  pyrometers  are 
supplied  that  make  one  record;  or  by  locating  two  galvanometers  side 
by  side,  the  one  recording  instrument  can  make  two  independent  records 
on  one  chart.  By  the  introduction  of  suitable  switching  mechanism, 
a  record  of  from  three  to  as  many  as  sixteen  thermocouples  can  be  pro- 
duced on  one  recording  sheet.  Multiple  recording  pyrometers  developed 
within  the  past  5  or  6  years  use  two  methods  of  producing  distinguishable 
records,  either  the  records  are  produced  in  different  colors,  or  each  record 
line  may  be  identified  by  a  number  printed  simultaneously  with  the 
operation  of  the  recorder.  Great  progress  has  been  made  in  the  develop- 
ment of  accurate  recording  pyrometers  in  the  last  few  years. 


200 


RECENT   IMPROVEMENTS   IN    PYROMETRY 


Recording  Pyrometers  for  Transformation  Points. — A  special  design 
of  recording  thermoelectric  pyrometer  is  required  for  determining 
accurately  the  transformation  points  that  metals  undergo  through  heat- 
ing or  cooling,  see  Fig.  5.  In  addition  to  the  test'  piece  of  steel,  the  transfor- 
mation point  of  which  is  to  be  determined,  a  neutral  body  of  a  nickel 
alloy  is  inserted  in  the  electric  furnace  with  the  test  piece.  One  thermo- 
couple is  installed  in  the  neutral  body  and  another  in  the  test  piece  and  as 
the  test  piece  goes  through  its  transformation  point  a  differential  is 
set  up,  due  to  the  continued  heating  of  the  neutral  body,  whereas  the  test 
piece  ceases  to  rise  in  temperature  at  these  transformation  points  although 
the  furnace  continues  to  heat  up. 


FIG.  5. — RECORDING  PYROMETERS  FOR  TRANSFORMATION  POINTS. 

Within  the  past  year,  the  writer  has  been  granted  two  patents  on  an 
improved  form  of  differential  transformation-point  recorder,  which  auto- 
matically plots  the  true  temperature  of  the  test  piece  and  the  record 
of  the  differential,  which  greatly  magnifies  the  jog  occurring  on  the  chart 
at  the  transformation  points. 

Automatic  Signaling  Pyrometers. — Instruments  to  automatically  ring 
a  bell  have  been  supplied  for  many  years,  operated  by  the  older  types 
of  expansion  pyrometers  or  mercurial  thermometers.  A  platinum  con- 
tact is  installed  in  the  mercurial  thermometer  tube  and  when  the  mercury 
rises  to  the  platinum  contact  the  bell  rings.  There  is  a  demand  for 
thermoelectric  pyrometers  for  measuring  higher  temperatures  that  will 
give  a  positive  and  reliable  alarm  under  certain  changes  in  temperature 


R.    P.    BROWN  201 

or  will  visually  indicate  changes  in  temperature  by  the  means  of  signal 
lights.  In  the  last  2  years  there  have  been  numerous  installations  of 
thermoelectric  signaling  pyrometers  operated  by  either  the  millivolt- 
meter  method  or  the  potentiometer  system. 

In  this  type  of  instrument  it  is  essential  that  positive  contacts  should 
be  made  and  no  current  should  pass  through  the  indicating  pointer 
normally  used  to  indicate  or  record  the  temperature.  In  the  standard 
form  of  signaling  pyrometer  we  construct,  the  pointer  is  periodically 
depressed  on  to  contacts  at  intervals  of  every  30  sec.  In  the  case  of  the 
pyrometer  operating  signaling  lights,  three  contacts  are  used  representing 
high,  correct,  and  low  temperatures,  corresponding  to  red,  white,  and 
green  lights. 

Many  of  the  men  employed  around  heat-treating  furnaces  are  unable 
to  read  or  write  and  a  temperature  scale  is  meaningless  to  them.  It  is 
a  comparatively  easy  matter  to  instruct  these  men  to  keep  the  white 
light  burning  all  the  time,  and  if  the  red  or  green  light  should  burn  to 
regulate  the  valve  accordingly.  This  is  a  much  easier  proposition  than 
to  tell  the  men  to  maintain  the  temperature  at  1450°  F.  (788°  C.)  and  if 
the  temperature  rises  to  1470°  (798°  C.)  he  should  partly  shut  the  valves 
or  if  it  falls  to  1430°  (777°  C.)  he  should  open  the  valve.  Besides  the 
advantage  to  the  workmen  of  being  able  to  instantly  observe  the  tem- 
perature of  the  signal  lights,  the  foreman  in  charge  of  the  department  can 
look  along  a  row  of  furnaces  from  one  end  of  the  room  and  instantly  note 
whether  the  white  light  is  burning  on  all  furnaces.  If  a  certain  furnace 
has  the  red  or  green  light  burning  too  often,  an  investigation  can  be 
made  as  to  the  difficulty  and  the  trouble  promptly  corrected. 

Automatic  Temperature-control  Pyrometers, — From  the  automatic  sig- 
naling of  the  temperature  by  lights,  it  is  a  very  short  step  to  the 
automatic  control  of  temperature.  The  problem  of  controlling  electric- 
furnace  temperatures  is  a  very  easy  one  as  a  switch  operated  by  solenoids 
or  electric  magnets  can  be  controlled  by  the  pyrometer.  Where  used  to 
operate  a  valve  to  control  furnaces  or  ovens  heated  by  steam,  gas,  or 
oil,  a  solenoid  or  magnet  must  be  applied  to  operate  the  valve  instead  of 
the  switching  mechanism.  Switches  for  electric-furnace  control  can 
be  installed  directly  in  the  main  circuit,  but  preferably  should  cut  in  or 
out  one  of  the  sections  or  heating  elements  of  the  furnace  or  a  certain 
part  of  the  resistance  in  series  with  the  furnace. 

Opening  and  closing  of  the  main  circuit  will  naturally  produce  con- 
stant rising  and  falling  of  the  temperature,  whereas  the  fluctuation  oc- 
curring in  the  voltage  or  temperature  conditions  in  the  furnace  can  be 
more  accurately  controlled  by  the  pyrometer  if  only  approximately  25 
or  50  per  cent,  of  the  current  is  controlled.  Likewise,  in  the  control  of 
oil  or  gas  furnaces,  if  the  valve  is  installed  in  the  main  line  the  supply  is 
either  all  on  or  cut  off  entirely,  and  this  must  naturally  produce  constant 


202  RECENT  IMPROVEMENTS  IN  PYROMETRY 

fluctuation  in  temperature.  If  the  valves  are  installed  in  a  by-pass  and  a 
proportion  of  gas  passing  through  the  by-pass  is  adjusted  so  as  to  have 
approximately  only  25  per  cent,  control,  these  constant  fluctuations 
are  eliminated  and  a  very  satisfactory  control  can  be  maintained. 

Radiation  Pyrometers. — A  radiation  pyrometer  measures  temperatures 
by  the  heating  of  a  thermocouple  subject  only  to  radiated  heat  instead 
of  to  the  direct  temperature.  Instead  of  placing  the  thermocouple 
directly  inside  the  furnace  where  the  temperature  would  be  so  high  as  to 
destroy  it,  it  is  placed  in  the  back  of  a  tube  in  the  focus  of  a  mirror.  The 
rays  of  heat  from  the  furnace  enter  the  tube  and  strike  the  mirror  and  are 
focused  on  the  hot  junction  of  the  thermocouple.  This  attains  a  heat 
of  only  200°  or  300°  F.  (93°  or  148°  C.). 

This  instrument  has  a  particular  field  where  temperature  must  be 
measured  from  2800°  F.  (1538°  C.)  up  to  3600°  F.  (1982°  C.)  or  more,  and 
it  is  possible  to  secure  an  accuracy  of  1  to  2  per  cent,  with  this  type  of 
pyrometer,  if  the  instructions  as  to  its  use  are  properly  carried  out. 
A  radiation  pyrometer  should  not  be  used  where  a  thermoelectric  pyrome- 
'  ter  can  be  applied  to  advantage. 

Optical  Pyrometers. — There  has  been  notable  improvement  in  the 
adaptation  in  the  past  few  years  of  optical  pyrometers  to  general  in- 
dustrial service.  Optical  pyrometers  are  not  a  recent  development,  like 
practically  every  other  type  of  pyrometer  in  use  today,  but  it  has  only 
been  within  the  past  few  years  that  optical  pyrometers  have  been  brought 
to  the  point  where  they  can  be  satisfactorily  used  in  the  industries.  The 
development  has  been  largely  along  the  lines  of  simplifying  the  design 
so  that  satisfactory  results  can  be  secured  by  almost  any  user.  The 
trouble  with  optical  pyrometers  in  the  past  has  been  that  no  two  opera- 
tors could  secure  the  same  reading.  There  are  some  types  of  optical 
pyrometers  used  to  a  considerable  extent  which  were  supposed  to  help  the 
eye  in  determining  the  temperature,  but  these  have  done  more  harm  than 
good. 

RESUME 

J  have  attempted  to  cover  the  subject  of  recent  improvements  in 
pyrometry  from  the  standpoint  of  the  application  of  pyrometers  to  the 
industries.  Many  special  types  of  pyrometers  have  been  built  for 
laboratory  and  experimental  use,  that  it  would  be  impossible  to  refer 
to  in  a  paper  of  this  character;  but  though  pyrometry  has  made  marked 
progress  in  the  last  few  years,  I  have  not  the  slightest  doubt  that 
much  greater  progress  will  be  made  in  the  next  few  years.  Most  in- 
dustrial plants  have  cooperated  to  the  utmost  with  the  pyrometer 
manufacturer  in  an  effort  to  perfect  temperature-measuring  apparatus 
to  meet  the  requirements  of  general  industrial  service.  Certain  foreign 
countries  were  formerly  recognized  as  the  leaders  in  the  manufacture  of 


DISCUSSION  203 

scientific  instruments,  and  particularly  pyrometers.  This  lead  has  now 
been  taken  by  our  country,  and  with  the  rapid  perfection  of  our 
pyrometers  and  the  great  amount  of  research  work  constantly  being 
performed,  we  shall  never  lose  the  lead. 

DISCUSSION 

E.  D.  TILLYER,*  Southbridge,  Mass,  (written  discussion f). — It  is 
quite  generally  known  that  there  is  very  little  that  is  standard  about  a 
mercurial  thermometer  at  temperatures  above  212°  F.  (100°  C.)  because 
so  many  precautions  must  be  taken — precautions  that  are  rarely  realized 
in  practice— such  as  depth  of  immersion,  aging,  elastic  fatigue,  separation 
of  mercury  column,  and  accidental  deformation  from  very  slight  excess 
temperature. 

One  cannot  help  wishing  that  Mr.  Brown  would  dwell  at  greater 
length  on  the  practical  side  of  both  radiation  and  optical  pyrometers, 
both  of  which  have  great  possibilities  as  commercial  instruments.  In 
the  radiation  pyrometer  we  have  ideal  conditions  for  a  permanent  instru- 
ment, no  materials  being  exposed  to  excessive  temperatures  or  subjected 
to  contamination  from  the  furnace  fumes  which  may  raise  such  havoc 
with  an  ordinary  pyrometer.  The  development  of  a  radiation  pyrometer 
requires  a  thermocouple  having  a  relatively  high  electromotive  force  but 
which  need  stand  temperatures  of  only  200°  F.  or  300°  F.  instead  of  1500 
to  2000°  F.,  which  is  required  of  even  a  base-metal  couple.  The  galva- 
nometer, if  located  near  the  thermocouple,  does  not  need  to  have  a  high 
resistance,  as  at  such  low  temperatures  no  changes  can  occur  in  the 
thermocouple  that  will  affect  its  resistance  and,  consequently,  the  in- 
dicated temperature. 

The  serious  source  of  error  is  the  lens  or  mirror,  which  images  the 
furnace  interior  on  the  hot  junction  of  the  thermocouple;  this  must 
always  have  the  same  transmission,  or  reflection,  and  dirt  and  tarnish 
must  be  avoided.  Another  source  of  error  in  many  instruments  is  the 
temperature  of  the  cold  junction,  which  heats  up  from  radiation.  How- 
ever, there  is  no  fundamental  reason  why  the  cold  junction  cannot  be 
carried  to  a  position  of  constant  temperature,  as  is  done  with  the  regular 
thermocouple  pyrometers. 

One  physical  defect  of  a  radiation  pyrometer  is  the  absorption  of  the 
longer  heat  rays  by  varying  amounts  of  water  vapor  in  the  atmosphere. 
Perhaps  this  could  be  overcome  by  using  selective  absorption  screens 
cutting  out  the  rays  absorbed  by  water  vapor.  Another  physical  defect 
of  a  radiation  pyrometer,  and  also  of  the  optical  pyrometer,  is  that  it 
indicates  the  radiation  temperature  of  the  object  on  which  it  is  focused. 


*  American  Optical  Co.  t  Received  Sept.  25,  1919. 


204  RECENT    IMPROVEMENTS    IN    PYROMETRY 

This  would  seldom  be  an  objection,  however,  as  the  most  that  is  desired 
is  to  reproduce  temperatures;  if  it  were  desired  to  obtain  true  tempera- 
tures, the  black-body  condition  could  be  obtained  for  a  small  part  of  the 
furnace  by  simply  inserting  a  heat-resisting  tube  for  a  considerable  dis- 
tance and  focusing  the  pyrometer  on  the  inside  end. 

The  optical  pyrometer  requires,  in  all  present  forms,  that  the  observer 
look  through  it  and  not  at  it.  Until  someone  overcomes  this  practical 
defect,  one  of  the  best  forms  of  instruments  for  higher  temperature 
pyrometry  is  probably  barred  out. 

A.  O.  ASHMAN,  Palmerton,  Pa.  (written  discussion*). — From  a  theo- 
retical point  of  view  the  best  method  to  maintain  the  cold  junction  at  a 
constant  temperature  is  by  means  of  a  pipe  driven  in  the  ground,  to  which 
the  so-called  cold-junction  compensating  leads  are  run,  and  of  course 
there  are  many  times  and  places  where  it  can  be  successfully  used.  There 
are,  however,  several  objections  to  this  method.  In  the  vicinity  of  any 
furnace  there  are  apt  to  be  underground  flues  for  preheated  air  or  gases, 
recuperators,  water  mains,  etc.,  which  will  eliminate  the  probability  of  a 
constant  temperature.  These,  of  course,  can  be  avoided  by  carrying 
the  leads  to  distant  locations  known  to  be  free,  but  this  is  not  good 
practice  because  the  so-called  compensating  leads  have  a  rather  high  re- 
sistance, which  materially  affects  the  accuracy  of  even  the  high-resistance 
instruments,  unless  they  are  specially  calibrated  for  it.  Moreover,  the 
cost  of  these  long  leads  is  almost  prohibitive.  A  better  way  is  to  run 
short  lengths  of  compensating  leads  to  electrically  heated  constant- 
temperature  boxes  conveniently  located  near  the  furnace  in  a  place  that 
is  of  fairly  uniform  temperature.  These  boxes  are  on  the  market  and 
can  be  procured  at  a  cost  that  is  approximately  equal  to  the  cost  of 
100  ft.  of  compensating  leads.  They  can  be  connected  to  any  power  or 
lighting  circuit  and  once  set  require  very  little  care.  One  box  will  ac- 
commodate a  number  of  couples. 

An  additional  disadvantage  of  the  buried  pipe  is  that  moisture  or 
water  may  accumulate  in  it.  This  is  hard  to  detect  and  almost  im- 
possible to  remove.  It  generally  gives  rise  to  galvanic  effects  which 
result  in  errors  that  are  greater  than  those  due  to  an  uncorrected  cold 
junction. 

While  the  term  "compensating  leads"  is  in  general  use,  it  is  not  justi- 
fiable, as  there  is  no  compensation  in  the  true  sense  of  the  word.  For 
example,  suppose  the  cold  junction  of  a  couple  was  exposed  to  a  tempera- 
ture of  100°  higher  than  the  couple  was  calibrated  for.  If  the  ends  of 
the  leads  connected  to  this  couple  were  in  the  same  region  there  would,  of 
course,  be  no  compensation  as  when  a  true  compensator  was  used.  A 
better  term,  I  think,  would  be  cold-junction  extension  leads.  Many 


*  Received  Sept.  25,  1919. 


DISCUSSION  205 

terms  in  pyrometry  are   more  or  less  loosely  used,  which   could   be 
standardized  by  suitable  action  on  the  part  of  some  interested  society 

W.  H.  BRISTOL,  Waterbury,  Conn. — There  should  be  some  standard 
way  of  speaking  of  what  are  sometimes  called  compensating  leads,  which, 
it  seems  to  me,  is  a  misnomer.  As  I  understand  it,  so-called  compensat- 
ing leads  are  an  extension  of  the  couple  itself,  a  continuation  of  the  same 
materials  of  which  the  couple  is  made,  so  as  to  carry  the  cold  end  to  a 
desired  point  where  the  temperature  is  more  uniform,  or  to  some  correct- 
ing device  where  it  can  be  held  at  constant  temperature  to  provide  for 
atmospheric  changes.  "Extension"  of  the  couple  would  be  a  very  good 
name. 

E.  F.  NORTHRUP,  Trenton,  N.  J. — We  have  found  it  is  possible  to 
do  away  with  all  of  these  cold-junction  corrections  by  using  the  all-steel 
thermos  bottles.  We  use  a  platinrhodium  couple  having  leads  just  long 
enough  to  reach  from  the  couple  down  into  the  steel  thermos  bottle; 
the  rest  of  the  way  copper  is  used.  These  bottles  can  be  set  within 
4  or  5  ft.  of  a  large  steel  bottle  filled  with  molten  steel  or  iron  without 
injury  to  the  bottle  and  where  it  will  hold  cracked  ice  for  24  hr.  We 
have  been  able  to  use  hundreds  of  couples  that  way  and  think  it  a  con- 
venient and  practical  method.  When  the  cold  junction  goes  into  the 
thermos  bottle,  it  must  not  pass  through  a  metal  tube,  for  that  will 
conduct  the  heat  into  the  bottle  and  melt  the  ice.  The  junction  should 
pass  either  through  a  non-heat-conducting  tube  or  it  must  be  separated 
from  the  neck  of  the  bottle  by  suitable  heat-insulating  material. 

ANTHONY  ZELENY,*  Minneapolis,  Minn. — In  1903,  I  experimented 
with  a  thermoelectric  installation  consisting  of  about  200  junctions 
connected  to  one  common  return  wire  about  400  ft.  in  length.  The 
common  cold-junction  was  placed  in  a  tube  buried  in  the  earth.  This 
was  in  a  steel-tank  grain  elevator ;  all  insulators  were  necessarily  attached 
to  iron  beams  and  were  soon  covered  with  grain  dust.  The  belt  con- 
veyors were  operated  by  direct-current  motors.  This  thermoelectric 
installation  operated  perfectly  except  when  the  motors  were  in  action. 
Sufficient  leakage  current  passed  through  the  system  and  the  galvanom- 
eter to  destroy  the  readings.  The  removal  of  the  cold  junction  from 
the  earth  remedied  the  trouble.  No  attempt  was  made  to  improve  the 
insulation.  It  appears  that  in  such  extraordinary  installations  special 
care  must  be  taken  to  prevent  leakage  when  the  cold  junction  is  buried 
in  the  earth. 


*  Professor  of  Physics,  University  of  Minnesota. 


206         AUTOMATIC   COMPENSATION   FOR   COLD-JUNCTION   TEMPERATURES 


Automatic  Compensation  for  Cold-junction  Temperatures  of  Thermo- 
couple Pyrometers 

BY   FELIX   WUNSCH,*   PHILADELPHIA,    PA. 
(Chicago  Meeting,  September,  1919) 

WHILE  the  effect  of  the  cold-junction  temperature  has  been  known  by 
many,  its  consideration  has  been  ignored  in  a  number  of  installations, 
resulting  at  times  in  a  very  considerable  error.  In  fact,  the  magnitude 
of  this  error  may  amount  to  over  100°  in  some  cases.  While  hand- 
operated  correcting  devices  for  portable  checking  pyrometers  may  be 
entirely  satisfactory,  it  is,  of  course,  desirable  to  have  such  apparatus  en- 


Milli  volfrne-her 


Wire 


Thermocoup/e 
•Mercury 


FIG.  1. — EARLY  METHOD   OF   AUTOMATICALLY   COMPENSATING   FOR    COLD-JUNCTION 

TEMPERATURE  CHANGES. 

tirely  automatic  when  applied  to  curve-drawing  or  printing  pyrometers, 
otherwise  there  is  no  assurance  that  the  record  is  correct. 

Numerous  methods  for  automatically  compensating  for  cold-junction 
temperature  changes  of  thermocouples  have  been  proposed  and  used  in 
connection  with  millivoltmeters.  One  of  the  earliest  proposed  was  that 
shown  in  Fig.  1.  A  bare  resistance  wire  was  immersed  in  a  column  of 
mercury  located  near  the  cold  junction  of  the  thermocouple.  An  in- 
crease in  temperature  near  the  cold  junction  resulted  in  a  rise  of  the 
mercury  column,  which  short-circuited  more  of  the  resistance  wire,  causing 
an  increase  in  the  potential  difference  across  the  millivoltmeter,  which 
compensated  for  the  decreased  electromotive  force  of  the  thermocouple. 

The  method  is  open  to  the  objection  that  the  compensation  is  accu- 
rate at  only  one  temperature  of  the  hot  junction,  for  the  voltage  change 
across  the  compensating  resistance  is  a  function  not  only  of  the  com- 
pensating resistance  but  also  of  the  current  passing  through  it.  As  the 

*  Electrical  Engineer,  Engineering  Department,  Leeds  &  Nor  thru  p  Co. 


FELIX   WUNSCH  207 

current  changes  with  the  hot-junction  temperature,  it  is  obvious  that 
accurate  compensation  for  cold-junction  temperature  can  be  obtained  for 
only  one  temperature  of  the  hot  junction.  As  an  example,  suppose  that 
the  cold-junction  temperature  is  0°  C.  and  the  hot-junction  temperature 
1000°  C.  and  that  the  e.m.f.  generated  is  60  millivolts.  Assume  the  re- 
sistance in  the  millivoltmeter  circuit  including  the  compensating  resist- 
ance at  0°  C.  to  be  600  ohms.  Then  the  current  in  the  millivoltmeter  is 
fin 

~  =  0.1  milliampere.     If  the  cold-junction  temperature  increases  to 
oUU 

50°  C.,  reducing  the  e.m.f.  by  3  millivolts  and  reducing  the  resistance  in 

•  fif\  Q  K7 

the  compensating  resistor  by  30  ohms,  the  current  is 


=  0.1  milliampere,  the  same  as  before,  and  perfect  compensation  has 
been  obtained  for  a  hot-junction  temperature  of  1000°  C.  But  if  the 
hot-junction  temperature  falls  to  500°  C.  and  the  cold-junction  is  at  0°  C., 
as  in  the  first  case,  and  the  e.m.f.  generated  is  30  millivolts,  the  current 

30 

is  ™~  =  0.05  milliampere.     On  the  other  hand,  should  the  cold  junction 
oUU 

rise  to  50°  C.  while  the  hot  junction  remains  at  500°  C.,  the  resistance  in 
the  circuit  is  reduced  to  570  ohms  as  before  and  the  voltage  is  reduced 
by  3  millivolts  as  before,  because  the  e.m.f.  generated  at  the  cold  junction 

is  independent  of  the  temperature  of  the  hot  junction.     The  current  will 

QQ  _  Q          27 
now  be  fi0f>  _  ofl  =  c?v  =  0.0474  milliamps.     This  current  is  5.2  per 


cent,  lower  than  that  obtained  with  the  cold  junction  at  0°  C.  Conse- 
quently the  reading  on  the  milhvoltmeter  will  be  5.2  per  cent,  low  at 
500°  C.  when  the  cold-junction  temperature  rises  from  0  to  50°  C.,  an  error 
of  26°  C.  In  practice,  the  error  may  be  considerably  greater  than  26°  C., 
since  the  location  of  the  cold  junctions  in  the  head  of  the  thermocouple 
protecting  tube  often  results  in  a  much  greater  rise  of  temperature  than 
50°  C.  at  the  cold  junctions. 

Fig.  2  shows  a  device  operating  on  the  same  principle  as  that  just 
described  but  employing  a  different  construction.  A  number  of  thin 
carbon  disks  are  tightly  packed  in  a  porcelain  cylinder,  which  has  a 
small  coefficient  of  expansion.  The  rod  is  made  of  a  metal  having  a 
large  coefficient  of  expansion,  such  as  zinc.  As  the  temperature  increases 
the  rod  expands  and  increases  the  pressure  between  the  carbon  disks, 
thus  decreasing  the  resistance.  The  device  is  connected  in  series  with 
the  thermocouple  and  millivoltmeter.  This  device  has  the  same  defect 
as  that  shown  in  Fig.  1,  namely,  it  compensates  accurately  for  varia- 
tions in  cold-junction  temperature  at  only  one  temperature  of  the  hot 
junction.  Fig.  3  shows  another  method  of  automatically  compensating 
for  cold-junction  temperature.  A  is  a  resistor  which  increases  its 
resistance  as  the  temperature  rises.  B  is  a  resistor  whose  resistance 
remains  constant  when  the  temperature  changes.  The  operation  is  as 


208        AUTOMATIC   COMPENSATION    FOR   COLD-JUNCTION   TEMPERATURES 


follows:  Assuming  a  constant  temperature  at  the  hot  junction,  the  cur- 
rents through  A  and  B  decrease  as  the  temperature  of  the  cold  junction 
increases.  This  would  tend  to  decrease  the  potential  across  A  but,  by 
proper  adjustment  of  A  and  B,  this  decrease  can  be  neutralized  by  the 
increase  in  resistance  of  the  coil  A,  the  net  result  being  that  the  potential 
difference  across  A  for  a  constant  temperature  at  the  hot  junction  re- 


2inc  Rod      \       Carbon  Disks 

Porcelain  Tube 
FIG.  2. — USE  OF  CARBON  DISKS  FOR  COMPENSATING  FOR  COLD-JUNCTION  TEMPERATURES 

mains  constant,  although  the  temperature  of  the  cold  junction  changes. 
The  compensation  can  be  made  correct  for  one  temperature  of  the  hot 
junction  only  and  consequently  is  not  better  than  the  devices  shown  in 
Figs.  1  and  2. 

All  devices  of  this  nature,  where  an  actual  current  is  drawn  from  the 
thermocouple,  are  defective  in  that  they  depend  for  accuracy  on  the 


A  -  Nickel  coil 
B  -  Mangan'm  coil 


Thermo  couple 


FIG.  3. — USE  OF  NICKEL  AND  MANGANIN  COILS  FOR  COMPENSATING  FOR  COLD-JUNC- 
TION TEMPERATURE  CHANGES. 

constancy  of  resistance  of  the  thermocouple.  The  thermocouple,  how- 
ever, gradually  changes  resistance  with  use  and  gives  no  external  evidence 
of  such  change  until  the  circuit  is  actually  broken. 

Fig.  4  shows  another  scheme  very  similar  to  that  just  described  which 
has  the  same  defect,  namely,  it  compensates  accurately  for  variations  of 
cold  junction  only  when  the  hot-junction  temperature  remains  constant. 
It  is  not  as  good  as  the  scheme  shown  in  Fig.  2  because  it  is  as  inaccurate 
and,  in  addition,  introduces  additional  resistance  in  the  millivoltmeter 


FELIX   WUNSCH 


209 


circuit,   which  is  objectionable  because  a  more  sensitive  millivoltmeter 
must  be  used. 

Fig.  5  shows  another  scheme  of  automatic  cold-junction  compensation. 
The  resistors  a,  b,  c,  and  d  are  connected  in  the  form  of  a  Wheatstone 
bridge,  as  shown,  and  placed  near  the  cold  junction  of  the  thermocouple. 
Three  arms  of  this  bridge  are  made  of  resistances  having  a  zero  tem- 
perature coefficient,  such  as  manganin.  The  fourth  arm  is  made  of  a 
metal  having  a  high  temperature  coefficient,  such  as  nickel.  These 


Mil  1 1  vo  It  meter 

Resistor  C  is  nickel 
Resistors  B-,Atoirid  D  are 

FIG.  4. — SECOND  METHOD  USING  NICKEL  AND  MANGANIN  COILS. 

resistances  are  adjusted  so  that  they  have  equal  resistances  at  some 
reference  temperature,  such  as  0°  C.  The  bridge  consequently  is  balanced 
at  this  temperature  and  no  difference  of  potential  due  to  the  battery 
Ba  appears  at  the  terminals  e  and  /.  If  the  temperature  of  the  cold  junc- 
tion changes,  the  resistance  of  the  nickel  coil  changes  and  throws  the 
bridge  out  of  balance.  This  changes  the  e.m.f.  across  the  terminals 
e  and  /  and  if  the  coils  are  properly  adjusted  will  exactly  neutralize  the 


Millivoltmeter 


.^   ,  ,    , =^^ Thermocouple 

Cola  Junctions 


FIG.  5. — METHOD  IN  WHICH  RESISTORS  ARE  CONNECTED  IN  FORM  OF  WHEATSTONE. 

BRIDGE. 

change  in  e.m.f.  at  the  cold  junction  of  the  thermocouple.  The  nickel 
resistance  c  is  made  low  in  comparison  to  the  total  resistance  in  the 
millivoltmeter  circuit,  so  that  resistance  changes  of  the  nickel  coil  do  not 
materially  change  the  total  resistance  in  the  millivoltmeter  circuit. 
Consequently  the  compensation  is  practically  correct  for  all  tempera- 
tures of  the  hot  junction  provided  the  voltage  at  the  terminals  of  the 
bridge  is  kept  constant.  Changes  in  the  voltage  of  the  battery  can  be 
corrected  for  by  means  of  a  rheostat  in  the  battery  circuit. 

14 


210        AUTOMATIC    COMPENSATION   FOR   COLD-JUNCTION   TEMPERATURES 

Other  methods  of  automatically  compensating  for  cold-junction  tem- 
perature changes  by  means  of  magnetic  shunts  and  other  devices  mounted 
on  the  millivoltmeter  to  shift  the  position  of  the  pointer  have  been  used 
more  or  less  successfully.  The  cold  junctions  are  brought  to  the  milli- 
voltmeter by  the  use  of  the  proper  lead  wire.  Fig.  6  shows  a  simplified 
diagram  of  connections  of  the  Leeds  &  Northrup  split-circuit  potenti- 
ometer system  with  automatic  cold-junction  compensator.  The  current 
from  the  battery  Ba  divides  at  a  and  b,  one-half  passing  through  the  upper 
branch  which  includes  the  slide  wire  S  and  resistances  G  and  B.  The 
lower  branch  includes  the  nickel  coil  D  and  the  resistance  C.  All  the 
resistances,  except  the  nickel  coil  D,  are  made  of  rnanganin  wire  having  a 
zero  temperature  coefficient.  The  current  in  the  two  branches  is  kept 
constant  by  adjusting  the  rheostat  R  until  the  drop  of  potential  across 


FIG.  6. — SIMPLIFIED   DIAGRAM   OP   CONNECTIONS   OF  LEEDS   &   NORTHRUP    SPLIT- 
CIRCUIT  POTENTIOMETER  SYSTEM. 

the  coil  C  is  equal  to  the  e.m.f.  of  the  standard  cell  W.  The  galva- 
nometer Ga  shows  when  a  balance  has  been  obtained.  In  the  recording 
instrument,  this  adjustment  is  made  automatically.  The  resistances  B 
and  C  are  high  and  are  so  chosen  that  the  resistances  in  the  two  branches 
are  equal.  The  nickel  coil  is  located  near  the  cold  junction  of  the  thermo- 
couple and  has  such  a  value  that  changes  in  cold- junction  temperature 
are  compensated  for  by  changes  in  the  drop  of  potential  across  this  coil 
caused  by  changes  in  its  resistance  with  temperature. 

The  value  of  the  compensating  coil  is  calculated  as  follows :  let  R  = 
resistance  of  compensating  coil  D  at  reference  temperature;  c  =  change 
in  e.m.f.  of  thermocouple  per  degree  change  in  temperature  of  cold 
junction;  K  =  temperature  coefficient  of  nickel  composing  compensating 
coil  per  ohm  per  degree;  t  =  temperature  change  of  cold  junction; 
i  =  current  in  branch  including  nickel  coil;  e  =  change  in  e.m.f.  of 


FELIX   WUNSCH  211 

thermocouple  due  to  temperature  change  t  of  cold  junction;  e\  =  change 
in  fall  of  potential  across  nickel  coil  due  to  temperature  change  t  of  cold 
junction. 

In  order  to  have  compensation  e  must  equal  e\. 

e  =  ei     e  =  et    e\  =  RKti 

ct  =  RKti        R  =  ~ 
Ki 

Therefore  the  resistance  D  is  made  equal  to  the  constant  quantity 

ft 

v>.  and  e  equals  e\  regardless  of  changes  in  temperature  of  the  cold 
Kt 

junction. 

The  value  of  the  resistance  G  is  so  chosen  that  the  scale  starts  at  0° 
or  at  any  other  desired  temperature.  For  example,  suppose  the  tempera- 
ture range  is  from  0  to  1000°  C.  The  coil  G  will  be  equal  to  the  nickel 
coil  D  at  0°  C.  Suppose  both  the  hot  junction  and  the  cold  junction  are 
at  0°  G.  The  e.m.f .  of  the  thermocouple  will  be  zero  and  the  difference  of 
potential  between-  K  and  J  will  also  be  zero  because  the  resistance  of  D 
equals  that  of  G.  If  the  temperature  of  the  hot  junction  increases  to 
1000°  C.  and  the  cold  junction  stays  at  0°  C.  the  galvanometer  will  show 
a  balance  when  the  contact  K  is  at  the  extreme  right  side  of  the  scale 
(side  marked  H).  Now  suppose  the  temperature  of  the  cold  junction 
increases.  The  e.m.f.  of  the  couple  will  decrease  but  the  resistance  of  the 
nickel  coil  increases  and  causes  a  decrease  in  the  e.m.f.  across  the  points 
J  and  K  that  exactly  compensates  for  the  decrease  in  the  e.m.f.  of  the 
thermocouple.  The  result  is  that  the  contact  K  remains  at  1000°  on 
the  scale,  which  is  the  temperature  at  the  hot  junction. 

If  base-metal  thermocouples  are  used,  the  leads  connecting  the 
thermocouple  with  the  measuring  instrument  are  usually  of  the  same 
material  as  the  thermocouples,  consequently  the  cold  junction  is  located 
at  the  measuring  instrument.  In  this  case  the  cold-junction  coil  is 
mounted  in  the  measuring  instrument.  If  noble-metal  couples  are  used, 
it  is  customary  to  use  copper  leads  to  the  thermocouple.  In  this  case- 
the  cold-junction  coil  is  mounted  in  the  head  of  the  thermocouple  and 
connected  to  the  measuring  instrument  by  means  of  three  copper  wires. 
The  connections  are  as  shown  in  Fig.  7.  It  will  be  noted  that  the  6  and 
c  leads  are  in  opposite  branches  of  the  potentiometer  circuit  and  being 
of  equal  length  the  resistance  causes  no  error,  because  the  drop  in  poten- 
tial in  one  lead  is  compensated  for  by  the  drop  in  the  other  lead. 

If  the  metals  of  the  leads  connecting  the  thermocouple  have  the  same 
characteristics  as  the  metals  of  the  thermocouple,  the  cold  junction  will 
be  located  at  the  measuring  instrument  and  the  compensating  coil  can 
also  be  located  in  the  measuring  instrument,  thus  avoiding  the  use  of  the 
three  copper  leads  that  must  be  run  to  the  cold-junction  coil  when  it  is 


212      AUTOMATIC    COMPENSATION    FOR    COLD-JUNCTION    TEMPERATURES 

mounted  in  the  head  of  the  thermocouple.  Patent  leads  composed  of 
base-metal  alloys  that  give  the  same  e.m.f.  as  platinum  platinum  -j-  10 
per  cent,  rhodium  wires  can  be  obtained.  These  leads  can  be  used  to 
connect  the  platinum  platinum-rhodium  couples  to  the  measuring 
instrument.  The  use  of  these  leads  is  increasing  rapidly  and  where 
used  they  have  given  satisfaction. 


FIG.  7. — CONNECTION  OF  COLD-JUNCTION  COIL  WITH  NOBLE-METAL  COUPLE. 

DISCUSSION 

W.  H.  BRISTOL,  Waterbury,  Conn. — Five  schemes  for  automatically 
taking  care  of  the  cold  end  of  the  thermocouple  are  shown  and  attention 
is  called  to  the  different,  you  might  say,  limitations  of  these  schemes. 
They  give  perfect  automatic  connections  with  a  cold  end  for  a  certain 
predetermined  point  on  the  scale,  and  either  above  or  below  such  points 
there  would  be  over-compensation  or  under-compensation,  as  the  case 


DISCUSSION  213 

may  be,  which  could  be  calculated  very  readily.  In  some  papers  pre- 
sented it  has  been  shown  what  the  limitations  are.  I  would  like  to  men- 
tion that  each  of  the  first  four  schemes  shown  was  developed  by  myself 
or  the  company  I  represent,  and  we  have  patented  the  basic  principles 
of  them  all.  Our  latest  development  is  a  thermoelectric  pyrometer  in 
which  the  cold  end  of  the  thermocouple  is  automatically  compensated 
for  the  entire  range,  which  can  be  anything  from  —0  through  the  work- 
ing range  of  a  thermoelectric  couple.  The  method  of  obtaining  this 
result  is  to  secure  to  the  controlling  spring  inside  of  the  instrument  a 
length  of  bimetallic  differential  thermostatic  metal  so  that  it  unwinds 
or  winds  the  control  spring  to  suit  the  changes  that  occur  in  the  instru- 
ment itself.  The  thermocouple  is  extended  entirely  from  the  hot  end 
to  the  inside  of  the  instrument. 


214       USE  OF  MODIFIED  ROSENHAIN  FURNACE  FOR  THERMAL  ANALYSIS 


Use  of  Modified  Rosenhain  Furnace  for  Thermal  Analysis 

BY    H.    SCOTT,*    A.    B.,    AND    J.    R.    FREEMAN,    JR.,*   B.    S.,    WASHINGTON,    D.    C. 
(Chicago  Meeting,  September,  1919) 

IN  a  paper  read  before  the  Institute  of  Metals,  Rosenhain1  described 
a  new  type  of  furnace  designed  primarily  for  the  thermal  analysis  of 
metals  by  the  inverse-rate  method  and  used  by  him  in  the  metallurgical 
department  of  the  National  Physical  Laboratory  with  considerable 
success.  In  his  discussion  of  this  furnace,  Rosenhain  pointed  out  some 
difficulties  met  with  in  its  operation,  such  as  uniformity  of  rate  of  heating 
or  cooling  being  inadequate  for  the  degree  of  accuracy  aimed  at.  To 
overcome  this  difficulty,  he  suggested,  in  place  of  motor  propulsion,  a 
gravity  drive  controlled  by  a  "  hydraulic  cylinder  with  a  relief  valve  whose 
width  of  opening  can  be  regulated  to  allow  of  any  desired  rate  of  motion." 
The  authors,  in  constructing  a  thermal-analysis  furnace  of  Rosenhain's 
type,  have  therefore  followed  this  suggestion  and  also  added  certain 
features  that  increase  somewhat  the  convenience  and  simplicity  of  its 
operation.  Requests  for  information  regarding  this  furnace  and  the 
highly  satisfactory  results  obtained  from  its  use,  it  is  believed,  justify 
describing  its  construction  and  operation  in  sufficient  detail  to  make  pos- 
sible its  duplication  or  improvement. 

Description  of  Furnace. — The  details  of  the  furnace  construction  are 
shown  in  Fig.  1,  which  is  drawn  to  scale.  The  heating  tube  is  of  ^  in. 
(6.35  mm.)  wall  "alundum"  heated  at  the  upper  end  by  seventeen  turns 
of  0.52  mm.  platinum  wire,  which  is  coated  with  "alundum"  cement 
supplied  for  this  purpose.  The  cement  coating  is  essential  when  a  high 
temperature  (over  1000°  C.)  is  required,  as  it  prevents  hot  spots  with  the 
resulting  burning  out  of  the  heater.  This  platinum-wire  winding,  unlike 
"nichrome, "  is  entirely  satisfactory  for  temperatures  of  at  least  1000°  C. 
It  has  been  maintained  at  that  temperature  continuously  for  2  months  and 
shows  no  signs  of  deterioration.  This  temperature  is  maintained  by  a 
current  of  5  amp.  drawn  from  30  volts  potential,  so  its  necessarily  con- 
tinuous operation  is  quite  economical. 

The  furnace  is  heated  at  the  top,  as  is  Rosenhain's,  to  avoid  convection 
currents,  but  the  sample  in  its  containing  tube  is  introduced  from  the  bot- 
tom, or  cold  end.  This  removes  the  disadvantages  of  his  method,  which 
consist  of  inconvenience  in  position  of  the  sample  and  control  apparatus 

*  Assistant  Physicist,  U.  S.  Bureau  of  Standards. 

1  Some  Appliances  for  Metallographic  Research.     Jril.  Inst.  Metals  (1915)  13,  160. 


H.   SCOTT   AND   J.   R.    FREEMAN,   JR. 


215 


Wire 


Monel   Metal 
Retoin/nq  Tube 

tf/undum   Tube 


Woter  Coo/er 


\ 

.^  1/-V"  Furnace     she/I  backet 


ivith    infusorial   earth. 


FIG.  1. — DIAGRAM  OF  FURNACE. 


216       USE  OF  MODIFIED  ROSENHAIN  FURNACE  FOR  THERMAL  ANALYSIS 


r 

J^ 

A 

vy  • 

i.-s 

v  -*:.-. 

y-  *>, 

y  ^ 

i 

L 
C 

i_i»_ 

y  . 

H 

II 

i 

:o'- 

// 

/* 

B 

C 

^J 

^m 

r^ 

n 

p 

^ 

n 

II 

•s 

5> 

)[ 

—? 

B 

lc 

wft= 

. 
-trr 

-^. 

^ 

•" 

y 

i 

1 

i 

1 

—  fl 

L, 

-. 

•j 

: 

JL 

K 

=  1 

C 

j 

-> 

J 

N 
Y 

<. 

1 

A 
B 
C 
D 

F 

fy 

^ 

i 

1 

* 

M 

G 
H 
1  i 
^ 
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—  - 

/ 

d 

•p 

i 

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/<7 

0  cr 

1. 

i 

'urn  ace 
B  E /era  for 
C  Quarte  Tube 
Holder 
Clamps 

F  Elevator   Guide 
G  Vacuum  Flash 
H  Thermocouple  Leads 
I  Vacuum  Pump 

Connecting  Tube 
J  Lowering    Weight 
i\  Elevating   Weights 
L  <2/  Cylinder 

Needle 
Valve 

N  Coarse  Valve 
O  Plunger  Head 


FIG  2  — DIAGRAM  OF  ELEVATING  MECHANISM 


H.    SCOTT    AND    J.    R.    FREEMAN,    JR. 


217 


and  the  heating  of  some  part  of  the  sample  tube  at  all  times  to  the  maxi- 
mum temperature  of  the  furnace.  The  latter  disadvantage  may  prove 
serious  in  the  event  of  slight  inhomogeneities  in  the  thermocouple  wire. 
Description  of  Elevating  Mechanism. — The  details  of  the  rate-control 
mechanism  are  shown  in  Fig.  2.  The  weights  K  (total  weight  15  Ib. 
—6.8  kg.)  operating  over  pulleys  supply  the  force  to  lift  the  elevator 
B  and  the  weight  J  (2  Ib.)  to  lower  it.  The  rate  of  motion  of  the  tube  C 
clamped  on  the  elevator  is  controlled  by  the  flow  of  oil  from  one  end  of  the 
cylinder  L  to  the  other  through  the  needle  valve  M.  The  oil  cylinder  is  kept 
open  to  the  air  and  filled  with  a  good  grade  of  engine  oil,  care  being  taken 
that  the  oil  is  free  from  dirt  and  air  bubbles,  which  might  easily  cause  vari- 


Quartz 
Tube 


Vacuum  Pump 
Connection 


"ass    Pluq 
Sea//ng  Quartz 
T^ube 


FIG.  3. — SAMPLE  MOUNTING. 


ations  in  the  rate  of  motion  of  the  plunger.  The  sample  tube  C  is  held 
and  centered  with  three«etscrews  in  a  sleeve  D,  which  fits  into  a  receptacle 
on  the  elevator  facilitating  rapid  changing  of  the  sample.  A  guide  rod 
F  prevents  rotation  of  the  elevator  and  steadies  its  motion. 

Details  of  Operation. — The  differential  method  of  obtaining  curves  may 
be  used  with  this  furnace,  but  the  experience  of  the  authors  has  been 
that  more  valuable  and  satisfactory  results  are  obtained  by  use  of  the 
inverse-rate  method,  which  accordingly  has  been  used  almost  exclusively. 
The  adoption  of  the  inverse-rate  method  limits  the  pyrometric  require- 
ments to  a  single  thermocouple  and  potentiometer.  This  permits  of  the 
use  of  a  somewhat  novel  method  of  mounting  samples,  first  used  by  Bur- 


218       USE  OF  MODIFIED  ROSENHAIN  FURNACE  FOR  THERMAL  ANALYSIS 

gess  and  Crowe2  in  their  researches  on  pure  iron.  This  method  is  illus- 
trated in  Fig.  3.  The  operations  involved  consist  simply  of  cutting  a 
0.5  mm.  slot  in  the  sample  with  a  small  hack  saw  and  riveting  in  this 
slot  the  flattened  head  at  the  hot  junction  of  a  platinum-90  platinum- 
10  rhodium  thermocouple  in  the  form  of  0.5-mm.  diameter  wire.  The 
mounted  sample  is  sealed  in  the  quartz  tube  and  a  vacuum  maintained 
through  the  brass-plug  connection. 


FIG.  4. — ASSEMBLED  FURNACE  AND  AUXILIARY  APPARATUS. 


This  method  of  mounting  has  the  advantages  of  good  thermal  con- 
tact between  sample  and  thermocouple,  use  of  small  samples — usually 
^8  by  %2  by  %  in.  (3.2  by  7.15  by  9.5  mm.)  weighing  about  1.7  gm. 
— with  the  consequent  elimination  of  detectable  thermal  gradients 
and  ease  of  preparation  of  samples.  Its  chief  disadvantage  is  the  slight 
contamination  of  the  thermocouple  resulting  from  close  contact  with  the 
sample  at  high  temperatures.  This  source  of  error  is  easily  avoided  by 
using  a  homogeneous  thermocouple  and  frequently  removing  the  short 
length  subject  to  contamination.  A  check  can  be  had  on  the  accuracy  and 


2  The  Critical  Ranges  A2  and  As  of  Pure  Iron. 
Paper  213. 


U.  S.  Bureau  of  Standards  Sci. 


H.    SCOTT   A-ND   J.    R.    FREEMAN,    JR. 


219 


sensitivity  of  the  apparatus  under  actual  operating  conditions  by  taking 
curves  on  pure  iron,  the  transformation  Az  of  which  has  a  maximum  heat 
effect  very  constant  at  768°  C.3  independent  of  rate  of  temperature  change. 


£<3%    Mc/fe/     5  tee/ 

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Curse                                        Curve 

bOO' 

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* 

500° 

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V) 
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l 

here              .  •  "I 

JS' 

W 

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in    Seconds     To   Heaf   or 
/o    ts          y    /0 

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Coo/ 

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FIG.  5. — THERMAL  CURVES  OF  28-PER  CENT.  NICKEL  STEEL. 

The  temperature  measurements  are  made  with  a  dial  potentiometer 
and  the  time  intervals  are  recorded  on  a  drum  type  chronograph,  which 

3  Burgess  and  Crowe :  loc.  cit. 


220      USE  OF  MODIFIED  ROSENHAIN  FURNACE  FOR  THERMAL  ANALYSIS 

instruments  have  already  been  described.4     The  assembled  apparatus 
is  shown  in  Fig.  4. 

A  heating  and  cooling  curve,  characteristic  of  the  furnace,  taken  on  a 
transformationless  (28  per  cent,  nickel)  steel  over  the  temperature  range 
50°  to  1000°  C.,  is  shown  in  Fig.  5,  each  curve  being  divided  into  two  sec- 
tions for  convenience  in  reproduction.  Curves  of  a  steel  showing  several 
critical  points  of  small  intensity  and  taken  with  this  apparatus  are 
available  in  the  work  of  one  of  the  present  authors.5  It  may  be  noted 
that  the  rate  of  temperature  change  is  somewhat  slower  at  the  lower 
temperatures  than  at  the  higher,  as  would  be  expected,  but  that  the 
change  is  not  sufficient  to  obscure  a  transformation  occurring  anywhere 
between  100°  and  1000°  C.  This  change  in  rate  is  emphasized  at  the 
lower  temperatures  by  the  parabolic  form  of  the  relation  between  the  tem- 
perature and  the  electromotive  force  of  the  platinum  couple,  for  the 
curves  are  plotted  with  time  of  unit  electromotive  force  change  as  abscis- 
sas as  a  matter  of  convenience.  The  actual  rate  change  can  be  reduced, 
and  probably  eliminated  for  a  given  rate,  by  using  a  metal  cylinder  or 
"alundum"  tube  tapered  to  increase  the  heat  conduction  at  the  lower 
temperatures. 

It  might  be  apprehended  that  the  gravity  drive  would  impart  an 
extended  acceleration  to  the  elevator  instead  of  a  uniform  velocity,  but 
that  the  time  required  for  the  rate  to  become  uniform  is  slight  is  shown  by 
the  short  curves,  on  the  right-hand  side,  obtained  by  bringing  the  sample 
to  the  constant  temperature  designated  and  then  taking  readings  from 
the  time  of  opening  the  valve.  The  time  required  for  the  rate  to  become 
normal  for  that  valve  setting  is  only  63^  min.  on  heating  and  4^  min. 
on  cooling,  while  the  corresponding  temperature  interval  is  only  33°  and 
35°  C.  This  is  an  extremely  useful  characteristic  of  the  furnace,  as  it 
enables  the  separation  of  one  transformation  superimposed  on  the  end 
of  another  by  holding  the  sample  at  a  temperature  at  which  the  first 
transformation  will  complete  itself  and  then  starting.  It  also  facilitates 
the  study  of  the  effect  of  time  of  holding  in  the  proximity  of  the  trans- 
formation temperature  on  its  position;  that  is,  determining  the  limits  of 
the  transformation  temperature  at  what  amounts  to  zero  rate  of  tem- 
perature change. 

The  authors  wish  to  acknowledge  the  skill  contributed  by  Mr.  F.  E. 
Mann  in  the  construction  of  this  furnace  and  the  assistance  of  Miss  H.  G. 
Movius  and  Mr.  H.  A.  Wadsworth  in  its  successful  development. 

*  4  Burgess  and  Crowe:  loc.  cit.  Dr.  P.  D.  Merica  has  substituted  a  pair  of  stop 
watches  for  the  costly  chronograph  with  good  results  providing  the  time  interval  is 
greater  than  15  sec. 

6  Scott:  Trans.  (1920)  62.     Also  U.  S.  Bureau  of  Standards  Sci.  Paper  335. 


A    HOT-WIRE    ANEMOMETER    WITH    THERMOCOUPLE  221 


A  Hot-wire  Anemometer  with  Thermocouple 

BY   T.    S.    TAYLOR,*   PH.    D.,    EAST   PITTSBURGH,    PA. 
(Chicago  Meeting,  September,  1919) 

THE  development  of  the  linear  hot-wire  anemometer  has  been  chiefly 
due  to  the  efforts  of  L.  V.  King1  and  A.  E.  Kennelly  and  H.  S.  Sanborn.2 
The  anemometers  used  by  these  investigators  consisted  essentially  of  a 
fine  heating  wire  having  attached  leads  for  resistance  measurements  at 
distances  of  10  or  more  centimeters  from  each  other.  In  using  such  an 
anemometer,  the  current  is  measured  that  is  necessary  to  maintain  the 
resistance  of  the  wire,  between  the  two  leads,  constant  for  different  air 
velocities.  This  resistance  is  always  so  chosen  that  the  temperature 
of  the  heating  wire  will  be  sufficiently  high  to  make  the  observations 
practically  independent  of  small  variations  in  the  temperature  of  the  gas 
in  which  the  anemometer  is  placed.  The  measurement  of  the  resistance 
of  the  anemometer  wire  requires  a  Kelvin  bridge  set  up,  which  for  com- 
mercial work  is  not  altogether  desirable.  The  cooling  effect  due  to  differ- 
ent air  velocities  depends  on  the  temperature  difference  between  the 
wire  and  the  gas  and  the  total  quantity  or  mass  of  gas  passing  the 
wire  per  unit  time.  Since  the  temperature  of  the  wire  is  maintained 
constant,  the  effect  observed  in  the  change  in  the  heating  current  for  a  given 
change  in  air  velocity  is  a  measure  of  the  difference  between  the  gas  flow  in 
the  two  cases.  Such  an  anemometer,  therefore,  measures  the  average  gas 
flow  for  a  length  depending  on  the  distance  between  the  two  resistance 
leads.  Therefore  the  instrument  in  the  form  used  thus  far  is  not  very 
satisfactory  for  measuring  gas  velocities  in  small  tubes  or  in  places  where 
the  velocity  varies  rapidly  across  the  line  of  flow. 

For  the  purpose  of  measuring  gas  flow  through  relatively  small  tubes, 
where  the  velocity  changes  rather  rapidly  across  the  tube,  the  hot-wire 
anemometer  has  been  modified  in  the  manner  shown  diagrammatically 
in  Fig.  1.  It  consists  essentially  of  a  platinum  heating  wire  H  about  0.007 
in.  (0.178  mm.)  in  diameter  and  ^  to  1  in.  (12.7  to  25.4  mm.)  long 
stretched  across  a  suitable  framework,  say  of  glass.  This  wire  has 
attached  at  its  mid-point  a  copper-constantan  thermocouple  made  of 
0.002-in.  (0.05  mm.)  wire,  C,  A. 

*  Research  Physicist,  Westinghouse  Research  Laboratory. 

1  L.  V.  King:  Phil.  Trans.  Roy.  Soc.  London  (1914)  A  214,  373-432.     Proc.  Roy. 
Soc.,  London  (1914)  A  90,  563-570. 

2  A.  E.  KenneDy  and  H.  S.  Sanborn:    Proc.  Amer.  Phil.  Soc.  (1914-15)  63,  55-77. 


222 


A    HOT-WIRE   ANEMOMETER   WITH   THERMOCOUPLE 


In  using  the  instrument,  it  has  been  found  quite  satisfactory  to  measure 
the  current  necessary  to  maintain  the  temperature  of  the  wire  as  deter- 
mined by  the  thermocouple,  say  200°  C.  above  the  temperature  of  gas  in 
which  it  is  placed.  The  temperature  can  be  measured  by  means  of  a 
thermocouple  potentiometer  indicator,  such  as  is  sold  by  Leeds  &  North- 
rup.  The  current  can  be  measured  either  by  means  of  a  first-class  amme- 
ter or  by  means  of  a  standard  resistance  and  potentiometer  system. 

This  anemometer  can  be  so  arranged  as  to  be  placed  in  various  posi- 
tions in  a  tube  of  any  size  and  still  not  interfere  seriously  with  the  air  velo- 
city. The  heating  wire  can  be  coiled  and  occupy  a  relatively  small  area 


H 


-2cm- 


FIG.  1. — MODIFIED  ARRANGEMENT  OF  HOT-WIRE  ANEMOMETER. 

or  can  be  stretched  straight,  as  indicated  in  the  figure.  By  use  of  the 
thermocouple,  the  temperature  of  the  gas  at  the  point  where  the  velocity 
is  desired  can  be  readily  obtained  when  there  is  no*  current  flowing  through 
the  wire.  If  direct  current  is  used,  the  mean  value  of  the  readings  ob- 
tained for  current  flowing  in  each  direction  should  be  used;  this  will  avoid 
the  possibility  of  any  error  arising  due  to  the  thermocouple  wires  not  being 
at  exactly  the  same  point  on  the  wire.  By  using  alternating  current  for 
heating  the  wire,  this  error  is  also  avoided. 

The  anemometer  is  calibrated  in  the  way  described  by  King3  and 
Kennelly.4  This  can  be  done  by  having  it  on  an  arm  that  can  be 
revolved  at  various  speeds.  A  curve  showing  the  relation  between  the 
speed  of  the  wire  through  air  at  room  temperature,  about  21°  C.,  and  the 
current  necessary  to  maintain  the  thermocouple  junction  at  a  tempera- 
ture of  200°  C.  is  shown  in  Fig.  2. 


3  Lot.  cit. 


4  Loc.  cit. 


T.    S.    TAYLOR 


223 


The  abscissas  represent  the  velocity,  in  feet  per  minute,  and  the  ordi- 
nates  the  corresponding  currents,  in  amperes,  necessary  to  maintain  the 
temperature  of  the  wire  at  200°  C.  This  curve  was  obtained  by  revolving 
the  anemometer  on  an  arm  4^  ft.  (1.37  m.)  long  in  a  horizontal  plane. 
Such  a  device  does  not  give  a  very  uniform  swirl  to  the  air.  More  recent 
work  has  been  done  by  having  the  anemometer  calibrated  when  it  is  on  a 
revolving  disk  2^  ft.  (0.762  m.)  in  diameter.  This  gives  a  very  uniform 
swirl  to  the  air  and  hence  it  can  be  accurately  corrected  by  revolving  the 
disk  when  the  anemometer  is  in  a  stationary  position.  The  revolving 
disk  can  also  be  placed  in  an  oven  so  that  the  anemometer  can  be  calibra- 
ted in  the  air  at  various  temperatures  up  to  100°  C.,  at  least,  and  possibly 


850 


3.25 


200     4UO      COO    800     1000   1200  1400    1600   1800  2lK>0  2200   2460  2600    2800  3000 
Velocity  Ft-  per  Min. 

FIG.  2. — RELATION  BETWEEN  SPEED  OF  WIRE  THROUGH  AIR  AT  ROOM  TEMPERATURE 
AND  CURRENT  NECESSARY  TO  MAINTAIN  THERMOCOUPLE  FUNCTION  AT  200°  C. 

200°  C.  Such  an  anemometer  has  the  advantage  of  really  measuring 
the  total  gas  flow  at  a  point  and  at  the  same  time  is  useful  in  measuring 
the  gas  flow  in  places  where  the  temperature  as  well  as  tjie  velocity  of 
the  gas  is  changing  from  point  to  point.  Such  cases  would  exist,  for 
example,  in  a  tube  having  a  heater  wound  around  it. 

In  the  calibration  of  the  anemometer,  the  current  and  thermocouple 
connections  are  made  to  the  anemometer  by  means  of  slip  rings  and  con- 
tact brushes  on  the  revolving  shaft.  It  was  not  found  necessary  to  use 
low-resistance  mercury  contacts,  as  is  the  case  when  the  resistance 
measurements  are  made  by  means  of  a  Kelvin  bridge,  as  was  done  by 
King  and  others.  Occasional  parasitic  currents,  due  to  imperfect  brush 
contact  and  temperature  changes,  arose  while  calibrating,  but  they  are 


224  A    HOT-WIRE   ANEMOMETER   WITH   THERMOCOUPLE 

readily  detected  and  eliminated  by  the  method  of  reversals  of  current  and 
by  noting  the  thermocouple  readings  where  the  anemometer  heating 
wire  has  no  current  through  it.  By  the  use  of  an  accurate  thermocouple 
potentiometer,  for  measuring  the  temperature  of  the  wire  and  a  potenti- 
ometer for  measuring  the  heating  current,  very  accurate  velocity  measure- 
ments can  be  obtained.  In  using  this  instrument,  it  is  only  necessary 
first  of  all  to  measure  the  temperature  of  the  air  by  means  of  an  attached 
thermocouple  at  the  point  in  question  before  putting  any  heating  current 
through  the  platinum  wire,  and  then  to  measure  accurately  the  current 
necessary  to  maintain  the  heating  wire,  say  200°  C.,  above  that  of  the  air 
in  which  it  is  placed.  Then,  by  referring  to  the  corresponding  calibration 
curve,  similar  to  the  one  shown  in  Fig.  2,  the  velocity  is  readily  obtained. 

In  addition  to  the  thermocouple-type  of  anemometer  described,  it  has 
been  found  that  a  resistance  type  similar  to  the  ones  used  by  King, 
Kennelly,  and  others,  can  be  used  quite  satisfactorily.  Instead  of  having 
the  leads  for  resistance  measurements  to  the  heating  wire  placed  far  apart 
(10  cm.  or  more)  they  are  placed  close  together.  Then,  in  calibrat- 
ing and  using  the  anemometer  the  potentiometer  is  used  to  measure 
the  drop  between  the  potential  leads.  This  drop  is  maintained  constant 
for  all  velocities  and  the  values  of  the  current  necessary  to  maintain  this 
potential-drop  constant  is  observed.  The  potential  leads  between  which 
the  potential  drop  is  measured  should  be  of  as  fine  wire  as  possible  in 
order  to  avoid  unnecessary  loss  of  heat  by  conduction.  The  latter  type 
is  a  little  easier  to  construct,  as  it  is  somewhat  easier  to  weld  one  small 
platinum  wire  to  a  larger  one  than  it  is  to  weld  a  small  copper-constantan 
thermocouple  junction  to  a  larger  platinum  wire  and  get  the  two  wires 
at  exactly  the  same  point  on  the  platinum  wire. 

Complete  details  of  the  experimental  results  employing  the  use  of 
these  instruments  in  measuring  the  gas  flow  through  tubes  of  various 
sizes  and  shapes  will  be  given  in  a  later  paper. 

SUMMARY 

1.  A  hot-wire  anemometer  consisting  of  a  small  platinum   heating 
wire  and  having  a  copper-constantan  thermocouple  attached  at  its  mid- 
point has  been  developed  and  found  to  be  useful  in  measuring  the  dis- 
tribution of  gas  flow  across  small  channels. 

2.  The  same  instrument  can  be  used  even  though  the  temperature 
may  vary  from  point  to  point  across  the  channel. 

3.  A  hot-wire  anemometer  of  the  usual  type  but  which  has  the  leads 
generally  used  for  the  resistance  measurements  placed  quite  close  to- 
gether has  been  found  very  satisfactory  when  it  is  used  so  that  observa- 
tions are  made  of  the  current  necessary  to  maintain  a  definite  drop 
between  the  potential  leads  for  various  velocities. 


HIGH-TEMPERATURE    THERMOMETERS  225 


High-temperature  Thermometers 

BY    R.    M.    WILHELM,*    WASHINGTON,    D.    C. 
(Chicago  Meeting,  September,  1919) 

HIGH-TEMPERATURE  thermometry,  as  treated  in  this  paper,  deals  with 
the  measurement  of  temperature  in  the  range  100°  to  550°  C.  The 
lower  limit  corresponds  to  the  temperature  of  boiling  water  at  normal 
atmospheric  pressure,  the  upper  limit  is  approximately  the  highest  tem- 
perature to  which  a  mercury-in-glass  thermometer  may  be  safely  sub- 
jected.1 In  this  range  the  domains  of  pyrometry  and  thermometry 
overlap  somewhat,  for  high-temperature  resistance  thermometers  and 
thermocouples  generally  classed  as  pyrometers  are  often  used  below 
550°  C.  This  paper  will  be  confined  to  high-temperature  mercurial 
thermometers,  and  indicating  and  recording  thermometers  of  the  vapor 
pressure,  liquid  or  gas  filled,  and  bimetallic  or  graphite-metal  expansion 
types,  a  classification  of  which  is  as  follows: 

CLASSIFICATION  OF  THERMOMETERS 

GENERAL  CLASSIFICATION  SUBDIVISIONS 

Mercury-in-glass  f  -,    ,  _  f  Etched  or  engraved  steins 

(mercurial)       \  \  Enclosed  scale  (Einschluss) 


Indicating  and 


[  Industrial 

( Vapor  pressure 
Pressure  \  Liquid  filled 
I  Gas  filled 


recording 

Bimetallic 

Graphite-metal  expansion 

HIGH-TEMPERATURE  MERCURIAL  THERMOMETERS 

In  the  laboratory  the  so-called  engraved  or  etched-stem  thermometer 
is  more  generally  used  than  the  enclosed  scale  or  "einschluss"  type. 
For  some  classes  of  work,  and  especially  under  high-temperature 
conditions,  the  enclosed-scale  thermometer  has  some  advantages  over  the 
etched-scale  type,  in  that  the  coloring  matter  in  the  graduation  lines 
cannot  be  removed  by  solvents,  the  thermometer  can  be  repaired  if  only 

*  Associate  Physicist,  U.  S.  Bureau  of  Standards. 

1  Quartz  glass  thermometers  filled  with  mercury  under  pressure  have  been  con- 
structed to  measure  temperatures  up  to  750°  C.;  they  are  not  used  in  this  country, 
however,  except  to  a  negligible  extent.  Reports  as  to  their  behavior  have  not  been 
promising. 

15 


226 


HIGH-TEMPERATURE   THERMOMETERS 


the  outer  tubing  is  broken,  and  parallax  may  be  avoided  by  a  simple 
procedure.  The  disadvantages,  however,  may  offset  the  good  features. 
The  scale  is  liable  to  become  loose,  and  this,  as  well  as  the  failure  of  the 
scale  and  capillary  tubing  to  be  in  close  contact  with  each  other,  may  in- 
troduce uncertain  errors.  The  computation  of  the  correction  for  emerg- 
ent stem  is  also  more  unreliable  by  reason  of  the  uncertainty  as 
to  the  actual  temperature  inside  the  glass  tubing  of  an  enclosed-scale 
thermometer. 

Fig.  1  shows  characteristic  types  of  etched-stem  high-temperature 
laboratory  thermometers.     The  first  instrument  a  is  the  type  used  as  a 


530  C 


400C 


750  F 


295 


20 


a  b  c 

FIG.  '1. — TYPES  OF  HIGH-TEMPERATURE  MERCURY-IN-GLASS  THERMOMETERS. 

standard  at  the  Bureau  of  Standards  in  the  range  300°  to  530°  C.  The 
second  6  is  a  continuous-scale  instrument  used  for  making  distillation 
tests  and  graduated  0°  to  400°  C.  in  1°  for  total  immersion.  The  third 
c  is  a  20°  to  750°  F.  thermometer  divided  into  2°  intervals  for  partial  im- 
mersion and  is  used  in  oil  testing  or  other  industrial  work. 

In  the  use  of  high-temperature  thermometers,  the  ice-point  gradua- 
tion (0°  C.,  32°  F.)  is  very  desirable  for  checking  purposes.  If  it  is  neces- 
sary to  use  a  thermometer  for  partial  immersion,  that  is  with  the  bulb 
and  only  a  part  of  the  stem  heated,  it  is  advisable  to  use  the  continuous- 


R.    M.    WILHELM 


227 


scale  type  6  or  c  rather  than  the  standard  type  a,  especially  if  the  enlarge- 
ment in  the  capillary  shown  between  the  0°  and  295°  marks  projects 
into  the  air,  which  is  much  cooler  than  the  bulb.  Large  and  uncertain 
errors  may  be  introduced  by  using  a  thermometer  under  these  conditions. 
It  is  not  necessary  or  advisable  to  graduate  thermometers  to  be  used 
in  the  range  200°  to  550°  C.  into  intervals  smaller  than  1°  or  2°  C.  The 
undesirability,  except  under  unusual  conditions,  of  specifying,  for  example, 
six  thermometers  graduated  in  0.1°  intervals  with  a  scale  range  of  25° 
each  to  cover  the  range  200°  to  350°  C.  instead  of  one  instrument  gradu- 
ated into  1°  or  0.5°,  cannot  be  too  strongly  emphasized.  These  short- 
range  thermometers  are  seldom  provided  with  ice-point  graduations  for 
checking,  and  for  this  reason  cannot  be  standardized  to  even  as  high  a 
degree  of  accuracy  as  thermometers  divided  into  1°  or  0.5°  intervals. 
Very  little  is  to  be  gained  therefore  by  attempting  to  read  these  short- 

TABLE  1. — Accuracy  Data  of  Laboratory  Thermometers 


Approximate  Scale 
Range,  Degrees 

Subdivision, 
Degrees 

Probable  Accuracy 
of  Unstandardized 
Thermometers,  Degrees 

• 

Accuracy  that  May 
be  Obtained  if 
Thermometer  is 
Standardized,  Degrees 

Centigrade  Scale 


100  to  200 

5 

2 

1 

100  to  200 

2 

1 

0.5 

100  to  200 

1 

1 

0.2 

100  to  200 

0.2or0.5 

0.5 

0.1 

200  to  300 

5 

2 

1 

200  to  300 

2 

2 

0.5 

200  to  300 

0.5  or  1 

1 

0.2 

300  to  400 

5 

3 

2 

300  to  400 

2 

3 

1 

300  to  400 

1 

2 

0.5 

400  to  500 

5 

5 

2 

400  to  500 

1  or  2 

2 

1 

Fahrenheit  Scale 


212  to  400 

5 

2 

1 

212  to  400 

2 

1 

0.5 

212  to  400 

0.5  or  1 

1 

0.2 

400  to  600 

5 

2 

1 

400  to  600 

2 

2 

0.5 

400  to  600 

1 

1 

0.2 

600  to  800 

5 

3 

2 

600  to  800 

2 

2 

1 

800  to  1000 

5 

5 

2 

800  to  1000 

2 

4 

2 

228 


HIGH-TEMPERATURE    THERMOMETERS 


scale  instruments  to  0.05  °  C.  or  0.01°  C.  when  the  maximum  obtainable 
accuracy  is  from  0.1°  to  0.5°  C. 

Table  1  gives  accuracy  data  for  laboratory  thermometers  in  the  range 
100°  C.  to  500°  C.,  and  212°  F.  to  1000°  F.  This  table  was  compiled  for  use 
at  the  Bureau  of  Standards,  and  is  applicable  only  to  thermometers  of  the 
laboratory  type  used  under  total-immersion  conditions;  i.e.,  with  bulb 
and  stem  containing  the  mercury  column  at  the  same  temperature. 
The  maximum  errors  allowed  in  the  table  represent  the  probable  magni- 
tude of  the  error  that  may  be  expected  of  the  best  grades  of  thermometers. 
This  degree  of  accuracy  cannot  always  be  expected  of  cheaper  grades  of 


Separable  Socket 


This   epace  filled 
with  mercury,  oil  01 
powdered  graphite 


FIG.  2. — INDUSTRIAL  TYPE  OF  MERCURY-IN-GLARS  THERMOMETER. 


thermometers,  or  of  partial-immersion  thermometers.  The  table  em- 
phasizes the  necessity  for  obtaining  and  applying  corrections  to  the  read- 
ings of  thermometers,  especially  if  high  accuracy  is  desired. 

Industrial  Thermometers. — The  use  of  the  chemical  or  laboratory 
type  of  mercurial  thermometer  is  restricted,  as  the  name  implies,  to 
the  laboratory  since  unprotected  glass  thermometers  are  much  too  frail 
to  withstand  the  rough  usage  of  the  plant  and  shop,  and  are  comparatively 
difficult  to  read  as  well. 

There  has,  therefore,  been  developed  what  is  generally  known  as  the 
industrial  type  of  mercurial  thermometer.  As  shown  in  Fig.  2,  it  is  char- 
acterized by  a  heavy  metal  back  and  protecting  tube  for  the  bulb,  large 
and  distinct  figures  and  graduation  marks,  and  threaded  connections  for 
attaching  the  instrument  firmly  and  quickly  to  some  part  of  the  apparatus. 
This  general  design  is  used  for  instruments  for  many  different  operations, 


R.    M.    WILHELM  229 

covering  ranges  of  temperature  from  —  40°  to  1000°  F.  The  instruments 
necessarily  must  be  graduated  and  standardized  for  the  condition  of 
use,  that  is  for  partial  immersion  of  the  mercury  column. 

It  will  be  noted,  from  the  cross-section  view  of  the  bulb  and  protecting 
case  of  the  industrial  thermometer,  that  the  bulb  does  not  come  into  direct 
contact  with  the  substance  the  temperature  of  which  is  to  be  measured 
and  that  the  bulb  is  surrounded  by  large  metal  parts  which  extend  into 
a  region  that  has  a  temperature  different  from  that  of  the  bulb.  These 
two  peculiarities  must  be  taken  into  consideration  in  the  use  of  these 
thermometers.  Since  the  bulb  is  not  in  direct  contact  with  the  heated 
substance,  the  time  that  it  requires  to  take  up  changes  in  temperatures 
is  greater  than  that  necessary  for  a  bare  bulb.  This  "lag, "  as  it  is  called, 
can  be  reduced  by  filling  the  space  between  the  bulb  and  the  outer  wall 
of  the  casing  with  a  good  conducting  medium.  The  most  satisfactory 
substance  to  use  for  temperatures  below  200°  C.  is  mercury.  Experiments 
made  at  the  Bureau  have  demonstrated  that  the  lag  of  an  instrument 
read  in  steam  at  100°  C.  with  mercury  surrounding  the  bulb  was  one-half  to 
one-third  as  great  as  when  powdered  graphite,  oil,  or  air  was  used. 
Obviously  mercury  cannot  be  used  at  the  higher  temperatures,  and  heavy 
oils  are  usually  substituted. 

This  lag,  however,  may  not  be  as  great  a  source  of  error  as  the  actual 
variance  that  may  be  noted  in  the  readings  of  these  thermometers  when 
used  to  measure  the  same  temperature,  but  under  varying  conditions, 
as  regards  construction  of  the  protecting  case  surrounding  the  bulb,  the 
material  into  which  the  bulb  is  immersed,  i.e.,  whether  liquid,  vapor,  or 
gas,  the  rate  of  flow  of  the  material  past  the  bulb,  and  the  exterior  con- 
ditions surrounding  the  protecting  stem.  These  variations  can  be  under- 
stood by  an  examination  of  the  construction  of  the  thermometer  and  an 
analysis  of  the  means  whereby  the  heat  is  either  conducted  to  the  bulb 
of  the  instrument  or  is  lost  by  conduction  to  the  air  by  way  of  the  heavy 
metal  parts.  There  are  no  published  results  of  reliable  investigations 
regarding  the  magnitude  of  the  influence  of  all  of  these  factors. 

If  industrial  thermometers  are  used  to  indicate  only  changes  in  the 
temperature  of  a  process  and  not  to  give  actual  temperatures,  it  may 
not  be  necessary  to  consider  the  various  sources  of  error  enumerated. 
But  these  instruments  cannot  be  depended  on  to  give  reliable  indications 
of  true  temperatures  unless  they  are  calibrated  or  standardized  under  the 
same  condition  as  that  of  use,  and  after  being  put  in  use  these  conditions 
must  not  be  changed. 

Emergent-stem  Error  of  Mercury-in-Glass  Thermometers. — In  discus- 
sing the  sources  of  error  and  precautions  to  be  observed  in  the  use  of 
high-temperature  mercurial  thermometers,  especially  of  the  laboratory 
type,  the  question  of  emergent-stem  error  is  perhaps  of  first  importance. 
It  is  common  practice  to  graduate  and  standardize  thermometers  for  the 


230  HIGH-TEMPERATURE    THERMOMETERS 

condition  that  the  bulb  and  the  part  of  the  stem  containing  the  mercury 
are  at  the  same  temperature.  This  process  is  called  graduation  and  stan- 
dardization for  total  immersion.  It  is  necessary  to  adopt  such  a  prac- 
tice, for  the  exact  conditions  of  immersion  to  which  a  thermometer  will  be 
subjected  are  not  generally  known.  Some  manufacturers  point  part 
of  their  stock  thermometers  for  other  conditions  of  immersion,  such  as 
3  in.,  claiming  that  this  condition  has  a  wider  application  than  that  of 
total  immersion.  For  lower  ranges  in  temperature  (below  100°  C.)  this 
method  of  pointing  may  have  some  advantages,  and  can  introduce  but 
a  small  error.  For  higher  temperatures,  it  would  seem  undesirable  to 
adopt  a  3-in.  immersion  for  a  standard  condition. 

In  the  actual  use  of  a  thermometer,  it  is  quite  often  impossible  to 
observe  the  conditions  of  total  immersion  of  the  mercury  column,  and 
many  instruments  must  be  used  with  the  bulb  only,  or  the  bulb  and  a 
very  small  part  of  the  stem  heated.  The  remainder  of  the  stem,  contain- 
ing the  mercury  column,  is  usually  at  a  temperature  considerably  lower 
than  the  bulb,  and  the  original  calibration  of  the  instrument,  if  for  total 
immersion,  will  not  hold.  The  thermometer  reads  too  low  under  these 
conditions.  It  is  always  possible  to  compute  the  approximate  correction 
to  apply  to  the  reading  to  reduce  it  to  standard  conditions  by  means  of 
the  formula2 

S  =  an  (T  -  f) 

where  S  =  correction  to  be  applied  to  reading;  a  =  factor  representing 
relative  expansion  of  mercury  in  glass;  n  =  number  of  degrees  of  mer- 
cury emergent  from  bath ;  T  =  temperature  of  the  bulb  or  bath ;  t  = 
average  temperature  of  emergent  mercury  column .  Of  these  factors,  a  can 
be  taken  to  be  0.00016  for  centigrade  temperatures  or  0.00009  for  Fahren- 
heit. The  value  of  n  is  observed.  T  can  be  approximated  by  using 
the  reading  of  the  thermometer  and  if  a  higher  degree  of  accuracy  is 
desired,  a  second  approximation  can  be  made  by  adding  the  correction 
first  found  to  the  reading  and  using  this  value  for  T  to  obtain  a  second 
correction.  The  value  of  t  is  most  difficult  to  secure  accurately,  but 
it  can  be  taken  to  be  approximately  the  reading  given  by  an  auxiliary 
thermometer,  the  bulb  of  which  is  placed  about  three-fourths  the  way 
down  the  exposed  mercury  thread.  A  more  accurate  method  for  obtaining 
t  is  to  use  what  is  known  as  a  "faden"  or  thread  thermometer.  This 
thermometer,  which  is  designed  to  give  the  average  temperature  of  a 
given  length  of  mercury  column,  is  indispensable  to  the  testing  laboratory 
where  accurate  determinations  of  stem  corrections  must  be  made.  The 

2  E.  Rimbach  [Zeit.  Inst.  (1890)  10,  153,  292]  gives  stem-correction  tables,  which 
have  been  widely  published.  Their  application  is  limited,  however,  as  the  data  were 
obtained  by  using  special  thermometers  in  distillation  apparatus. 


R.    M.    WILHELM  231 

theory  of  stem  correction  and  the  use  of  faden  thermometers  are  discussed 
more  in  detail  in  a  paper  by  Buckingham.3 

For  the  processes  in  which  the  immersion  of  the  thermometer  is 
definitely  known,  thermometers  can  be  previously  graduated  for  the 
required  immersion.  It  should  be  understood  that  partial-immersion 
thermometers  are  subject  to  error  unless  the  conditions  under  which  the 
thermometer  is  used  exactly  correspond  to  those  of  pointing  or  standard- 
ization. Thus,  changes  in  the  room  temperature  or  temperature  condi- 
tions surrounding  the  stem  may  introduce  errors  of  several  degrees. 
However,  when  a  high  degree  of  accuracy  is  not  desired,  it  is  probably 
more  satisfactory  to  use  these  partial-immersion  instruments  than  to 
attempt  a  stem  correction  with  a  total-immersion  thermometer.  Ther- 
mometers so  graduated  should  be  marked  preferably  with  a  ring  around 
the  stem  indicating  the  depth  of  immersion  and  also  with  a  statement 
to  this  effect  on  the  stem.  It  can  be  assumed  generally  that  the  ther- 
mometers not  so  definitely  marked  will  give  erroneous  results  if  used 
other  than  total  immersion. 

A  few  tests  in  which  thermometers  are  used  under  partial-immersion 
conditions  have  been  investigated  at  the  Bureau  of  Standards  and  stem 
correction  data  obtained.  The  results  are  given  in  Tables  2  "to  4.  The 

TABLE  2. — Stem  Correction  Data  for  Cleveland  Open-cup  Flash  and 

Fire-point  Tester.     Thermometer  Range  —20°  to  760°  F. 

in  2°  Intervals,  Length  About  15  In. 


Rpadinsr 
Degrees  F. 

Degrees  of  Mercury 
Column  Emergent, 
Degrees  F. 

Mean  Temperature 
of  Emergent  Mercury 
Column,  Degrees  F. 

Stem  Correction, 
Degrees  F. 

200 

208 

174 

0.5 

300 

308 

177 

3.5 

400 

408 

177 

8.5 

500 

508 

187 

15.0 

600 

608 

203 

23.0 

diameter  of  the  bulb  of  the  flask  used  in  the  experiments,  the  results  of 
which  are  given  in  Table  3,  was  8  cm.,  diameter  neck  2  cm.,  height  of 
neck  15  cm.,  distance  of  bottom  of  outlet  tube  to  top  8  cm.  The  ther- 
mometer range  was  0°  to  400°  C.,  in  1°  intervals,  the  length  of  the  ther- 
mometer was  40  cm.  The  thermometer  used  for  obtaining  the  results 
given  in  Table  4,  up  to  150°  C.,  was  graduated  from  40°  to  160°  C.  in  1° 
intervals  and  had  a  scale  length  of  9.5  cm.  The  thermometer  used  from 
200°  to  300°  had  a  range  from  200°  to  360°  in  1°  intervals  and  had  a  scale 
length  of  12  cm.  It  can  be  seen,  from  these  tables,  that  if  the  thermometers 
had  been  graduated  for  total  immersion,  appreciable  errors  would  have 

»  U.  S.  Bureau  of  Standards  Sci.  Paper  170  (1911). 


232 


HIGH-TEMPERATURE    THERMOMETERS 


TABLE  3. — Stem  Correction  Data  for  Distillation  Flask  Used  in  Distilla- 
tion of  Creosote  Oil* 


Reading 
Degrees  C. 

Degrees  of  Mercury 
Column  Above  Bulb, 
Degrees  C. 

Mean  Temperature 
of  Mercury  Column, 
Degrees  C. 

Stem  Correction, 
Degrees  C. 

200 

208 

59 

4.7 

250 

258                               111 

6.0 

300 

308 

98 

10.5 

350    , 

358 

95 

15.5 

TABLE  4. — Stem  Correction  Data  for  Pensky-Martin  Flash-point 

Apparatus 


Reading 
Degrees  C. 

Degrees  of  Mercury 
Column  Emergent, 
Degrees  C. 

Mean  Temperature 
of  Emergent  Mercury 
Column,  Degrees  C. 

Stem  Correction, 
Degrees  C. 

50 

30 

35 

0.1 

100 
150 

80 
130 

45 

55 

0.7 
2.0 

200 

10 

75 

0.2 

250 

60 

85 

1.6 

300 

110 

100 

3.5 

been  introduced,  if  the  stem  correction  had  been  neglected.  Empirical 
methods  are  sometimes  used  for  certain  more  or  less  standard  operations, 
such  as  distillation,  in  which  a  total-immersion  thermometer  is  specified 
to  be  used  without  regard  to  the  emergent-stem  correction. 

Other  Sources  of  Error  in  Use  of  High-temperature  Mercurial  Ther- 
mometers.— Aside  from  the  error  that  may  be  introduced  in  the  use  of 
high-temperature  mercurial  thermometers  by  a  failure  to  observe  the 
proper  condition  of  immersion,  there  are  two  common  sources  of  error 
that  can  be  attributed  to  actual  defects  in  the  manufacture  of  the  instru- 
ment. These  are,  first,  insufficient  pressure  above  the  mercury  to  pre- 
vent breaking  of  the  mercury  column,  or  evapora-tion  at  the  surface  of 
the  mercury  and,  second,  improper  or  insufficient  annealing  for  use  at 
high  temperatures.  This  latter  defect  may  result  in  a  rise  of  the  reading 
with  continued  heating  amounting  to  as  much  as  20°  C.  in  extreme  cases. 

Mercury  boils  at  approximately  357°  C.  and  evaporates  fairly  rapidly 
at  much  lower  temperatures.  Experiments  have  shown  that  even  for 
use  above  100°  C.  the  filling  under  pressure  of  that  part  of  the  capillary 
above  the  column  with  an  inert  gas,  such  as  nitrogen,  is  desirable.  The 
amount  of  pressure  that  must  exist  above  the  surface  of  the  mercury  to 
prevent  evaporation  or  breaking  of  the  mercury  column  varies  with  the 
temperature  and  the  construction  of  the  upper  portion  of  the  stem.  This 

4  See  Wilhelm:  "U.  S.  Bureau  of  Standards  Tech.  Paper  49  (1915), 


R.    M.    WILHELM 


233 


pressure  may  be  anywhere  from  two  to  twenty  atmospheres.  The  failure 
to  fill  high-temperature  thermometers  under  the  proper  pressure  is  often 
the  cause  of  large  errors.  This  defect  can  be  detected,  sometimes,  by  the 
broken  appearance  of  the  mercury  column,  and  by  drops  of  mercury  that 
have  distilled  from  the  top,  but  often  the  column  breaks  in  a  part  of  the 
stem  not  visible,  and  the  defect  is  not  detected. 

Improper  annealing  can  be  detected  only  by  the  checking  of  the  indi- 
cations of  the  instruments  from  time  to  time.  Laboratories  usually  have 
facilities  for  checking  the  readings  either  in  melting  ice  or  in  steam,  or 
perhaps  comparing  the  thermometers  from  time  to  time  with  a  thermo- 
meter known  to  give  reliable  readings. 


FIG.  3. — DISTANCE  READING 
INDICATING  THERMOMETER. 


FIG.  4. — RECORDING   THER- 
MOMETER. 


INDICATING  AND  RECORDING  THERMOMETERS 

The  term  " indicating"  is  usually  employed  to  designate  thermometers 
of  the  dial  and  pointer  type,  rather  than  those  constructed  of  mercury 
and  glass.  Indicating  thermometers  may  or  may  not  be  distance 
reading,  that  is,  so  constructed  as  to  allow  the  indicator  to  be  placed 
at  some  distance  from  the  bulb.  A  distance-reading  indicating  ther- 
mometer is  shown  in  Fig.  3.  A  recording  thermometer,  as  the  name 
indicates,  employs  a  mechanism  for  making  a  continuous  record  of  tem- 
peratures on  a  suitable  chart,  as  shown  in  Fig.  4. 

Indicating  and  recording  thermometers  may  be  divided  into  three 
general  classes,  electrical,  pressure,  and  bimetallic.  Electrical  ther- 
mometers are  generally  classed  as  pyrometers  and  will  not  be  discussed 
here.  Pressure  thermometers  comprise  a  bulb  containing  a  liquid  or  gas 
or  both  connected  by  means  of  capillary  tubing  to  some  form  of  pressure 


234  HIGH-TEMPERATURE    THERMOMETERS 

gage.  Bimetallic  thermometers  utilize  the  turning  force  produced  when 
a  strip  consisting  of  two  metals  having  different  coefficients  of  expansion 
and  brazed  to  each  other  is  heated.  Graphite-metal  thermometers  in- 
dicate temperatures  as  a  result  of  the  relatively  large  difference  in  thermal 
expansion  of  these  two  substances.  Although  of  simple  construction, 
the  accuracy  and  adaptability  of  these  types  of  instruments  have  not 
been  sufficiently  investigated  to  allow  a  more  detailed  discussion  of  them 
here. 

Pressure  Thermometers. — Pressure  thermometers  may  be  subdivided 
under  vapor  pressure,  liquid-filled,  and  gas-filled.  From  outward  ap- 
pearance, vapor-pressure  thermometers  may  be  distinguished  from  liquid- 
or  gas-filled  thermometers  by  the  form  of  the  temperature  scales,  since 
the  change  in  vapor  pressure,  with  respect  to  temperatures,  is  not  linear 
but  increases  as  the  temperature  rises.  Vapor-pressure  thermometers 
thus  have  an  unequally  divided  scale,  the  length  of  the  intervals  for  a 
given  number  of  degrees  increasing  with  increasing  temperatures. 
Liquid-  and  gas-filled  instruments  have  equally  divided  scales,  since  their 
action  is  based  on  thermal  expansion  which  is  approximately  linear 
with  respect  to  temperature.  The  bulbs,  capillary  tubing,  and  form  of 
pressure  gage  of  any  one  type  may  be  identical  in  outward  appearance  to 
that  of  any  other. 

Liquids  Used  in  Vapor-pressure  Thermometers. — The  volatile  liquid 
used  in  a  vapor-pressure  thermometer  must  possess  certain  well-defined 
characteristics.  It  should  be  stable,  readily  obtained,  and  purified, 
and  should  not  act  upon  the  metals  with  which  it  will  be  in  contact.  It 
must  have  a  sufficiently  high  pressure  at  the  lower  temperatures  to  which 
it  will  be  subjected  to  insure  a  readable  temperature  scale,  and  its  critical 
temperature  must  be  higher  than  the  highest  temperature  to  be  measured. 

The  vapor  pressures  of  the  liquids  available  for  use  in  vapor-pressure 
thermometers  have  been  determined  over  wide  ranges  and  it  is  usually 
possible,  by  making  use  of  the  data  found  in  the  literature  on  the  subject, 
to  select  the  most  suitable  liquid  for  a  given  range  of  temperature;  or  if  a 
liquid  is  given,  to  predict  the  temperature  range  within  which  it  can  be 
used  and  the  pressures  that  will  be  developed.  In  general,  the  tempera- 
ture and  pressure  range  for  a  given  liquid  will  be  included  between  its 
boiling  point  at  a  pressure  of  one  atmosphere,  and  its  critical  temperature. 
Although  these  limits  are  very  arbitrary,  they  are  convenient  for  reference. 

Table  5  gives  the  boiling  points,  at  a  pressure  of  one  atmosphere, 
and  the  critical  temperatures  and  pressures  for  various  liquids  suggested 
for  use  in  vapor-pressure  thermometers. 

Liquids  and  Gases  Used  and  Pressures  in  Liquid-  and  Gas-filled  Ther- 
mometers.— The  liquids  used  in  liquid-filled  thermometers  do  not  need  to 
possess  such  definite  characteristics  as  those  used  in  vapor-pressure 
thermometers.  It  is  obvious  that  pure  liquids  are  to  be  desired,  that 


R.    M.    WILHELM 


235 


TABLE  5. — Boiling  Points  and  Critical  Data  of  Liquids  Suggested  for  Use 
in  Vapor-pressure  Thermometers 


Substance 

Boiling  Point, 
Degrees  C. 

Critical 
Temperature, 
Degrees  C. 

Critical 
Pressure, 
Atmospheres 

Ethyl  alcohol  

80 

243 

63 

Benzine  

80 

288 

48 

Water     .                 

100 

365 

195 

Toluene'  

111 

320 

Aniline  

184 

426 

52 

they  should  not  react  chemically  upon  the  metals  with  which  they  come 
in  contact,  and  that  the  highest  temperature  to  which  they  are  to  be 
subjected  should  not  exceed  their  critical  temperature.  Alcohol,  which 
has  been  used  in  liquid-filled  thermometers  for  lower  temperatures  would 
not  be  satisfactory  for  temperatures  much  above  200°  C.  Aniline  has 
been  suggested  for  temperatures  as  high  as  400°  C.  Mercury  is  used 
almost  exclusively  in  this  country  in  liquid-filled  thermometers  reading 
up  to  1000°  F. 

Any  inert  gas  could  be  used  to  fill  gas-filled  thermometers,  but  nitrogen 
has  been  preferred  in  this  country.  Gas-filled  thermometers  are  not 
used  in  general  above  1000°  F.,  since  the  metal  bulbs  become  permeable 
to  the  gas  at  high  temperatures. 

The  initial  pressure  and  the  pressure  range  of  a  liquid-  or  gas-filled 
thermometer  will  vary  according  to  the  strength  of  the  gage  used  and 
the  temperature  range.  The  initial  pressure  is  made  relatively  high 
(10  or  15  atmospheres). 

Principles  Underlying  Action  of  Pressure  Thermometers. — The  action 
of  the  vapor-pressure  thermometer  depends  on  the  fact  that  the  pressure 
inside  the  thermometer  is  determined  solely  by  the  temperature  of  the 
free  surface  of  the  liquid.  It  follows,  therefore,  that  the  thermometer 
must  be  so  constructed  that  one  free  surface  is  always  in  the  bulb.  If 
this  condition  is  fulfilled  the  reading  of  the  instrument  will  not  be  sensi- 
bly affected  by  changes  in  the  temperature  of  the  gage  and  capillary. 
This  is  a  decided  advantage  over  other  types,  if  the  connecting  tubing 
and  gage  are  both  to  be  subjected  to  considerable  changes  in  temperature. 
If  the  vapor-pressure  thermometer  is  not  filled  properly  (i.e.,  the  propor- 
tion of  liquid  is  too  great  or  too  small  as  compared  with  the  volume  of 
the  bulb,  capillary  and  gage)  very  large  and  uncertain  errors  may  be 
introduced.  As  an  example,  take  the  condition  met  with  in  the  use  of 
high-temperature  vapor-pressure  thermometers.  The  bulb  is  usually 
much  hotter  than  the  capillary  and  is  filled,  or  nearly  filled,  with  vapor. 
The  liquid  is  condensed  in  the  capillary  and  there  should  be  sufficient 
liquid  in  the  capillary  to  entirely  fill  it  and  partly  fill  the  bulb.  If  the 
liquid  only  partly  fills  the  capillary  and  there  is  no  liquid  in  the  bulb,  the 


236 


HIGH-TEMPERATURE    THERMOMETERS 


capillary  will  contain  the  free  surface  of  the  liquid  and  the  temperature 
indicated  will  be  that  of  the  portion  of  the  capillary  containing  the  free 
surface  of  the  liquid,  as  shown  in  Fig.  5.  This  temperature,  which  may 
be  several  hundred  degrees  lower  than  that  of  the  bulb,  will  be  indicated 
instead  of  the  true  temperature  of  the  bulb. 

It  is  desirable  to  use  capillary  tubing  of  as  small  a  bore  as  possible 
for  vapor-pressure  thermometers,  for  the  smaller  the  volume  of  the  capil- 
lary and  gage  the  smaller  is  the  required  volume  of  the  bulb,  since  the 
combined  volume  of  the  capillary  and  gage  must  be  less  than  that  of  the 
bulb.5 

'  r  Bourdon  Gage 


' Liquid 


-Free  surface  of  liquid 
instrument  will  indicate 
temperature  of  this  point 


Vapor 


FIG.  5. — VAPOR-PRESSURE  THERMOMETER  WITH  INSUFFICIENT  LIQUID.     BULB  HOTTER 

THAN  TUBING  AND  GAGE. 

Gas-  and  liquid-filled  thermometers  operate  on  the  principle  of  ther- 
mal expansion.  They  are  entirely  filled  with  either  the  liquid  or  the 
gas.  The  expansion  of  the  liquid  or  gas  in  the  bulb  is  transmitted  through 
capillary  tubing  to  the  pressure  gage.  These  instruments  are  subject 
to  error  if  the  gage  and  capillary  are  heated  or  cooled  to  temperatures 
differing  from  those  under  which  they  were  calibrated.  This  error  may 
be  made  negligible,  in  many  instances,  by  reducing  the  volume  of  the 
capillary  and  gage  as  compared  with  that  of  the  bulb,  or  by  using  a  com- 
pensator in  the  head.  Such  a  compensator,  however,  will  not  eliminate 
errors  due  to  the  heating  or  cooling  of  the  capillary  alone. 

Accuracy  o/  Pressure  Thermometers. — On  account  of  the  mechanical 
construction  of  pressure  thermometers,  both  as  regards  the  mechanism 
for  indicating  and  recording  and  the  necessity  for  using  comparatively 
large  and  heavy  bulbs,  the  accuracy  that  can  be  secured  with  this  type 
of  thermometer  is  lower  than  that  generally  obtained  with  mercury  in 
glass  thermometers. 

6  This  requirement  is  not  necessary  if  the  bulb  is  at  all  times  hotter   than  the 
capillary  and  gage. 


R.  M.  WILHELM  237 

TESTING  HIGH-TEMPERATURE  THERMOMETERS 

The  testing  of  high-temperature  thermometers  for  scale  errors  in- 
volves, primarily,  either  the  comparison  of  the  test  thermometer  with 
standard  thermometers  at  various  temperatures  in  suitably  designed 
comparators,  or  the  reading  of  the  thermometers  in  the  vapors  of  various 
boiling  liquids.  The  100°  point  may  thus  be  checked  in  an  apparatus 
known  as  a  hypsometer  or  steam-point  apparatus,  which  provides  for 
the  immersion  of  the  thermometers  directly  in  the  vapor  of  boiling  water. 
Thermometers  may  also  be  tested  in  the  vapors  of  naphthalene,  benzo- 
phenone,  anthracene,  and  sulfur,  the  boiling  points  of  which  are  218°, 
306°,  340°,  and  444.6°  C.,  respectively.  The  boiling-point  method  of 
testing  is  less  frequently  used  than  the  comparison  method,  and  is,  in 
general,  less  convenient  and  subject  to  greater  errors  than  the  latter. 

For  testing  in  the  range  100°  to  320°,  an  electrically  heated  oil  bath  is 
most  convenient  to  use.  This  bath  should  be  well  stirred  to  give  a  uni- 
form distribution  of  temperature.  The  thermometers  are  immersed 
directly  in  the  oil. 

For  obtaining  higher  temperatures  than  those  that  can  be  secured  in 
the  oil  bath,  two  different  types  of  comparators  have  been  designed  and 
used  at  the  Bureau  of  Standards.  One  of  these,  which  has  been  in  use  up 
to  very  recently,  is  an  electrically  heated  stirred  air  bath.  The  air  is 
rapidly  circulated  around  a  block  of  copper,  into  which  holes  were  drilled 
for  insertion  of  the  thermometers.  Two  heating  coils  are  used,  one  of 
which  is  wound  around  the  cylinder  in  which  the  air  circulates,  the  other 
is.  wound  horizontally  on  an  asbestos  plate  covering  the  top.  By  means  of 
a  differential  thermocouple,  of  which  one  junction  is  placed  near  the 
bottom  of  the  bath,  the  other  at  the  top,  the  difference  in  temperature 
between  the  top  and  the  bottom  of  the  bath  can  be  observed  and  this 
difference  can  be  reduced  to  a  minimum  by  regulating  the  amount  of 
energy  supplied  to  either  the  upper  or  the  lower  coil. 

The  latest  type  of  bath  designed  for  high  temperatures  up  to  550°  C. 
uses  molten  lead  or  tin  as  a  comparison  liquid.  This  bath  is  constructed 
of  iron  throughout  and  contains  a  stirrer  for  circulating  the  molten  metal. 
The  thermometers  are  inserted  in  thin  steel  tubes  closed  at  the  bottom 
and  immersed  in  the  liquid.  This  molten  metal  bath  has  proved,  from 
many  standpoints,  more  satisfactory  for  testing  than  the  air  bath.  It 
can  be  heated  and  regulated  more  rapidly,  has  a  more  uniform  tem- 
perature distribution,  and  possesses  less  vibration  than  the  air  bath. 

Precautions  to  be  Observed  in  Testing  High-temperature  Thermome- 
ters.— It  is  not  possible  to  read  thermometers  totally  immersed  in  the 
comparators  mentioned,  and  since  the  stem-correction  error  becomes 
larger  with  increasing  temperatures,  every  precaution  must  be  taken  to 
eliminate  this  error  or  to  compute  it  accurately.  This  error  is  made 


238  HIGH-TEMPERATURE   THERMOMETERS 

negligible  if  both  standard  and  test  thermometers  have  the  same  dimen- 
sions and  the  same  number  of  degrees  of  mercury  column  emergent 
when  tested.  It  is  seldom  possible  to  fulfill  these  conditions,  however, 
since  the  standard  and  the  test  thermometers  usually  have  different 
dimensions,  and  hence  it  is  necessary  to  compute  stem  corrections  for 
both  unless  the  latter  are  to  be  standardized  for  partial  immersion. 
Thermometers  graduated  for  short  lengths  of  immersion,  as  for  example 
1  or  2  in.,  should  not  be  tested  immersed  to  these  short  lengths  on  account 
of  the  uncertainty  as  to  the  temperature  of  the  top  part  of  the  baths. 
Such  thermometers  are  usually  tested  immersed  to  a  greater  length  and  a 
correction  computed  to  allow  for  the  difference  in  reading  due  to  the 
fact  that  part  of  the  mercury  column  is  at  a  higher  temperature  than 
it  would  be  in  use. 

In  testing  high-temperature  thermometers,  it  should  be  borne  in  mind 
that  the  observed  variations  from  standard  readings  will  not  remain 
constant  if  the  glass  of  which  the  bulb  is  made  was  not  properly  annealed. 
Even  the  best-made  thermometers  change  with  time  and  on  continued 
heating.  As  a  result,  the  correction  observed  for  a  thermometer  at  a 
given  temperature  will  not  hold  if  the  bulb  contracts  at  a  higher  tempera- 
ture. If  the  thermometer  is  so  graduated  that  it  can  be  read  in  ice  or 
steam,  the  amount  that  the  readings  have  changed  can  be  easily  observed 
by  taking  check  readings  in  ice  or  steam  directly  after  reading  at  the 
higher  temperature.  From  the  results  of  these  observations,  allowance 
can  be  made  for  changes  that  may  have  taken  place,  due  to  a  contraction 
of  the  bulb.  Where  no  ice  point  is  provided,  it  is  advisable  to  test  the 
highest  point  first  and  the  lower  points  afterward. 

If  a  preliminary  test  of  a  thermometer  shows  that  a  considerable 
change  in  the  readings  occurs  with  moderate  heating,  it  is  advisable  to 
subject  the  thermometer  to  an  annealing  test,  which  may  be  made  in 
an  electrically  heated  annealing  oven.  The  construction  of  this  oven  and 
instructions  as  to  annealing  are  given  in  a  paper  by  Dickinson.6 

This  brief  description  of  the  testing  of  high-temperature  thermometers 
pertains  more  especially  to  laboratory  thermometers  of  the  etched-scale 
or  enclosed-scale  types.  The  comparators  mentioned  were  designed 
with  the  view  of  obtaining  the  highest  attainable  precision.  These 
comparators  are  sometimes  used  to  test  industrial  thermometers  and  the 
other  types  of  high-temperature  instruments  mentioned  in  this  article. 
In  many  instances,  however,  they  do  not  reproduce  conditions  of  use 
sufficiently  well  for  testing  certain  types  of  thermometers  designed  for 
special  purposes,  such  as  the  measurement  of  the  temperature  of  gases  or 
superheated  steam,  and  special  testing  apparatus  must,  therefore,  be 
built. 


6  U.  S.  Bureau  of  Standards  Sd.  Paper  32  (1906). 


DISCUSSION  239 

DISCUSSION 

A.  O.  ASHMAN,  Palmerton,  Pa.  (written  discussion*). — Owing  to 
the  conflict  of  the  terms  thermometry  and  pyrometry,  in  numerous 
cases  the  entire  field  of  temperature  measurements  has  been  divided 
under  these  two  headings.  This  confusion  is  probably  due  to  the  fact 
that  originally  a  pyrometer  was  understood  to  be  an  instrument  for 
measuring  temperatures  above  the  range  of  a  mercury  thermometer. 
In  modern  practice,  however,  pyrometers  are  not  only  used  to  measure 
the  temperature  over  the  range  of  the  thermometer,  but  also  to  a  much 
lower  temperature,  thereby  eliminating  the  basis  of  the  earlier  division. 
It  seems  that  the  modern  meaning  of  the  term  pyrometry  is  understood 
to  include  the  entire  field  of  temperature  measurements,  of  which  ther- 
mometry is  one  subdivision.  The  fact  that  this  paper  is  presented  at 
a  pyrometer  symposium  would  bear  out  this  fact. 

The  following,  I  believe,  gives  the  modern  idea  of  the  divisions  of 
pyrometry,  and  shows  the  relation  of  thermometry  to  pyrometry : 

1.  Expansion  pyrometry :  4.  Optical  pyrometry 

(a)  Expansion  of  gases.  5.  Radiation  pyrometry 

(6)  Expansion  of  liquids,  6.  Calorimetric  pyrometry 

(c)  Expansion  of  solids.  7.  Melting  point  pyrometry 

2.  Thermoelectric  pyrometry  8.  Transpiration  pyrometry 

3.  Electrical  resistance  pyrometry  9.  Miscellaneous  pyrometric  methods 

R.  M.  WILHELM  (author's  reply  to  discussion  f). — Although  certain 
types  of  pyrometers  may  be  used  to  measure  temperatures  below  the 
upper  range  of  the  mercurial  thermometer,  it  would  hardly  seem,  ad- 
visable or  logical  to  make  so  radical  a  change  as  Mr.  Ashman  suggests 
in  the  nomenclature  of  the  subject. 

*  Received  Sept.  25,  1919.  f  Received  Jan.  19,  1920. 


240  PORCELAIN    FOR    PYROMETRIC    PURPOSES 


Porcelain  for  Pyrometric  Purposes* 

BY   FRANK   H.    RIDDLE,  f    PITTSBURGH,    PA. 
(Chicago  Meeting,  September,  1919) 

THE  life  of  thermocouples  is  governed,  to  a  large  extent,  by  the  pro- 
tection they  receive  when  in  use;  particularly  when  the  temperatures 
being  measured  are  high  and  the  products  of  combustion  are  reducing  in 
character.  Several  types  of  protection  tubes  are  being  used  for  this  work; 
they  are  made  of  ordinary  and  aluminous  porcelain,  fused  quartz,  car- 
borundum, fused  alumina,  fireclay,  and  some  kinds  of  metal.  All  of 
them  may  render  good  service  under  certain  conditions  but  they  must  be 
used  for  the  purpose  for  which  they  are  intended.  For  example,  fused- 
quartz  tubes  are  very  good  when  excessive  changes  in  temperature  occur, 
but  under  certain  conditions  they  will  crystallize  and  lose  their  strength. 

Porcelain  is  used  in  several  ways  in  connection  with  the  use  of  thermo- 
couples: for  insulating  the  two  wires  of  the  couple  and  to  prevent  short 
circuits;  for  protecting  the  entire  couple  in  a  refractory  gas-tight  tube, 
which  is  ordinarily  fastened  to  and  is  considered  part  of  a  properly 
equipped  thermocouple;  and  for  protecting  the  complete  thermocouple 
from  the  possibility  of  physical  injury  and  to  support  it.  This  tube  is 
made  porous  and  strong  and  is  ordinarily  built  right  into  the  wall  or  crown 
of  the  furnace,  as  the  case  may  be. 

The  insulating  tubes  must  be  small  and  sufficiently  refractory  to 
withstand  the  temperatures  at  which  they  are  to  be  used.  The  length 
of  the  tubes,  porosity  of  the  material  from  which  they  are  made,  etc.  does 
not  particularly  matter.  The  tubes  are  sometimes  made  with  two  or 
even  four  holes,  the  latter  being  used  where  it  is  desirable  to  have  two 
couples  as  close  together  as  possible  for  calibration  work. 

The  thermocouple  protection  tube  must  be  of  proper  size  to  permit 
the  thermoelements  and  insulating  tubes  to  be  inserted  or  taken  out 
easily.  It  must  be  porous  enough  to  withstand  sudden  changes  in  tem- 
perature and  yet  gas-tight,  as  well  as  refractory  enough  to  withstand  the 
furnace  temperature,  even  if  the  tube  protrudes  into  the  furnace  in  a 
horizontal  position  without  support  for  a  reasonable  distance.  If  the 
tubes  are  dense  enough  to  be  gas-tight,  they  do  not  require  glazing  but 
are  not  so  resistant  to  temperature  changes  as  the  porous  tubes. 

The  protection  tubes  for  the  noble  metals  are  normally  about  ^  in. 


*  Published  by  permission  of  Director,  U.  S.  Bureau  of  Standards, 
t  Chemist,  Clay  Products  Division,  U.  S.  Bureau  of  Standards. 


PRANK   H.    RIDDLE  241 

(12  mm.)  inside  diameter,  have  ^-in.  (3.17-mm.)  walls,  are  closed  at  one 
end,  and  flanged  at  the  other.  Their  length  varies  from  one  to  several 
feet.  For  base  elements,  the  tubes  are  usually  1  in.  inside  diameter, 
otherwise  the  same  as  for  the  noble  metals.  Glazed  Marquardt  mass 
tubes  are  commonly  employed  for  the  noble  elements  and  ordinary 
unglazed  vitreous  porcelain  for  the  base  elements. 

The  outer  protection  tubes  must  be  refractory  and  preferably  porous. 
They  are  usually  rather  heavy  walled  and  closed  on  the  exposed  end. 
Their  inside  diameter  is  great  enough  to  permit  the  thermocouple 
protection  tube  to  go  in  and  out  with  ease.  The  walls  are  usually  ^  in. 
or  more  in  thickness.  Alundum  and  carborundum  make  excellent  tubes 
for  this  purpose  for  high  temperatures.  Sillimanite  is  also  a  good  mate- 
rial. Fireclay  bodies  are  not  so  good  but  satisfactory  for  temperatures 
equivalent  to  the  fusing  temperature  of  cone  10  or  less. 

As  with  a  good  many  other  products,  a  shortage  of  pyrometer  tubes 
was  felt  shortly  after  the  outbreak  of  the  war,  when  importations  were 
cut  off.  After  the  shortage  was  overcome,  the  quality  of  the  product 
was  soon  made  practically  equal  to  that  of  Germany.  As  conditions 
are  at  present,  there  is  no  reason  why  it  should  be  necessary  to  resume 
the  importation  of  pyrometer  tubes. 

MARQUARDT  MASS  BODY  No.  1 

About  4  years  ago  the  Bureau  of  Standards,  in  its  Clay  Products 
Laboratory,  at  Pittsburgh,  Pa.,  undertook  the  duplication  of  the  Mar- 
quardt mass  tubes.1  The  body  composition  adopted  was  as  follows: 

BODY  PER  CENT.                             CALCINE  No.  1  PER  CENT. 

Calcine  No.  1 45 . 7  Calcined  alumina 70. 0 

Calcine  No.  2 7.3  North  Carolina  kaolin 22.0 

North  Carolina  kaolin 17 . 0  Potash  feldspar 8.0 

Florida  kaolin 5.0 

100.0 

CALCINE  No.  2 

Tennessee  ball  clay  No.  5.         15.0  Potash  feldspar 64.0 

English  china  clay 10.0  Calcined  alumina i        36.0 


100.0  100.0 

Each  calcine  mixture  is  ground  dry  in  a  ball  mill,  to  insure  thorough 
mixing  of  the  materials.  It  is  then  tempered  with  water  to  the  consist- 
ency of  a  very  thick  paste  and  molded,  by  hand,  into  balls  about  \Y±  in. 
(3.17  cm.)  in  diameter.  An  alternate  method  is  to  grind  the  mixture  in 


1  W.  L.  Howat:  Manufacture  of  Porcelain  Pyrometer  Tubes.     Trans.  Am.  Cer. 
Soc.  (1916)  18,  268. 

F.  H.  Riddle:  Marquardt  Porcelain.     Trans.  Am.  Cer.  Soc.  (1917)  19,  397. 

16 


242  POECELAIN   FOR   PYROMETRIC   PURPOSES 

a  ball  mill  in  the  wet  state,  having  sufficient  water  to  make  a  creamy  slip. 
When  ground  sufficiently,  the  slip  is  run  through  a  fine  silk  lawn  and  filter 
pressed  until  sufficient  water  has  been  eliminated  to  permit  forming  the 
body.  It  is  then  wedged  until  homogeneous  and  made  into  balls  as 
before.  The  thoroughly  dried  balls  are  then  placed  in  a  kiln  and  cal- 
cined to  a  temperature  equivalent  to  the  fusing  point  of  Orton  pyrometric 
cone  20.  This,  at  the  time-temperature  rate  used  in  our  case,  is  about 
1525°  C.  At  this  temperature  calcine  No.  2  will  fuse  sufficiently  to  allow 
the  balls  of  the  material  to  flatten  out  of  shape  and  stick  together; 
calcine  No.  1  has  the  shrinkage  well  taken  out  of  it  but  it  does  not  vitrify. 
The  calcined  materials  are  then  crushed  until  sufficiently  fine  for  ball 
milling.  This  pebble-mill  grinding  is  continued  until  the  powder  is 
fine  enough  to  pass  dry  through  a  lawn  with  120  meshes  to  the  square 
inch. 

The  body  is  prepared  by  ball  milling  the  calcines  and  raw  materials 
in  the  proper  proportion  in  the  wet  state,  screening  them  through  a  120- 
mesh  lawn,  passing  them  over  a  magnetic  separator,  and  then  filter 
pressing.  -The  body  is  then  in  the  proper  condition  for  manufacture. 

MARQUARDT  MASS  BODY  No.  2 

For  particularly  high  temperatures,  it  is  advantageous  to  eliminate 
calcine  No.  2  from  the  mixture  and  to  use  only  calcine  No.  1.  This  gives 
a  body  of  the  composition 

PER  CENT. 

Calcine  No.  1 53 . 0 

North  Carolina  kaolin 17.0 

Florida  kaolin 5.0 

Tennessee  ball  clay  No.  5 15 . 0 

English  china  clay. 10. 0 


100.0 


Calcine  No.  1  is  the  same  as  is  used  in  Marquardt  mass  body  No.  1. 
Tubes  of  this  refractory  quality,  as  stated,  are  necessary  only  in  extreme 
cases,  considered  from  the  ceramic  standpoint.  In  making  these  tubes, 
if  the  product  is  to  hold  its  shape  in  use,  it  must  be  burned  in  manufac- 
ture to  a  point  at  which  the  shrinkage  is  eliminated  as  much  as  possible. 

Both  Marquardt  mass  bodies  are  porous  at  the  burning  temperature 
used  and  are  dependent  on  the  glaze  to  make  them  gas-tight.  They 
have,  however,  the  excellent  quality  of  being  able  to  resist  reasonable 
changes  in  temperature.  If  care  is  used,  it  is  possible  to  safely  insert  a 
cold  tube  into  a  furnace  heated  at  1500°  C.  in  a  few  minutes  time. 


FRANK  H.  RIDDLE  243 

ORDINARY  PORCELAIN  TUBES 

These  tubes  are  not  as  refractory  as  Marquardt  or  sillimanite  tubes 
but  are  good  at  low  temperatures  where  the  temperature  changes  are  not 
too  great.  However,  they  should  be  supported  if  inserted  horizontally 
into  furnaces  far  enough  to  project.  Tubes  of  this  sort  are  vitrified  and  so 
are  not  dependent  on  a  glaze  to  make  them  gas-tight.  This  eliminates  the 
possibility  of  the  tube  sticking  where  in  contact.  They  are  also  cheaper 
to  manufacture.  Numerous  compositions  may  be  used  for  this  purpose; 
two  that  are  satisfactory  are : 

No.  1.  No.  2, 

PER  CENT.         PER  CENT. 

Potash  feldspar 18  12 

Flint 32  38 

North  Carolina  kaolin 30  25 

Florida  kaolin 8  8 

Tennessee  ball  clay  No.  5 12  17 

100  100 

These  bodies  are  prepared  in  a  similar  manner  to  the  Marquardt 
bodies;  that  is,  by  pebble  milling,  screening,  passing  over  the  magnetic 
separator,  and  filter  pressing. 

SILLIMANITE  TUBES 

As  is  well  known,  pure  dehydrated  kaolin,  Al203.2Si02,  has  a  melting 
temperature  of  about  1750°  C.±  while  sillimanite  Al2Os.Si02  has  a  well- 
defined  melting  point  of  1816°  C.  + ,  which  makes  it  useful  for  temperature 
measurements  around  the  melting  point  of  kaolins.  Tubes  of  this  sort 
are  difficult  to  glaze  or  vitrify  except  by  the  use  of  very  high  temperatures 
of  firing.  For  this  reason  they  are  usually  not  gas-tight.  The  material 
is  useful,  however,  for  Arsem  furnace  tubes  and  particularly  good  for 
outer  protection  tubes.  The  coefficient  of  thermal  expansion  of  silliman- 
ite is  very  low  and  uniform,  as  compared  with  that  of  clay,  which  is 
greater  and  variable  at  the  different  temperatures.  The  addition  of  free 
silica  (flint)  increases  the  coefficient  and  causes  it  to  be  variable  at  differ- 
ent temperatures.  A  workable  mixture  is  as  follows: 

PER  CENT.  CALCINE  No.  1  PEB  CENT. 

Calcine  No.  1 68.3  Calcined  alumina 28.33 

Georgia  kaolin 21 . 2  Florida  kaolin 71.67 

Tennessee  ball  clay 10.5  100.00 

100.0 

The  calcine  is  prepared  in  the  same  manner  as  the  Marquardt  mass 
calcines  and  burned  to  cone  20.  The  longer  this  temperature  can  be 
held  in  the  burning,  or  the  oftener  the  calcine  can  be  burned,  the  better 
the  conversion. 


244  PORCELAIN    FOR    PYROMETRIC    PURPOSES 

• 

MANUFACTURE  OF  TUBES  BY  PRESSING 

It  is  possible  to  make  all  sizes  of  tubes  from  2  mm.  outside  diameter 
by  0.6  mm.  inside  diameter  up  to  practically  any  size  desired  by  this 
method.  We  have  not  made  tubes  more  than  15  mm.  outside  diameter. 
The  machine  used  is  a  miniature  hydraulic  press  built  similar  to  a  sewer- 
pipe  press.  The  water  cylinder  is  8  in.  (20  cm.)  in  diameter,  the  clay 
cylinder  is  3^  in.  (8.9  cm.)  in  diameter  and  the  stroke  is  10  in.  (25  cm.). 
All  dies  are  made  of  brass  and  accurately  finished.  The  water  pressure 
used  to  operate  the  press  is  80  Ib.  to  the  sq.  in.  This  machine  works 
well  and  makes  very  dense  tubes.  The  troubles  encountered  are  the 
ordinary  ones  that  would  be  expected  with  a  machine  of  this  type;  viz., 
necessity  of  maintaining  absolute  water  content  and  uniform  pugging 
to  prevent  clay  running  faster  on  one  side  of  the  die  than  on  the  other. 
Ring  cracks  also  form  at  the  point  of  the  die  when  the  column  is  held  so 
as  to  throw  back  the  plunger  to  recharge  the  clay  cylinder.  The  longer 
the  stroke,  consequently,  the  fewer  the  ring  cracks  formed  and  the  greater 
will  be  the  production.  Hence  a  stroke  of  24  in.  (60.9  cm.)  would  be 
preferable.  If  the  press  is  vertical,  the  length  of  the  tube  is  limited  by  the 
tensile  strength  of  the  clay,  but  this  will  usually  permit  the  making  of  at 
least  5-ft.  (1.5  m.)  tubes.  If  the  press  is  inclined  or  horizontal,  it  is  neces- 
sary to  use  a  hand-controlled  off-bearing  belt.  We  have  found  that  the 
press  is  satisfactory  for  all  sizes  of  tubes  we  require,  and  that  its 
capacity  is  sufficient.  However,  on  account  of  the  greater  advan- 
tages of  casting,  the  only  tubes  made  on  the  press  are  those  that  cannot 
be  cast  on  account  of  their  small  bore.  In  our  case  all  tubes  with  an 
internal  diameter  over  5  mm.  are  cast.  This  eliminates  all  tubes  but 
the  inner  or  insulating  tubes.  It  is,  however,  possible  to  make  the  small- 
bore tubes  very  successfully  and  to  make  tubes  with  several  holes  in 
them. 

MANUFACTURE  OF  TUBES  BY  CASTING 

Cast  tubes  above  5  mm.  inside  diameter  are  superior  to  pressed  ones 
in  many  ways.  They  are  much  straighter,  do  not  have  to  be  handled 
until  stiff,  and  are  more  homogeneous.  The  speed  and  trueness  of 
this  work,  of  course,  depends  primarily  on  the  plaster  molds.  For 
long  small-diameter  molds  for  tubes  closed  on  one  end,  the  mold  should  be 
cast  around  a  cold-drawn  steel  rod,  on  account  of  its  trueness.  The  end 
of  the  rod  should  be  rounded  off  as  desired,  and  a  hole  about  ^6  m-  (1-5 
mm.)  in  diameter  drilled  in  the  center  at  this  end,  probably  %  in.  (9.5 
mm.)  deep.  The  other  end  of  the  rod  should  be  mounted  in  the  center 
of  a  plaster  block  about  6  in.  (15  cm.)  high  and  4  in.  in  diameter.  The 
molds  are  cast  by  placing  the  rod  vertically  on  the  mounting,  and  wrap- 
ping a  "coddle"  of  roofing  paper,  or  similar  flexible  material,  around  the 
mounting  and  maintaining  this  measure  up  to  a  height  of  a  few  inches 


FRANK    H.    RIDDLE  245 

above  the  top  of  the  rod.  Then  run  a  Hs-in.  pin,  about  3  in.  long,  down 
into  the  small  hole  in  the  top  end  of  the  rod.  Prepare  the  plaster  and  fill 
the  "coddle"  so  that  the  pin  sticks  out  of  the  plaster  about  %  in.  This 
M6'm-  hole  through  the  bottom  of  the  mold,  made  by  drawing  out  the 
pin,  serves  as  an  air  inlet,  making  it  possible  to  pull  the  rod  out  of  the 
mold;  it  also  serves  the  same  purpose  in  pulling  out  the  cast  tubes. 

Casting. — The  casting  process  consists  in  pouring  clay  suspended  in 
water  (slip)  into  a  dry  plaster-of-Paris  mold  and  permitting  it  to  re- 
main there  a  sufficient  time  for  the  plaster  to  absorb  the  water  from 
the  slip  in  direct  contact  with  the  mold.  The  inside  of  the  mold  is, 
of  course,  the  shape  of  the  outside  of  the  piece  of  ware.  As  this  absorp- 
tion continues  the  clay  becomes  stiff;  the  longer  it  continues,  the  thicker 
is  the  layer  of  hard  clay  next  to  the  mold.  When  this  layer  has  hardened 
for  a  sufficient  distance  from  the  inner  face  of  the  mold  to  make  a  wall 
of  the  proper  thickness,  the  remaining  slip  is  poured  out  by  turning  over 
the  mold.  During  casting,  it  is  necessary  to  keep  the  mold  full  of  slip. 
The  mold  absorbs  the  water  from  the  slip  and  sufficient  slip  must  be 


FIG.  1. — PALLET  FOR  DRYING  TUBE. 

run  in  to  replace  this.  Casting  necessitates  the  use  of  considerable  care 
that  is  not  at  first  apparent,  particularly  in  pieces  of  the  shape  of  tubes. 

The  small  hole  left  in  the  bottom  of  the  mold  as  an  air  inlet,  when 
drawing  out  the  metal  core,  is  also  essential  in  casting.  Before  the  slip 
is  poured  into  the  mold  a  steel  rod  of  the  same  diameter  as  the  air  vent 
is  placed  in  this  hole  and  run  in  until  it  projects  into  the  mold  proper 
about  %  in.  (12  cm.).  It  is  bent  over  on  the  outside  of  the  mold  and  cut 
off  about  1  in.  from  the  center,  and  is  held  in  place  with  soft  clay.  After 
the  slip  is  poured  and  it  is  time  to  empty  the  mold,  the  pin  must  be 
taken  out  while  the  mold  is  being  turned.  If  the  pin  is  taken  out  too 
soon,  the  slip  will  run  down  and  plug  the  hole;  if  it  is  left  in  after  the 
slip  begins  to  run  out  at  the  other  end,  the  tube  will  collapse. 

Considerable  care  must  be  used  in  drying  the  tubes,  and  proper 
pallets  are  necessary;  plate  glass  is  ideal  for  the  purpose.  Where  wood 
is  used  there  should  be  a  stiff  reinforcement  running  lengthwise  of  the 
pallet  underneath,  as  well  as  cleats  crossways.  The  boards  should  be 
of  the  same  length  as  the  green  tubes  and  have  a  stop  block  running  the 


246 


PORCELAIN  FOR  PYROMETRIC  PURPOSES 


entire  length,  along  one  edge.  The  first  tube  is  placed  against  this  stop 
block,  and  the  next  one  rolled  close  to  the  first,  but  with  the  head  on  the 
opposite  end  of  the  pallet,  etc.  This  method  permits  the  necessary 
clearance  for  the  heads  and  each  tube  is  held  on  both  sides  for  its 
entire  length,  see  Fig.  1.  If  the  pallets  are  longer  than  the  tubes,  the 
head  end  of  each  tube  will  be  without  a  brace  on  either  side,  for  a  distance 
equal  to  the  difference  between  the  length  of  the  tubes  and  pallet.  The 
last  tube  from  the  stop  block  is  held  in  place  by  a  ^  in.  square  steel  rod. 
Twenty-four  hours  is  sufficient  for  drying. 


18— 

FIG.  2. — COLLAR  OF  UNIVERSAL  JOINT  FROM  WHICH  TUBES  ARE  SUSPENDED. 

Close  attention  is  required  in  preparing  the  casting  slips  as  they 
must  always  have  the  same*  water  content  and  the  same  casting  time  in 
order  to  secure  proper  uniformity  of  production.  To  keep  the  shrinkage 
low,  the  water  content  must  be  kept  as  low  as  possible  and  still  have 
proper  fluidity.  This  decrease  in  water  content  is  facilitated  by  the 
use  of  alkaline  electrolytes.2 

Setting. — Setting  must  be  done  with  great  care,  particularly  if  the 
tubes  are  very  long.  Tubes  5  ft.  long  or  over  are  difficult  to  pick  up  while 
green,  without  straining  or  breaking,  unless  a  small  trough  is  used  to 
support  them  for  their  entire  length.  They  must  be  hung  from  the  top 
of  the  furnace  by  the  collar  on  a  universal  joint.  Fig.  2  shows  the  collar 
as  used.  The  tensile  strength  of  the  clay  must  be  great  enough  to  hold 

1  A.  V.  Bleininger:  U.  S.  Bureau  of  Standards  Tech.  Paper  51. 


FRANK    H.    RIDDLE 


247 


the  weight  of  the  tube,  but  all  jars  must  be  avoided  in  setting  as  the 
tubes  are  easily  broken.     Figs.  2  and  3  also  show  the  kiln  used  in  burn- 


FIG.  3. — SECTION  OF  FURNACE  FOR  BURNING  TUBES. 

ing.     It  will  be  noted  that  the  cover  of  the  kiln  lifts  off  so  that  the  ware 
can  be  lowered  down  into  the  saggers.     The  kiln  and  saggers  are  so 


248  PORCELAIN  FOR  PYROMETRIC  PURPOSES 

arranged  that  there  is  an  even  temperature  distribution  on  both  sides 
of 'every  row  of  tubes.  This  is  essential  if  straight  tubes  are  to  be 
produced,  as  an  irregular  temperature  causes  uneven  shrinkage  and, 
consequently,  warped  tubes.  Where  a  larger  capacity  is  desired,  it  is 
possible  to  make  the  kiln  longer,  thus  permitting  the  placing  of  more 
bungs  of  saggers.  Each  additional  sagger  will  require  an  additional 
vertical  row  of  burners.  It  is  important  that  the  refractories  used  in 
the  kiln  and  saggers  be  of  high  grade.  A  sagger  mixture  that  has  proved 
satisfactory  is, 

PER  CENT. 
Calcined  flint  fireclay,  sized  to  pass  through  an  8-mesh 

and  be  retained  on  a  30-mesh  screen 55 

Georgia  kaolin 35 

Tennessee  ball  clay  No.  5  or  No.  9 10 

100 

It  is  essential  that  a  high-grade  flint  fireclay  be  used,  that  is,  one 
having  a  softening  point  of  at  least  cone  32. 

The  Bisque  Burn. — If  the  tubes  are  to  be  glazed,  they  should  be 
burned  twice.  The  glaze  is  not  applied  until  after  the  first  burn.  This 
first,  or  bisque,  burn  need  be  carried  to  a  temperature  only  sufficiently 
high  to  expel  its  combined  water  and  make  the  body  hard  enough  to 
handle,  that  is,  about  cone  1.  Practice  has  shown,  however,  that  a 
straighter  product  is  obtained  by  eliminating  part  of  the  fire  shrinkage 
in  the  bisque  burn  but  still  leaving  the  tube  porous  enough  to  make  it  easy 
to  apply  the  glaze.  For  Marquardt  tubes,  this  temperature  is  equiva- 
lent to  the  fusion  point  of  Orton  cone  12  or  14  while  cone  8  or  10  is 
sufficient  for  the  ordinary  porcelain  tubes. 

If  the  tubes  are  to  be  left  unglazed  the  burning  temperature  should 
be  sufficient  to  mature  the  body;  that  is,  the  temperature  at  which  the 
body  has  the  proper  physical  properties  for  the  work  for  which  it  is  to  be 
used. 

The  inner  insulating  tubes  and  the  extreme  outer  protection  tubes,  if 
1^  in.  (3.8  cm.)  or  greater  in  diameter,  can  be  burned  in  any  kiln  in 
which  the  proper  temperature  can  be  reached.  The  small  tubes  should 
be  burned  in  a  refractory  cradle.  Care  should  be  used  in  covering  the 
tubes  well  and  burning  slowly  in  order  to  prevent  the  top  tubes  from  curl- 
ing up,  due  to  uneven  shrinkage.  The  larger  diameter  tubes  can  be 
burned  horizontally  without  bedding  or,  if  short  enough,  stood  close 
together  on  end.  It  is  not  necessary  to  burn  the  insulating  tubes  to  as 
high  a  temperature  as  the  thermocouple  protection  tubes.  They  should, 
however,  be  burned  to  a  temperature  high  enough  to  remove  most  of  the 
shrinkage. 

Glazing. — The  ground  glaze  suspended  in  water  may  be  applied  to 
the  bisque  tubes  in  several  ways,  chiefly  by  spraying  and  dipping.  Dip- 


FBANK   H.    RIDDLE  249 

ping  is  not  as  satisfactory  as  spraying  but  much  cheaper  and  preferable 
from  a  manufacturing  standpoint,  if  done  properly.  A  third,  and  very 
satisfactory,  method  is  to  arrange  a  small  container  with  a  bottom  con- 
sisting of  a  rubber  sheet.  A  hole  is  cut  in  the  center  of  the  rubber  large 
enough  to  permit  the  tube  which  is  to  be  glazed  to  slide  through  it,  but 
tight  enough  so  that  the  container  will  hold  liquid  when  the  tube  fills 
the  hole.  The  tube  is  hung  in  a  vertical  position  and  the  container 
pushed  up  the  tube  to  the  top,  the  tube,  of  course,  passing  through  the 
rubber  bottom  of  the  container.  The  container  is  then  filled  with  the 
liquid  glaze,  of  proper  viscosity,  and  drawn  down  the  tube  at  a  rate  great 
enough  to  allow  a  coat  of  glaze  of  just  the  proper  thickness  to  adhere  to 
the  porous  walls  of  the  tube.  This  gives  a  very  uniform  thickness  of 
glaze  as  every  part  of  the  tube  is  exposed  to  the  glaze  application  for  the 
same  length  of  time,  and  it  is  applied  slowly  enough  to  prevent  running. 
This  is  a  defect  that  is  apt  to  occur  in  dipping.  Glazes  of  the  character 
used  for  this  work  are  of  such  a  nature  that  they  will  not  run  or  mend 
any  defects  caused  by  improper  application. 

Glazes  for  use  in  pyrometer  work  should  be  sufficiently  refractory  so 
that  they  will  not  soften  at  working  temperatures  and  stick  to  the  furn- 
ace or  the  outside  protection  tubes.  Most  of  the  glazes  used  are  of  such 
a  character  that  they  are  absorbed  into  the  body  of  the  porous  tube  when 
the  heat  is  increased  too  much.  This  prevents  sticking  and  keeps  the 
tube  gas-tight  but  eventually  decreases  its  refractoriness  and  tends  to 
vitrify  the  tube  and  make  it  less  resistant  to  temperature  changes. 

Two  refractory  glazes  that  absorb  into  the  body  when  the  heat  is  too 
great  are  of  the  chemical  composition : 

1.0  CaO,  2.0  A1203,  4.0  SiO2  (1) 

The  batch  weights  are  whiting,  100  parts  by  weight;  clay,  516  parts. 
KNaO  0.047 ] 


CaO  0.953  f  AM),  2.23,  SiO,  5.123 

Of  the  above,  the  CaO,  2  A12O3,  4  SiO2  are  fritted  at  cone  20,  ground 
and  added  to  the  rest  of  the  batch.  » 

BATCH  OF  FKITT  PARTS  BATCH  OF  GLAZE  PARTS 

Whiting 100.0  Fritt 100 

Kaolin 516 . 0  Tennessee  ball  clay 15 

Boric  acid 1.3  Potash  feldspar 6 

Flint 6 

617.3 

127 

The  glazes  are  ground  wet  in  ball  mills  and  lawned  through  120-mesh 
screens  before  being  used.  Glaze  No.  1  is  a  beautiful  cream  matt  at  cone 
17  down  while  No.  2  is  rather  harder  but  vitreous  enough  to  be  gas- 
tight. 


250  PORCELAIN  FOR  PYROMETRIC  PURPOSES 

Bright  glazes  for  use  on  tubes  that  will  be  exposed  to  temperatures 
under  1000°  C.  are: 

1.0  CaO,  1.0  A12O3,  10  Si02  (3) 

0.15    K20 

0.20  MgO    1.0  A1203,  10.0  SiO2  (4) 

0.65    CaO 

These  glazes  are  very  beautiful  at  cone  17  but  soften  at  cone  10  and 
are  not  absorbed  into  the  body  and  hence  stick  to  anything  the  tubes  may 
be  touching. 

A  harder  bright  glaze  that  is  more  satisfactory  up  to  the  fusing  tem- 
perature of  cone  14  is: 

1  CaO,  1.0  A1203,  4  Si02 

The  batch  weights  are  whiting,  100  parts  by  weight;  clay,  258  parts; 
flint,  120  parts.  In  all  the  glazes,  the  thickness  of  application  has  con- 
siderable effect  on  the  sticking  and  should  be  carefully  watched. 

The  Glost  Burn. — This  burn  is  handled  in  the  same  manner  as  the 
bisque  burn,  the  temperature  being  brought  to  the  proper  maturing 
point  of  the  body  and  glaze.  The  body  and  glaze  should  mature  at 
the  same  temperature.  Tubes  have  been  made  at  the  Bureau  meas- 
uring 7  ft.  (2.1  m.)  in  length  when  finished.  For  these,  body  No.  1  and 
glaze  No.  1  were  used.  The  manufacturing  loss  was  9  per  cent,  and 
the  tubes  were  a  good  quality  marketable  product.  They  were  used 
for  experimental  work. 

One  very  important  point  in  burning  is  to  allow  sufficient  time  at  the 
maturing  temperature  to  permit  the  heat  to  penetrate  the  walls  of  the 
ware  thoroughly  so  that  it  has  a  uniform  effect  upon  all  parts  of 
the  ware.  It  is  also  essential  to  burn  the  ware  to  a  temperature  higher 
than  that  at  which  it  is  to  be  used,  if  it  is  necessary  for  the  tubes  to 
remain  straight  in  use,  particularly  when  the  tubes  are  projected  into 
the  furnace  horizontally  with  the  inner  end  unsupported.  The  reason 
for  this  is  evident.  There  is  some  shrinkage  in  the  tube  if  the  body 
is  still  porous  and  was  burned  to  a  lower  temperature  when  manufac- 
tured. When  the  tube  starts  to  shrink  gravity  assists  the  shrinkage 
on  the  lower  side  of  the  projected  tube  and  opposes  shrinkage  on  the  top. 
The  result  will  be  a  crooked  tube. 

Other  Refractory  Materials. — Zirconium  oxide  and  magnesium  alumi- 
nate  (Spinel)  are  both  very  refractory  and  tubes  made  of  these  materials 
should  prove  very  useful.  The  Bureau  of  Standards  has  done  some  work 
upon  these  materials  but  the  manufacture  of  pyrometer  tubes  made  from 
them  has  not  yet  been  commercialized. 

In  conclusion,  the  writer  wishes  to  express  his  thanks  to  his  associates 
for  many  helpful  suggestions. 


PYROMETER    PORCELAINS   AND    REFRACTORIES  251 


Pyrometer  Porcelains  and  Refractories 

BY   R.    W.    NEWCOMB,   B.  S.,    NEW   YORK,    N.    Y. 
(Chicago  Meeting,  September,  1919) 

THE  constancy  of  calibration,  and  to  a  great  extent  the  life,  of  a 
thermoelement  is  dependent  on  the  suitability  of  the  primary  protecting 
tube  in  which  the  wires  are  mounted,  particularly  when  used  at  high 
temperatures.  An  ideal  thermocouple  protecting  tube  would  be  com- 
posed of  materials  that  would  not  contaminate  the  thermoelement  wires 
contained  in  it — one  that  would  remain  absolutely  gas-tight  at  all  tem- 
peratures of  usage,  that  will  not  be  attacked  by  gases,  or  other  surrounding 
agencies,  that  is  not  destroyed  by  heat,  that  withstands  sudden  and  ex- 
treme temperature  changes,  that  affords  a  high  degree  of  mechanical 
protection  and  does  not  deform  at  high  tern  peratures,  that  is  a  good  heat 
conductor  and  obtainable  in  small  diameters  so  as  to  keep  down  lag  factors. 
It  has  not  yet  been  possible  to  produce  tubes  of  any  known  mate- 
rials that  will  meet  all  of  these  conditions.  In  selecting  protecting  tubes, 
therefore,  one  should  be  chosen  the  characteristics  of  which  best  fit  it  to 
the  particular  conditions  of  usage.  Frequently  conditions  are  such 
that  two  tubes  have  to  be  used,'  a  primary  gas-tight  protecting  tube 
inside  of  a  secondary  protecting  tube,  because  certain  agencies  will  attack 
the  primary  gas-tight  tube  unless  it  is  guarded  by  the  outside  tube. 

PRIMARY  PROTECTING  TUBES 

Of  primary  protecting  tubes,  there  are  two  classes:  those  of  quartz 
which  are  obtainable  in  three  grades,  transparent  quartz,  drawn  silica, 
and  molded  silica ;  and  those  of  refractory  porcelain  (alundum  included) , 
which  are  divided  into  several  grades. 

Primary  protecting  tubes  of  quartz  (silica)  are  apparently  gas-tight 
when  well  made  and  remain  gas-tight  if  not  used  at  too  high  temperatures. 
At  temperatures  where  divitrification  is  considerable,  tubes  of  quartz  soon 
become  crystallized  and  are  not  impervious  to  gases.  The  principal 
advantage  of  quartz  tubes  as  a  thermoelement  protection  is  in  its  ex- 
tremely low-temperature  coefficient  of  expansion.  It  can  be  subjected  to 
violent  temperature  changes  without  danger  of  breaking.  Because 
silica  is  easily  reduced,  care  should  be  taken  when  the  tube  is  used  over  a 
platinum  thermoelement,  because  silicon  is  a  very  bad  contaminating 
agent.  Results  in  practice  seem  to  indicate  that  for  permanent  instal- 


252  PYROMETER  PORCELAINS  AND  REFRACTORIES 

lation  quartz  tubes  should  not  be  used  above  1000°  C.  and  for  intermittent 
service  not  above  1300°  C.  At  the  latter  range  deformation  should  be 
guarded  against. 

In  the  refractory  porcelain  group  of  primary  protecting  tubes,  it  is 
difficult  to  separate  the  classes.  They  range  from  vitrified  porcelains  to 
very  high  refractory  porcelains.  Alundum  tubes,  perhaps,  should  not 
be  included  with  this  group;  the  body  mixture  is  different,  but  in  charac- 
teristics they  are  not  dissimilar.  Before  the  war,  practically  all  porcelain 
primary  tubes  were  imported.  Within  the  last  3  years,  however,  Ameri- 
can porcelain  tubes  have  been  produced  that  excel  in  most  respects 
those  previously  imported. 

Vitrified  porcelains  that  stand  sudden  temperature  changes  remark- 
ably well  are  now  available  and  they  can  be  used  continuously  at  tempera- 
tures up  to  1200°  C.  Vitrified  porcelain  tubes  are  usually  gas-tight 
without  being  glazed,  but  are  frequently  glazed  as  an  added  precaution. 
Vitrified  porcelain  tubes  are  valuable  as  a  thermoelement  protection  for 
permanent  installation  above  the  practical  range  of  quartz  tubes  and 
where  moderate  temperature  changes  take  place  over  a  short  time  interval. 

Refractory  porcelain  tubes  are  obtainable  for  the  range  to  1600°  C. 
The  glaze  softens  and  is  absorbed  by  the  body  of  the  tube  if  subjected  to 
this  heat  for  some  length  of  time.  Tests  made  seem  to  indicate,  however, 
that  the  tubes  remain  impervious  to  gases.  The  best  American  refractory 
porcelain  tubes  are  now  provided  with  a  glaze  that  does  not  soften  below 
1350°  C.  and  at  this  temperature  does  not  flow. 

SECONDARY  PROTECTING  TUBES 

Secondary  pyrometer  tubes  are  not  ordinarily  gas-tight.  Their 
purpose  is  to  give  mechanical  protection  to  the  primary  tube,  protect  the 
glaze  of  the  primary  tube  from  abrasive,  corrosive,  and  fluxing  conditions, 
and  to  prevent  deformation  at  high-temperature  ranges.  Also,  frequently, 
in  the  case  of  refractory  porcelain,  to  introduce  sufficient  lag  to  prevent 
a  too  rapid  temperature  change,  which  otherwise  might  cause  the  primary 
tube  to  crack. 

There  are  many  secondary  tubes  besides  those  of  metal;  those  most 
commonly  used  are  fireclay,  plumbago,  carborundum,  and  unglazed 
refractory  porcelain. 

Those  of  refractory  porcelain  offer  the  greatest  advantage  for  most 
conditions  other  than  in  molten  metal  and  baths.  First,  because  they 
stand  up  under  the  most  severe  temperatures  at  which  thermocouples  are 
used  and,  second,  because  in  themselves  they  are  not  a  contaminating 
agent  that  will  attack  the  thermoelement  wires  in  case  the  primary  tube 
becomes  broken.  Most  of  the  refractory  porcelain  secondary  tubes  have 
a  body  with  a  lower  temperature  coefficient  of  expansion  than  that  of  the 


DISCUSSION  253 

primary  tubes,  and  they  are  less  liable  to  crack  under  severe  temperature 
changes.  There  are  few  fluxing  agents  that  act  upon  them.  They 
deform  only  at  the  very  highest  temperatures.  In  glass-melting  tanks, 
where  conditions  are  very  trying  on  a  thermocouple  protecting  tube,  these 
tubes  remain  apparently  unaffected  under  normal  conditions. 

Fireclay  secondary  tubes  can  be  used  satisfactorily  in  very  large 
slow-heating  furnaces  such  as  brick  kilns,  continuously  operated  fur- 
naces, etc.  Their  characteristics  are  similar  to  other  fireclay  products. 
For  the  most  part  they  must  be  large  to  give  the  required  mechanical 
protection,  which  causes  them  to  be  of  a  size  where  the  lag  they  introduce 
is  a  serious  disadvantage. 

Plumbago  secondary  tubes  are  ordinarily  used  only  in  molten  metals 
and  baths.  When  exposed  to  oxidating  conditions,  they  are  rapidly 
destroyed.  It  is  best  to  frankly  admit  that  for  use  in  molten  metals, 
lead  heat-treating  baths  excepted,  no  tubes  known  to  the  writer  give 
what  can  be  called  satisfactory  service.  •  Plumbago  tubes  last  some 
length  of  time  in  molten  brass, .  bronze,  copper  and  aluminum,  but  the 
service  cannot  be  compared  to  the  service  given  by  the  other  tubes  under 
ordinary  furnace  condition. 

Carborundum  tubes  appear  to  be  a  very  good  secondary  protec- 
tion from  the  standpoint  of  heat  conduction,  mechanical  protection, 
and  refractory  qualities.  The  fact  that  these  tubes  are  so  refractory 
permits  them  to  be  used  as  a  secondary  protection  for  high  temperatures. 
When  this  is  done,  a  primary  protecting  tube  free  from  silica  must  be 
used  on  account  of  the  strong  reducing  condition  produced  by  the  second- 
ary tube,  which  causes  a  reduction  of  the  silica  in  the  primary  tube, 
resulting  in  a  contamination  of  the  thermoelement  combined  with  a  very 
rapid  crystallization.  The  same  results  could  be  expected  from  plum- 
bago secondary  tubes  were  they  used  at  the  same  temperature  range; 
but  this  is  not  usually  the  case. 

While  material  progress  has  been  made  in  the  perfection  of  primary 
and  secondary  pyrometer  tubes,  there  is  still  much  to  be  desired.  As  yet 
no  tube  has  been  developed  for  molten  steel  or  molten  glass  nor  has 
there  been  a  tube  produced  of  great  mechanical  strength  that  can  be  used 
at  the  higher  temperature  ranges.  Of  the  tubes  now  available,  a  careful 
selection  must  be  made  to  obtain  a  combination  of  primary  and  second- 
ary tubes  of  which  the  qualities  are  such  as  will  best  fit  them  to  the  par- 
ticular conditions  under  which  they  are  to  be  used. 

DISCUSSION 

A.  O.  ASHMAN,  Palmerton,  Pa.  (written discussion*). — Mr.  Newcomb's 
paper  has  interested  me  greatly,  as  I  have  had  numerous  experiences 

*  Received  Sept.  25,  1919. 


254  PYROMETER    PORCELAINS    AND    REFRACTORIES 

along  this  line.  I  do  not  think  enough  emphasis  can  be  put  on  his  warn- 
ing to  keep  platinum  couples  free  from  contact  with  quartz  tubes,  as 
silica  shows  a  tendency  to  alloy  with  platinum  to  a  surprising  degree 
even  at  low  temperatures.  I  have  frequently  had  evidence  of  silica 
contamination  in  a  perfectly  good  tube,  in  which  there  was  seemingly 
no  possibility  of  a  reducing  atmosphere. 

This  is  rather  important  as  it  seems  to  be  common  practice  to  insulate, 
as  well  as  protect,  the  wires  by  means  of  capillary  silica  tubing,  thus 
allowing  the  entire  wire  to  be  in  contact  with  the  silica.  The  best  way 
to  mount  a  couple  in  a  silica  protecting  tube  is  by  means  of  double-bore 
porcelain  insulating  tubes,  the  protecting  tube  being  slightly  longer  than 
the  couple  so  as  to  leave  a  space  between  the  couple  and  the  end  of  the 
tube.  In  this  way  there  is  no  possibility  of  contamination  from  silica. 
In  no  case  should  silica  capillary  tubes  be  used  to  insulate  platinum  wires. 

Mr.  Newcomb  states  that  there  is  still  much  to  be  desired  in  pyro- 
meter protecting  tubes;  this  is  in  keeping  with  my  experience.  From 
a  practical  standpoint  there  is  not  a  satisfactory  pyrometer  tube  on  the 
market  for  high  temperatures;  with  all  due  respect  to  the  many  improve- 
ments and  good  work  recently  done  in  this  line.  I  believe  that  the  whole 
future  development  of  pyrometry  is  dependent  on  the  development  of 
suitable  refractories.  With  suitable  refractories,  for  example,  Darling's1 
work  with  liquid  couples  could  be  developed  to  a  practical  basis,  making 
possible  the  use  of  base-metal  couples  to  replace  platinum. 

CARLETON  W.  HUBBARD,  Greenwich,  Conn,  (written  discussion*). 
This  paper  would  have  been  more  valuable  if  the  information  in  it  had 
been  tabulated,  giving  the  author's  recommendations  for  primary  and 
secondary  tubes  for  various  temperature  ranges  and  uses.  The  danger  of 
thermoelement  contamination  is  generally  not  sufficiently  appreciated. 
This  point  is  touched  on  several  times  in  the  paper,  but  the  actual  danger 
points  as  to  temperatures  and  conditions  of  use  are  not  given  as  elaborately 
as  they  should  be  to  be  of  value  to  the  purchaser  or  engineer,  who,  at  the 
same  time,  is  not  a  chemist.  There  is  need  for  a  definite  body  of  infor- 
mation regarding  temperatures  at  which  various  kinds  of  tubes  begin  to 
deform.  Some  test  standards  for  this  kind  of  work  should  be  set,  and  I 
would  suggest  various  lengths  of  overhang  for  tubes  of  different  diameters 
and  wall  thicknesses. 


*  Received  Oct.  8,  1919. 

\C.   B.    Darling:   Base-metal   Thermpelectric   Pyrometers.    Jnl.   Faraday  Soc., 
Meeting  Nov.  7,  1917. 


PYROMETER    PROTECTION    TUBES  255 


Pyrometer  Protection  Tubes 

BY   F.    A.    HARVEY,*  PH.  D.,    SYRACUSE,  N.  Y. 
(Chicago  Meeting,  September,  1919) 

DURING  the  last  few  years,  there  has  been  a  constant  tendency  toward 
increasingly  high  temperatures  in  many  lines  of  industry.  The  necessity 
for  increased  production  of  coke  gave  a  16-hr,  coking  period  where  we 
used  to  have  24  or  even  30-hr,  periods.  Mechanical  stokers  have  in- 
creased the  temperatures  in  boiler  furnaces.  The  use  of  a  chain-grate 
stoker  with  coke-breeze  fuel  necessitates  a  low  arch  in  the  furnace  and 
temperatures  run  much  higher  than  firebrick  were  formerly  called  upon 
to  stand.  These  higher  temperatures  have  necessitated  more  rigid  tests 
and  the  separation  of  even  high-grade  firebrick  into  separate  classes.  More 
severe  tests  mean  higher  temperatures  and  closer  control,  and  this,  in 
turn,  means  better  pyrometer  tubes.  Platinum  couples  do  not  have  a 
very  long  life  when  used  at  temperatures  exceeding,  say,  1200°  C.,  but 
the  proper  selection  of  a  high-grade  brick  to  be  used  in  a  boiler  setting 
will  save  the  cost  of  several  couples;  and  with  freight  rates  continually 
increasing  it  becomes  increasingly  important  to  know  where  brick  may 
be  bought,  nearest  the  job,  that  will  prove  satisfactory.  Couples  that  will 
stand  continuous  use  at  higher  temperatures,  of  tungsten,  molybdenum, 
or  other  metal,  will  doubtless  be  forthcoming  as  soon  as  the  need  becomes 
sufficiently  urgent. 

There  are  on  the  market  several  tubes  that  are  entirely  satisfactory 
for  temperatures  up  to  1300°  C.,  if  properly  protected  from  heat  shock. 
The  Semet-Solvay  Co.  makes  a  practice,  however,  of  testing  silica  and 
clay  firebricks  at  1450°  C.  for  72  hr.,  and  so  far  we  have  been  unable  to 
find  a  tube  that  is  entirely  satisfactory  for  this  purpose.  A  graphic 
record  of  the  temperature  during  the  entire  test  is  extremely  desirable 
and  hence  we  have  been  trying  out  anything  that  gave  promise  of  success. 

Several  years  ago  we  were  using  insulating  tubes,  made  purely  for 
electrical  work,  of  vitrified  porcelain.  These  tubes  stood  up  under  any 
temperature  we  were  able  to  reach  and  were  apparently  impervious  to 
gases.  When  this  stock  gave  out,  due  to  mechanical  breakage,  we  tried 
other  insulation  tubes  only  to  have  them  melt  down  like  glass.  Marquardt 
Masse  tubes  were  tried  next  under  a  statement  from  the  distributers' 
catalog,  copied  from  German  circulars  without  verification,  that  these 
tubes  "can  be  used  up  to  3000°  F.  (1650°  C.)  without  the  slightest  risk." 


*  Laboratory  Physicist,  Semet-Solvay  Co. 


256  PYROMETER   PROTECTION   TUBES 

This  statement  is  about  300°  C.  from  the  truth,  as  1350°  C.  is  as  high  as 
our  experience  indicates  they  can  be  used.  The  tubes  are  uneven  in 
their  heat  resistance.  Many  broke  through  heat  shock,  but  this  may 
be  prevented  by  using  an  outer  protection  tube  of  silfrax.  Usalite  tubes, 
Royal  Worcester  tubes  (England),  and  several  unbranded  tubes  from 
different  makers  failed. 

The  S.  C.  P.  Japan  tubes  have  been  tried  and  have  so  far  been  the  most 
successful.  They  have  a  slightly  too  low  softening  point  and  will  oc- 
casionally stick  to  the  silfrax  tube  or  warp  so  that  the  inner  insulating 
tube  cannot  be  withdrawn  to  anneal  the  couple.  They  seem,  however, 
to  be  completely  impervious  to  gases.  Even  when  the  glaze  has  been 
chipped  off,  the  body  of  the  tube  seems  to  be  impervious. 

Alundum  tubes  have  been  tried.  Without  a  glaze  they  are  too 
porous  and  so  far  the  glazes  tried  seem  to  lower  the  fusion  point  too  far. 
Impervite  tubes  were  tried  and  found  to  be  anything  but  impervious 
under  the  conditions:  viz.,  an  impervite  tube  inside  a  silfrax  tube.  The 
firm  from  which  these  tubes  were  purchased  recommends  a  secondary 
tube  of  unglazed  impervite  and  very  kindly  supplied  us  with  new  outer 
and  inner  tubes  for  experiment.  At  1250°  C.,  there  seems  to  be  no  attack 
on  the  tube  or  couple;  at  1300°  C.,  for  two  72-hr,  runs  the  tube  stood  up 
all  right.  At  1350°  C.,  during  one  run  of  72  hr.  the  tube  lost  its  glaze  but 
apparently  the  couple  was  not  contaminated.  At  the  time  of  writing 
it  has  not  been  tried  at  1450°  C.  It  seems  probable  that  the  rate  of  dif- 
fusion of  the  waste  gases  will  be  slow  enough  to  permit  the  use  of  the  tube 
in  an  atmosphere  that  is  not  particularly  hard  on  couples.  The  body 
of  the  tube  certainly  has  a  very  high  fusion  point. 

Mr.  F.  H.  Riddle  of  the  Bureau  of  Standards  has  sent  us  five  tubes, 
but  the  tests  on  these  tubes  have  not  yet  been  completed. 

At  present  our  method  of  recording  the  temperature  of  our  furnaces 
dodges  the  difficulty  by  using  two  couples.  Two  silfrax  sheaths  are  used, 
one  just  above  the  other,  the  top  one  projecting  about  7  in.  (17  cm.)  into 
the  furnace  chamber,  while  the  lower  projects  only  2%  in.  (6.35  cm.). 
It  has  been  found  by  trial  that  a  couple  in  this  lower  tube  does  not  attain 
the  full  temperature  of  the  furnace,  but  runs  about  100°  lower.  In 
spite  of  this  the  lag  over  the  actual  changes  in  furnace  temperature  is 
slight.  Two  couples  are  connected  to  a  recording  meter,  the  lower  one 
being  left  in  continuous  run.  The  upper  is  protected  by  a  fused  quartz 
tube  and  is  pushed  clear  into  the  upper  silfrax  tube,  where  it  attains  the 
full  temperature  of  the  furnace  in  about  5  min.  After  10  min.,  it  is 
withdrawn  and  the  recorder  chart  thus  carries  a  calibration  of  the  lower 
couple  made  every  2  or  3  hr.  The  workmen  run  the  furnace  by  the 
continuous  record. 


DISCUSSION  257 

DISCUSSION 

F.  H.  RIDDLE,*  Pittsburgh,  Pa. — I  understand  that  these  tests  at 
1450°  were  for  continuous  periods,  that  is,  over  several  hours.  Is  it 
possible,  for  short  periods  of  time,  to  go  to  higher  temperatures  than 
1450°?  The  apparently  vitreous  tubes  that  appeared  to  resist  the  effect 
of  iron  the  best  are  made  to  vitrify.  The  Marquardt  mass  bodies  are 
very  porous.  When  examined  after  use  at  high  temperatures,  the  glaze 
seems  to  have  entirely  disappeared.  This  is  due  to  the  fact  that  when 
it  softens  on  the  porous  body  it  is  absorbed  into  the  body.  This  can  be 
shown  by  holding  a  broken  portion  of  a  tube  in  a  colored  solution  and  then 
examining  a  cross-section. 

The  vitreous  tubes  are  made  to  resist  detrimental  gases,  but  they  will 
not  withstand  sudden  temperature  changes,  as  a  rule,  as  well  as  the  low- 
coefficient  porous  tubes. 

F.  A.  HARVEY. — We  have  not  tried  these  tubes  for  shorter  periods  at 
higher  temperatures  for  we  have  no  occasion  to  go  above  1450°.  There 
is  one  definite  problem  we  are  trying  to  solve;  that  is  for  a  continuous 
run  of  72  hr.  at  1450°.  We  need  a  more  highly  refractory  tube  than  we 
have  at  the  present  time.  You  can  protect  against  heat  shocks  by  put- 
ting a  carborundum  tube  on  the  outside.  If  you  are  dealing  with  a  con- 
tinuous run,  the  temperatures  can  be  raised  as  gradually  as  you  wish. 

W.  E.  FORSYTHE,  Nela  Park,  Cleveland,  0. — Our  experience  has  been 
that  if  you  are  going  to  measure  temperatures  as  high  as  1550°  C.  with  a 
platinum  platinum-rhodium  couple  in  anything  approaching  practical 
conditions,  the  e.m.f.  is  very  questionable.  We  have  never  had  a  plati- 
num platinum-rhodium  thermocouple  that  would  keep  its  calibration 
when  mounted  in  an  ordinary  platinum-wound  furnace  operated  at  this 
temperature  for  any  length  of  time. 

R.  B.  SOSMAN,  Washington,  D.  C. — We  measure  temperatures  with 
platinum  platinum-rhodium  couples,  regularly,  up  to  1755°,  the  melting 
point  of  platinum,  and  get  comparative  accuracy,  but  the  platinum 
must  be  pure. 

*  Chemist,  Clay  Products  Division  U.  S.  Bureau  of  Standards. 


17 


258  PROTECTING   TUBES    FOR   THERMOCOUPLES 


Protecting  Tubes  for  Thermocouples 

BY   R.   B.    LINCOLN,*   DETROIT,    MICH. 
(Chicago  Meeting,  September,  1919) 

THE  function  of  a  pyrometer  protecting  tube  is  to  maintain  an  atmos- 
phere about  the  thermocouple  most  favorable  to  its  continued  accuracy 
and  long  life,  and  at  the  same  time  permit  the  weld  of  the  couple  to  attain 
the  full  temperature  of  the  area  or  object  being  measured.  In  addition  to 
protecting  the  thermocouple  from  the  chemical  or  alloying  effects  of  the 
products  of  combustion  or  bath  being  measured,  the  tube  must  also  pro- 
tect the  couple  from  mechanical  injury.  The  conditions  met  in  practice 
vary  so  much  that  no  one  material  is  suitable  for  all  applications.  The 
usefulness  as  well  as  the  expense  of  maintenance  of  a  pyrometer  system  are 
influenced  greatly  by  a  choice  of  the  most  suitable  protecting  tube.  The 
tube  must  protect  the  couple  without  itself  becoming  too  great  an 
expense  item. 

The  protecting  tube  should  have  the  following  qualities:  (1)  A  melt- 
ing point  well  above  the  highest  temperature  to  be  encountered.  (2) 
Sufficient  strength  through  the  entire  range  of  temperatures  to  hold  up  its 
own  weight  and  resist  such  shock  and  jar  as  are  unavoidable.  (3)  It 
must  be  impervious  to  the  atmosphere  or  bath  to  which  it  will  be  subject. 
(4)  It  must  not  give  off  any  vapor  injurious  to  the  couple.  (5)  It 
must  not  form  any  oxides  fusible  below  the  highest  temperature  to  be 
measured. 

The  proper  location  of  the  tube  in  the  furnace  is  almost  as  important 
as  the  choice  of  material.  The  tube  must  be  placed  as  near  to  the  work  to 
be  heated  as  possible.  It  must  be  kept  out  of  the  direct  path  of  flame, 
both  because  that  would  give  too  high  temperature  readings  and  because 
partly  burned  fuel  will  destroy  the  tube  faster  than  the  quiet  products  of 
combustion. 

Pyrometer  tubes  are  sometimes  buried  in  the  floor  or  embedded  in  the 
wall  of  the  furnace,  but  usually  the  lag  is  so  great  that  the  readings  are  of 
little  use.  When  rare-metal  couples  and  expensive  porcelain  tubes  are 
used,  there  is  a  temptation  not  to  project  the  couple  far  enough  into  the 
furnace.  Where  only  approximate  results  are  required,  a  couple  may  be 
projected  into  a  depression  in  the  furnace  wall;  but  for  the  heat  treat- 
ment of  steel,  the  couple  must  be  near  the  work.  The  tube  must  always 
project  into  a  furnace  far  enough  to  become  fully  heated  to  the  furnace 


*  Hoskins  Mfg.  Co. 


R.   B.    LINCOLN  259 

temperature.  A  refractory  tube  will  require  two  or  three  times  its  diam- 
eter exposed  to  the  heat  to  get  away  from  the  cooling  of  the  tube  by  the 
colder  parts.  A  nickel-chromium  tube  will  require  twice  that  much 
immersion  and  a  steel  tube  slightly  more  than  the  nickel-chromium. 

Neither  platinum  nor  platinum-rhodium  are  injured  by  oxidation  but 
both  absorb  carbon,  hydrogen,  and  many  metal  vapors  at  high  tempera- 
tures. Porcelain,  kaolin,  or  fused  silica  are  about  the  only  substances 
that  may  be  safely  allowed  to  touch  a  rare-metal  couple  at  high  tempera- 
tures. Since  "chemists  triangles"  made  of  nickel-chromium  wire  do  not 
injure  platinum  dishes,  it  may  be  that  if  drawn  nickel-chromium  tubes 
could  be  obtained  the  fragile  porcelain  tubes  could  be  entirely  dis- 
pensed with,  but  as  long  as  cast  tubes  only  are  available,  porcelain  must 
be  interposed  between  the  outer  tube  and  the  couple.  A  rare-metal 
couple  must  be  protected  from  contamination  even  when  cold,  otherwise 
bits  of  metal,  salt  or  paint,  and  charcoal  from  a  wooden  bench  may  be- 
come attached  to  it  when  cold  and  later  destroy  the  accuracy  of  the  couple 
when  placed  in  the  furnace. 

Porcelain  was  one  of  the  first  materials  to  be  used  for  pyrometer 
protecting  tubes.  The  best  grades  for  this  work  soften  about  1800°  C. 
and  are  practically  impervious  to  gas.  It  is  one  of  the  few  materials 
that  will  not  alloy  with  or  contaminate  platinum.  This  material  is  very 
brittle  at  temperatures  below  1200°  C.,  and  its  high  coefficient  of  expan- 
sion causes  it  to  break  from  too  rapid  changes  of  temperature.  It  is 
destroyed  by  fused  alkalies  or  metallic  oxides.  This  combined  with 
its  first  cost  has  limited  its  use  to  a  protection  for  rare-metal  couples, 
and  it  is  usually  protected  by  an  outer  metal  tube. 

Fused  silica,  artificial  quartz,  has  a  very  low  coefficient  of  expansion 
and  is  much  cheaper  than  porcelain.  It  is  suitable  for  protecting  rare- 
metal  couples  subject  to  sudden  changes  of  temperature.  When  subject 
to  temperatures  above  1200°  C.,  it  undergoes  a  recrystallization  which 
causes  it  to  become  cloudy  and  weak.  It  finally  breaks  from  its  own 
internal  strains. 

Thermocouples  made  of  nickel-chromium,  known  under  the  trade 
name  of  chromel,  and  nickel-aluminum,  known  as  alumel,  oxidize  very 
slowly  and  maintain  their  accuracy  best  when  subjected  to  oxidizing  con- 
ditions. They  fail  quickly  when  subjected  to  strong  reducing  condi- 
tions and  even  more  quickly  when  subjected  to  alternative  reducing  and 
oxidizing  conditions.  These  materials,  when  used  in  an  electric  furnace 
heated  by  nickel-chromium  or  platinum  wire,  usually  require  no  protect- 
ing tube.  When  used  in  a  fuel-fired  furnace,  probably  the  best  and  at  the 
same  time  most  economical  protecting  tube  is  one  made  of  nickel-chro- 
mium. Tubes  made  of  an  alloy  of  80  per  cent,  nickel  and  20  per  cent, 
chromium,  known  under  the  trade  name  of  chromel  A,  are  most  economi- 
cal for  temperatures  between  600°  C.  and  1100°  C.,  in  gas-  or  oil-fired 


260  PROTECTING   TUBES   FOR   THERMOCOUPLES 

furnaces.  A  cheaper  tube  is  made  with  somewhat  less  chromium  and  an 
addition  of  about  25  per  cent,  of  iron.  This  tube  will  stand  a  slightly 
higher  temperature  than  the  straight  nickel-chromium  but  lasts  about 
one-half  as  long  at  1000°  C.  The  life  of  nickel-chromium  tubes  is  greatly 
reduced  by  alternately  strongly  oxidizing  and  strongly  reducing  conditions 
such  as  are  encountered  near  the  bridge  wall  of  a  coal-fired  furnace.  The 
tube  containing  25  per  cent,  of  iron  is  rather  more  satisfactory  under 
these  conditions  than  the  straight  nickel-chromium,  but  the  best  solution 
is  to  so  locate  the  tube  that  it  will  be  subjected  to  a  dead  heat.  Nickel- 
chromium  tubes  are  very  satisfactory  as  an  outer  protection  for  rare- 
metal  couples. 

At  temperatures  around  1200°  and  1300°  C.,  alundum  tubes  are  quite 
satisfactory  but  are,  of  course,  very  fragile. 

For  measuring  high  temperatures  in  large  furnaces,  porcelain  kilns, 
and  glass  tanks,  where  the  conditions  are  very  severe  but  the  change  in 
temperature  is  very  slow,  tubes  made  of  fireclay  with  heavy  walls,  % 
to  1^  in.  (19  to  38  mm.)  thick  give  good  service. 

Iron  and  steel  protecting  tubes  seem  to  allow  furnace  gases  to  diffuse 
through  the  walls  of  the  tube  at  red  heats  and,  when  used  with  nickel- 
chromium  couples,  should  always  be  open  to  the  air  at  the  end  away  from 
the  heat.  If  the  outer  end  is  plugged,  the  couples  will  behave  very  much 
as  though  not  protected  from  the  furnace  gases.  I  consider  it  safest  to 
have  all  protecting  tubes  open  to  the  air  at  the  end  away  from  the  heat. 
Care  should  be  taken  to  see  that  this  end  is  not  surrounded  by  flame  or  gas 
from  the  furnace.  In  the  case  of  a  bath  of  molten  metal  or  salt,  the  tube 
should  be  long  enough  to  prevent  drops  of  metal  or  salt  or  small  pieces 
of  charcoal  from  dropping  down  the  tube. 

Couples  of  iron  versus  constantan,  iron  versus  commercial  nickel,  or 
nickel-chromium  versus  nickel-copper  (chromel-copel)  may  be  used  up  to 
300°  or  400°  C.  without  any  protection,  and  to  500°  or  600°  C.  with  light 
steel  tubes.  The  use  of  iron-constantan  couples  at  a  temperature  around 
800°  and  900°  C.  requires  somewhat  different  treatment  from  that  given 
either  rare  metals  or  chromel-alumel  couples,  since  the  iron  and  constan- 
tan oxidize  rapidly  at  this  temperature,  while  the  other  couples  are 
injured  most  by  a  reducing  condition.  Iron-constantan  couples  are 
frequently  installed  in  iron  tubes  closed  as  tightly  as  possible-  in  order  to 
allow  the  oxidation  of  the  iron  tube  and  the  diffusion  of  gas  through  the 
walls  of  the  tube  to  create  a  reducing,  or  at  least  non-oxidizing,  atmosphere 
in  the  tube.  Iron-constantan  will  oxidize  very  rapidly  when  used  in  a 
nickel-chromium  protecting  tube  that  allows  free  access  to  the  air,  and 
when  it  is  desired  to  use  a  tube  of  this  material,  an  inert  material  (such 
as  alundum)  mixed  with  a  few  per  cent,  of  charcoal  is  packed  around  the 
couple  to  retard  oxidation. 

Until  this  time  I  have  considered  the  protecting  tube  in  its  relation  to 


R.   B.    LINCOLN  261 

the  couple  and  assumed  that  the  tube  is  exposed  to  products  of  combus- 
tion in  a  furnace.  When  the  tube  is  exposed  to  a  molten  bath  a  tube 
must  be  chosen  that  will  withstand  the  bath.  A  nickel-chromium-iron 
tube  will  give  good  results  in  molten  lead  around  700°  to  800°  C.  Seam- 
less steel  or  extra  heavy  wrought  iron  pipe  is  sometimes  used  because  of 
its  low  first  cost. 

Molten  cyanide  of  potash  is  one  of  the  most  difficult  baths  to  control. 
It  is  very  injurious  to  any  kind  of  a  couple,  it  fluxes  refractory  tubes,  and 
passes  through  steel  tubes.  A  nickel-plated  steel  tube  has  given  fair 
results,  and  nickel-chromium  alloy  is  good  if  the  casting  is  entirely  free 
from  defects. 

An  alloy  of  75  per  cent,  iron  and  25  per  cent,  chromium  known  under 
the  trade  name  of  "chromon"  has  been  developed  to  withstand  the  action 
of  molten  brass  and  bronze.  A  light  protecting  tube  of  this  material 
is  used  with  a  fairly  light  couple  to  take  readings  in  a  crucible  of  molten 
metal.  A  reading  is  secured  in  40  sec.  to  1  min.  Such  a  tube  will  give 
between  100  and  200  readings  before  failing,  depending  on  the  tempera- 
ture and  composition  of  the  melt,  and  on  the  perfection  of  the  casting. 

In  conclusion,  to  intelligently  select  a  protecting  tube  the  following 
factors  must  be  considered:  Whether  the  couple  should  be  used  under 
oxidizing  or  reducing  conditions;  the  atmosphere  or  bath  to  which  the 
tube  will  be  subjected;  the  maximum  temperature  to  be  encountered. 
Care  taken  in  the  selection  of  the  proper  tube,  locating  it  in  the  most 
favorable  place  in  the  furnace,  and  then  inspecting  and  replacing  it 
before  it  has  deteriorated  enough  to  injure  the  couple,  will  result  in 
increased  accuracy  and  decreased  up-keep  charges. 


262  PYROMETER   PROTECTION   TUBES 


Pyrometer  Protection  Tubes 

BY   OTIS   HUTCHINS,*  B.   S.,    NIAGARA   FALLS,    N.    Y. 
(Chicago  Meeting,  September,  1919) 

IT  is  intended  to  discuss  in  this  paper  protection  appliances  used  for 
high-temperature  pyrometer  installations  involving  the  use  of  platinum 
couples  and  to  describe  some  of  the  characteristics  of  a  new  carborun- 
dum protection  tube.  Of  all  the  components  that  go  to  make  a  complete 
pyrometer  installation,  the  one  given  the  least  attention  and  the  one 
about  which  there  is  the  least  exact  information  is  the  outer  protection 
tube.  This  condition  is  unfortunate,  as  much,  and  in  some  cases  even 
the  success  of  the  equipment  as  a  whole,  depends  on  the  proper  type  of 
protection.  In  the  majority  of  cases,  the  installation  is  allowed  to  operate 
without  attention  until  some  part  of  the  apparatus  breaks  down.  The 
outer  tube  is  the  component  that  usually  fails  and,  more  often  than  not, 
the  failure  causes  breakage  of  the  porcelain  protection  and  ruin  of  the 
couple.  This  condition  should  be  recognized  and  studied  as  it  has  a 
very  important  bearing  upon  the  upkeep  cost  of  pyrometer  equipment. 
The  cost  of  the  outer  protection  tube  is  small  compared  with  the  cost 
of  the  platinum  couple  and  porcelain  protection  and  it  would  well  repay 
users  of  this  type  of  equipment  to  replace  their  outer  protection  tubes  at 
certain  definite  periods,  which  experience  shows  is  necessary  to  prevent 
destruction  of  the  platinum. 

Speaking  generally,  platinum  thermocouple  equipment  is  used  for 
measuring  temperatures  over  2000°  F.  (1094°  C.),  and  is  used  extensively 
for  the  control  of  brick  and  pipe  kilns,  glass  pot  furnaces,  glass  tank 
furnaces,  heat-treating  furnaces,  open-hearth  furnaces,  and  forge  furnaces. 
A  satisfactory  protection  tube  must  be  sufficiently  refractory  not  to  soften 
at  the  maximum  temperature  within  the  furnace.  It  must  be  resistant 
to  cracking  due  to  temperature  changes,  should  be  as  nearly  gas-tight  as 
possible,  and  should  be  made  of  a  material  with  a  high  thermal  conduc- 
tivity and  great  resistance  toward  erosion  by  the  furnace  atmosphere. 

Carborundum  is  a  refractory  that  possesses  these  characteristics  to  a 
marked  degree.  Recent  manufacturing  improvements  hav.e  made  pos- 
sible the  production  of  protection  tubes  composed  entirely  of  carborun- 
dum without  the  addition  of  any  binding  material.  These  tubes  possess 
all  the  desirable  properties  of  carborundum  including  great  refractoriness, 
low  coefficient  of  expansion,  resistance  toward  the  action  of  furnace  gases 
and  slags,  and  high  thermal  conductivity.  The  effect  of  the  thermal 

*  Metallurgical  Engineer,  The  Carborundum  Co. 


OTIS   HUTCHINS  263 

conductivity  of  the  protection  tube  is  of  great  importance  and  has  a 
very  decided  bearing  on  the  accuracy  with  which  the  pyrometer  tempera- 
ture indication  follows  the  actual  temperature  condition  within  the 
furnace.  Carborundum  having  a  thermal  conductivity  of  about  eight 
times  that  of  fireclay  and  three  to  four  times  that  of  fused  alumina  re- 
fractories would  be  expected,  when  used  as  a  protection,  to  show  a  con- 
siderable advantage  in  this  respect. 

To  prove  this  point  a  series  of  experiments  was  conducted  with  car- 
borundum and  fireclay  tubes  to  determine  the  lag  in  the  pyrometer 
reading  caused  by  these  types  of  protection.  Four  chromel-alumel 
type  P  couples  were  prepared  and  standardized.  The  first  couple  was 
used  in  the  form  of  bare  wire  without  any  protection,  the  second  was  pro- 
tected by  a  porcelain  tube  and  a  carborundum  tube  1  in.  (2.5  cm.)  inside 
diameter,  1%  in.  (4.7  cm.)  outside  diameter,  length  outside  19  in.  (47 
cm.),  length  inside  17^  in.  (45cm.);  the  third  was  protected  by  a  porcelain 
tube  and  a  fireclay  tube  1  in.  inside  diameter,  2  in.  outside  diameter,  length 
outside  18  in.,  length  inside  17J4  in.,  and  the  fourth  couple  was  protected 
by  a  porcelain  tube  and  a  fireclay  tube  1^  in.  inside  diameter,  3  in.  outside 
diameter,  length  outside  18  in.,  length  inside  17^  in.  A  large  electric 
pot  furnace  was  maintained  at  a  constant  temperature  by  means  of  a 
rheostat.  The  temperature  of  this  furnace  was  determined  by  means 
of  a  platinum  thermocouple,  which  was  allowed  to  remain  in  the  furnace 
throughout  the  experiment. 

Experiment  1. — The  furnace  was  maintained  at  approximately  450°  C. 
and  the  four  chromel-alumel  couples  were  plunged  into  it,  one  at  a  time, 
and  allowed  to  remain  there  until  the  millivolt  reading  of  the  couple 
became  constant.  Readings  of  millivolts  and  time  were  taken  and 
curves  plotted. 

Experiment  2. — The  procedure  was  repeated  with  the  furnace  tem- 
perature maintained  at  approximately  950°  C.  The  following  results 
were  obtained : 

FURNACE  AT     FURNACE  AT 

450°  C.,  950°  C., 

MINUTES  MINUTES 

Time  for  reading  of  bare  couple  to  become  constant.  ......         5  2j£ 

Time  for  reading  of  couple  in  carborundum  protection  to  be- 
come constant 25  12j<j 

Time  for  reading  of  couple  in  2  in.  diameter  fireclay  protec- 
tion to  become  constant 50  35 

Time  for  reading  of  couple  in  3  in.  diameter  fireclay  protec- 
tion to  become  constant 70  50 

It  was  recognized  that  while  the  above  results  were  interesting  they 
did  not  indicate  what  could  be  expected  from  commercial  installations 
where  changes  of  temperature  were  less  rapid.  As  measuring  the  tem- 
perature of  the  gases  of  the  open-hearth  steel  furnace  is  a  promising  field 
for  pyrometry,  it  was  decided  to  make  certain  tests  to  determine  the  effect 
of  couple  protection  on  this  work.  Three  calibrated  platinum  thermo- 


264 


PYROMETER   PROTECTION  TUBES 


couples  were  installed  side  by  side  in  the  slag  pocket  of  an  open-hearth 
steel  furnace  just  above  the  checkerwork.  The  first  couple  had  porcelain 
and  a  1%  in.  diameter  carborundum-tube  protection,  the  second  had 
porcelain  and  a  2  in.  diameter  fireclay-tube  protection,  and  the  third 
couple  had  porcelain  and  a  3  in.  diameter  fireclay-tube  protection. 
Leads  and  switches  were  arranged  so  that  any  one  of  the  couples  could 
be  connected  with  a  millivolt  meter.  Readings  were  taken  of  millivolts 
and  time  for  each  couple  for  seven  reversals  of  the  furnace  gases.  The 


500 


)       5        10       15      20      25       30      35      40       45       50       55      60 
Time  in  Minutes 


5        10      15      20      25      30      35      40      45       50      55      CO 
Time  in  Minutes 

FIG.  1. — CURVES  SHOWING  LAG  IN  PYROMETER  READING  FOR  DIFFERENT  PROTECTION 

TUBES. 

thermocouple  protected  by  the  2-in.  fireclay  tube  did  not  operate  satis- 
factorily throughout  the  entire  test  and  no  curve  of  the  readings  of  the 
couple  is  included.  The  general  shape  of  the  curve  was  the  same  as  that 
of  the  couple  protected  by  the  3-in.  fireclay  tube,  the  time  and  temper- 
ature lag  being  less.  The  average  results  for  this  thermocouple  are  in- 
cluded in  the  tabulated  data.  No  record  was  made  of  the  exact  time  of 
changing  the  valves  on  the  furnace  so  that  no  data  are  available  to  show 
the  time  lag  of  the  couple  protected  by  the  carborundum  tube.  However, 


OTIS   HUTCHINS 


265 


judging  from  the  shape  of  the  curve  for  this  couple,  the  time  lag  must 
have  been  small.  The  data  given  in  Table  1  are  obtained  from  the 
curves  shown  in  Fig.  2. 


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It  wijl  be  seen  from  these  results  that  the  use  of  couple-protecting 
material  having  high  heat  conductivity  is  of  very  considerable  importance. 

In  the  glass-furnace  field  carborundum  tubes  are  rapidly  becoming 
the  standard  type  of  protection.  In  Fig.  3  is  shown  a  photograph  of  a 
carborundum  tube  that  has  had  4  mo.  service  in  an  oil-fired  glass  tank 


266 


PYROMETER   PROTECTION    TUBES 


TABLE  1 


Re- 
versal 

Indicated  Time  of  Reversal 
by  Couple  with 

Lag  of  3-in. 
Fireclay 
Behind 
Carborundum, 
Minutes 

Indicated  Temperature 
at  Time  of  Reversal  by 
Couple  with 

Temperature 
Lag  of  3-in. 
Diam.  Fireclay 
Couple  as 
Compared  with 
Carborundum 
Couple, 

Carborundum     3-in.  Fireclay 
Protection,         Protection, 

Carborundum     3-in.  Fireclay 
Protection,         Protection, 

Minutes 

Minutes 

Degrees  F. 

Degrees  F. 

Degrees  F. 

1 

7 

1QH 

3H 

1923 

2010 

87 

2 

24^ 

27M 

3 

2382 

2282 

100 

3 

40% 

44 

3K 

1978 

2065 

87 

4 

60 

61 

1 

2385 

2288 

103 

5 

78K 

81 

2H 

1962 

2062 

100 

6 

100H 

104                    3^ 

2435 

2340 

95 

7 

117 

120                   3 

2037 

2125 

88 

FIBECLAT  PROTECTION  TUBE 
3  INCH  2  INCH 


Average    time    lag   of   couple   protected   by  fireclay   tube 

over  carborundum  protected  couple 2  min.  49  sec.  1  min.  9  sec. 

Average  temperature  lag  of  couple  protected  by  fireclay  over 

carborundum  protected  couple 94 . 3°  F.       56 . 0°  F. 


FIG.  3. — CARBORUNDUM  TUBE  AFTER  4  MO.  SERVICE  IN  AN  OIL-FIRED  GLASS  TANK 

FURNACE. 

furnace.  It  will  be  noticed  that  the  fireclay  supporting  tube  has  been 
very  badly  melted  away  while  the  carborundum  tube  shows  only  slight 
signs  of  wear.  A  life  from  6  to  8  mo.  is  usual  for  these  tubes  and  it  is 
not  uncommon-  to  find  them  giving  satisfactory  service  for  a  very  much 
longer  period. 

Due  to  the  high  thermal  conductivity  of  carborundum  tubes  it  is  not 
necessary,  when  installing  this  equipment,  to  allow  the  tube  to  project 
more  than  a  short  distance  into  the  furnace.  In  some  cases  it  is  even 
desirable  to  keep  the  end  of  the  tube  flush  with  the  furnace  wall;  this 
method  of  installation  will  add  very  materially  to  the  life  of  the  thermo- 
couple protection. 


MELTING    POINT    OF   REFRACTORY   MATERIALS  267 


Melting  Point  of  Refractory  Materials 

BY   LEO   I.    DANA,*   B.    8.,    WASHINGTON,    D.    C. 
(Chicago  Meeting,  September,  1919) 

THE  object  of  this  paper  is  to  discuss  the  factors  and  conditions  that 
affect  the  observed  values  of  the  melting  points  of  refractory  materials 
and  to  describe  practical  methods  for  the  determination  of  these  points. 
While  it  appeared  to  be  necessary  to  discuss  some  of  the  general  pro- 
perties of  silicates  and  refractories,  these  subjects  have  been  entered 
into  only  in  so  far  as  they  relate  to  the  melting  point  and  its 
determination. 

Refractory  materials  such  as  fireclays,  firebricks,  and  minerals  gener- 
ally, may  be  considered  to  be  composed  of  compounds  of  metallic  oxides, 
solid  solutions  of  the  oxides,  the  pure  oxides,  or  mixtures  of  the  three 
classes  together  with  small  amounts  of  a  variety  of  chemical  compounds. 
The  oxides  silica  and  alumina  occur  most  commonly  in  refractory  ma- 
terials; in  combination  with  these,  oxides  of  the  alkali  earth,  the  alkali, 
the  iron  group,  and  the  rare  earth  metals  are  frequently  found. 

MEANING  OF  MELTING  POINT 

In  its  strictest  sense,  the  term  melting  point  is  applied  to  the  tempera- 
ture at  which  the  solid  and  liquid  phases  of  a  pure  crystalline  substance 
can  remain  in  equilibrium ;  at  the  melting  point,  there  is  usually  a  discon- 
tinuous change  of  a  number  of  its  physical  properties.  la  the  case  of 
those  refractory  materials  that  are  either  amorphous  or  heterogeneous 
mixtures  or  compounds  of  oxides  or  other  substances,  the  term  melting 
point  is  not  a  definite  temperature;  the  change  from  the  solid  condition 
to  one  in  which  the  material  will  flow  is  gradual  over  a  temperature  and 
time  interval.  In  addition,  physical  and  chemical  reactions,  which  are 
not  equilibrium  reactions,  often  take  place  during  melting. 

With  the  rise  in  temperature  of  a  refractory  material,  the  first  phe- 
nomenon of  importance  usually  observed  is  the  sintering  or  vitrification  of 
the  particles;  that  is,  the  edges  of  the  particles  first  become  soft  and  liquid 
and  the  particles  stick  together  even  though  all  parts  of  the  material  are 
at  the  same  temperature.1  As  a  result  of  surface  tension,  the  soft  edges 

*  Assistant  Physicist,  U.  S.  Bureau  of  Standards. 

1  J.  W.  Mellor:  "Clay  and  Pottery  Industries,"  32,  247;  37,  309.  Griffin,  London, 
1914. 


268  MELTING    POINT   OF   REFRACTORY    MATERIALS 

of  the  particles  become  rounded  off.  Sintering  may  also  be  produced  by 
the  fusion  of  the  lower  melting-point  constituents,  the  formation  of 
eutectics  and  solutions,  and  the  chemical  reaction  of  the  constituents.  If, 
as  a  result  of  these  phenomena,  a  substance  of  sufficient  fluidity  to  diffuse 
through  the  mass  is  formed,  the  whole  body  may  flow  while  the  solid 
particles  are  held  in  suspension.  From  the  point  at  which  the  substance 
begins  to  sinter,  it  may  become  more  viscous  gradually,  over  a  long  range 
in  temperature,  until  it  flows,  or  it  may  soften  and  flow  distinctly  over  a 
short  range  in  temperature.  While  these  materials  have  a  more  or  less 
definite  temperature  interval  of  melting,  or  melting  range,  their  soften- 
ing is  also  a  function  of  the  time. 

If  the  refractory  material  is  a  pure  crystalline  compound,  it  will  have 
a  definite  melting  point,  in  the  strict  sense.  Nevertheless,  there  are  such 
pure  refractory  compounds  as  quartz  and  albite,  which  soften  very  slowly 
and  the  melting  interval  of  which  is  an  interval  of  time;  that  is,  a  long 
time  is  necessary  to  attain  equilibrium  between  the  crystalline  solid  and 
the  liquid.2  For  these  substances,  the  temperature-time  method  is  not 
suitable  for  the  determination  of  the  melting  point.  For  practical  and 
technical  purposes,  the  criterion  of  marked  flow  is  used  for  such  substances. 
On  account  of  superheating  while  melting,  the  melting  point  found  by 
this  method  may  be  much  higher  than  the  true  melting  point,  as  in  the 
case  of  quartz,  where  it  is  about  50°  C.  higher.3 

By  the  term  refractoriness  of  a  refractory  material  is  usually  implied 
its  resistance  to  the  action  of  heat  or,  more  definitely,  its  ability  to  retain 
its  shape  at  high  temperatures  under  accurately  specified  conditions. 
The  term  "refractoriness"  is  broader  and  more  general  in  meaning  than 
"fusibility,"  which  is  usually  measured  by  the  softening  point  or  fusing 
point.  The  latter  refers  to  the  temperature  at  which  the  material  begins 
to  lose  its  shape  and  flows.  The  last  mentioned  phenomena  ordinarily 
take  place  in  several  stages  over  a  range  in  temperature.  The  sintering 
of  the  material  produces  shrinkage  and  bending;  but  this  phenomenon 
should  not  be  considered  as  taking  place  in  the  melting  range. 

Assuming  that  the  sample  is  in  the  form  of  a  cone  or  cylinder,  the 
beginning  of  the  deformation,  bending,  or  squatting  of  the  specimen 
marks  the  first  stage  of  melting;  the  second  stage  begins  when  the  mate- 
rial has  fused  into  a  lump  or  ball  or,  in  the  case  of  a  cone,  when  the  apex 
has  touched  the  base;  the  third  stage  begins  when  the  lump  has  flattened 
out  and  is  fluid.  These  melting  stages  occur  over  temperature  intervals 
of  varying  magnitude,  depending  on  the  substance;  in  many  cases  of  more 
or  less  pure  compounds,  the  material  melts  at  a  definite  temperature  or 

2  Day  and  Sosman:  Amer.  Jnl.  Sci.  [41  (May,  1911)  31,  341. 

3  The  temperature  at  which  silica  begins  to  flow  is  about  1750°  C.     See  C.  W. 
Kanolt:    U.    S.    Bureau    of  Standards    Tech.    Paper    10.     True   melting   point   of 
crystobalite  is  1710°  C.     See  Ferguson  and  Merwin:  Amer.  JnL  Sci.  (Aug.,  1918)  46. 


LEO    I.    DANA  269 

over  a  very  small  temperature  interval  and  thus  it  does  not  serve  a  useful 
purpose  to  demarcate  the  melting  stages.  That  particular  stage  in 
the  melting  range  which  is  to  be  considered  as  the  melting  point  or  soften- 
ing point  depends  on  the  material  and  the  extent  of  softening  that  will 
manifest  the  most  information  concerning  some  limitations  of  use  of  the 
material  or  the  conclusions  to  be  derived  from  the  melting-point  test. 
From  a  general  and  practical  standpoint  and  wherever  the  conditions 
of  use  of  the  material  are  not  specifically  known,  the  knowledge  of  the 
temperature  at  which  a  marked  and  distinct  flow  of  the  sample  begins  is 
most  important  and  useful  as  a  fixed  temperature  as  well  as  a  comparison 
temperature. 

The  temperature  at  which  a  marked  flow  begins  usually  occurs  after 
the  start  of  the  above-mentioned  first  stage  of  melting.  Especially  with 
samples  in  the  form  of  a  cone  and  cylinder  and  with  rapid  rates  of  heating, 
the  beginning  of  the  marked  and  distinct  flow  is  seen  to  occur  when  the 
sample  is  about  half  bent  over  or  halfway  between  the  first  two 
stages. 

In  general,  then,  the  practical  definition  of  the  melting  point  of  a 
refractory  material  is  identical  with  that  of  its  softening  or  fusing  point 
and  is  arbitrarily  stated  to  be  the  temperature  at  which  a  marked  flow 
of  the  material  begins.  In  terms  of  the  deformation  of  a  cone  or  cylinder, 
the  melting  point  is  halfway  between  the  temperature  at  which  the  de- 
formation begins  and  the  temperature  at  which  the  material  fuses  into  a 
lump  or  ball  or  is  completely  bent  over.  For  most  refractory  materials, 
the  melting  point  is,  under  specified  conditions,  reproducible  and  definite 
enough  to  be  worth  determining. 

FACTORS  AND  CONDITIONS  AFFECTING  OBSERVED  MELTING  POINT 

Chemical  Composition. — It  is  evident  that  the  chemical  composition 
of  a  refractory  material  will  affect,  to  a  large  extent,  the  observed  melt- 
ing point.  While  attempts  have  been  made  to  determine  a  relationship, 
between  the  melting  point  and  composition  of  fireclays,  no  definite  and 
complete  connection  has  been  found.  In  fact,  no  equilibrium  diagram 
can  be  established  for  such  complex  and  heterogeneous  mixture  as  fire- 
clays and  firebricks;  first,  because  of  the  large  number  of  components, 
and,  second,  because  of  the  inhomogeneity  of  the  chemical  constituents. 
For  the  same  reasons  it  is  difficult  to  determine  empiric  relations  between 
the  observed  melting  point  (which  is  not  an  equilibrium  temperature) 
and  the  composition.  Nevertheless,  chemical  analyses  will  often  indi- 
cate the  relative  refractoriness  of  different  materials.  Of  course,  where 
we  have  combinations  of  chemically  pure  oxides  in  which  are  formed 
definite  chemical  compounds,  solid  solutions  or  eutectics,  the  tempera- 
ture versus  composition  or  equilibrium  diagrams  have  been  established 


270  MELTING    POINT    OF   REFRACTORY    MATERIALS 

for  a  number  of  groups  of  oxides,  such  as  for  the  combinations  of  lime, 
alumina,  magnesia,  and  silica.4 

The  addition  of  an  impurity  to  a  refractory  material  usually  lowers 
its  melting  point.  For  instance,  in  fireclay  substances,  the  addition  of 
sodium,  potassium,  iron,  titanium,  calcium,  or  magnesium  compounds 
produces  a  very  marked  depression  of  the  melting  point;  the  addition  of 
silica  to  fireclay  materials  decreases  the  refractoriness  while  the  addition 
of  alumina  increases  it. 

Size  of  Particles  and  Shape  and  Position  of  Body. — It  is  well  known 
that,  within  certain  limits,  the  smaller  the  particles  of  a  refractory  mate- 
rial the  lower  may  be  its  melting  point.  The  softening  of  the  surface 
of  the  particles  takes  place  at  a  lower  temperature  than  the  softening  of 
the  whole  body  en  masse;  in  other  words,  the  particles  sinter  together 
before  the  body  flows.  It  is  readily  apparent  that  the  smaller  the  parti- 
cles or  the  finer  the  texture,  the  greater  is  the  surface  area  exposed  to 
softening.  The  fine  division  of  the  particles  also  allows  a  wider  and 
more  thorough  distribution  of  the  fluxing  agents;  consequently,  the  vitrifi- 
cation will  proceed  more  rapidly,  the  solution  and  reaction  of  the  constitu- 
ents will  be  facilitated,  and  the  material  will  flow  at  a  lower  tempera- 
ture. On  the  contrary,  a  finer  division  of  the  particles  may  produce  a 
wider  and  more  thorough  distribution  of  the  higher  melting-point  con- 
stituents to  the  extent  of  raising  the  melting  point. 

The  total  effect  on  the  melting  point  of  varying  the  size  of  the  parti- 
cles ordinarily  is  not  large.  For  example,  in  the  case  of  a  large  number  of 
samples  of  coal  ash,  those  specimens  ground  "to  an  impalpable  powder 
tended  to  soften  at  a  slightly  lower  temperature  than  ash  that  would 
pass  a  100-mesh  screen.  The  difference  averaged  6°  C.  and  in  no  test 
exceeded  40°  C."5  Experiments  made  in  the  pyrometry  laboratory  at 
the  Bureau  of  Standards  on  the  melting  points  of  silica  foundry  sands  of 
particles  just  passing  a  10-mesh  screen  showed  no  differences  in  melting 
point  larger  than  the  experimental  error  when  the  particles  were  ground 
to  pass  an  80-mesh  screen.  Other  experiments  on  a  fireclay  brick  gave  a 
melting  point  of  1655°  C.  when  ground  to  pass  an  80-mesh  screen  and 
1640°  C.  when  ground  to  pass  a  200-mesh  screen.  The  melting  point  of 
the  unground  brick  was  found  to  be  1630°  C.  In  this  case  it  appears  that 
the  grinding  served  to  modify  the  distribution  .of  the  different  constitu- 
ents in  addition  to  reducing  their  size.  All  of  these  experiments  were 
made  under  the  same  conditions. 

Because  the  melting  of  a  refractory  material  is  accompanied  by  a 
more  or  less  gradual  decrease  in  viscosity,  the  temperature  of  marked  flow 

4R.  B.  Sosman:  The  Common  Refractory  Oxides.     Trans.  Faraday  Soc.  (1916- 
17)  12,  254;  Jnl.  Ind.  •&  Eng.  Chem.  (Nov.,  1916)  985. 

'Fieldner,  Hall,  and  Field:  U.  S.  Bureau  of  Mines  Bull.  129,  114. 


rLEO   I.  DANA  271 

will  be  dependent  on  the  original  geometrical  form  and  position  of  the 
substance.  For  example,  pyrometric  cones  in  the  shape  of  a  tetrahedron 
with  the  axes  at  various  angles  from  the  vertical  will  be  subject  to  different 
bending  moments  while  softening  and  falling  over;  thus  when  bent  over, 
the  degree  of  fluidity  attained  will  not  be  the  same.  If  the  same  sub- 
stance were  in  the  form  of  a  short  cylinder,  it  is  probable  that  one  would 
not  be  able  to  judge  by  the  squatting  of  the  cylinder  the  temperature  at 
which  the  same  degree  of  viscosity  occurs  as  in  the  case  of  the  cone; 
hence  the  melting  point  observed  with  a  cylinder  may  be  different  from 
that  observed  with  a  cone.  However,  experiments  made  at  this  Bureau 
showed  no  difference  between  the  melting  points  of  a  cone  and  cylinder 
of  the  same  height  placed  vertically  and  heated  under  the  same  condi- 
tions. The  cylinder  measured  2.5  cm.  in  height  and  1.2  cm.  in  diameter; 
the  cone  was  in  the  shape  of  a  tetrahedron,  being  2.5  cm.  high  and  having 
8-mm.  sides  for  the  base. 

Time  and  Rate  of  Heating. — The  process  of  vitrification  and  melting 
of  refractories  is  a  matter  of  time  as  well  as  of  temperature.  Obviously, 
the  longer  the  time  during  which  the  substance  is  held  within  its  vitri- 
fication range  the  greater  the  extent  of  sintering;  that  is,  the  softening, 
melting,  solution,  or  reaction  of  the  components.  Thus,  if  a  refractory 
is  held  for  a  long  time  within  its  vitrification  range,  its  fusibility  will  be 
increased;  if  a  refractory  is  kept  for  a  long  time  below  the  vitrification 
range,  the  sintering  will  not  be  appreciable.  The  melting  point  may  be 
increased  by  the  occurrence  of  a  chemical  reaction  that  results  in  the 
formation  of  a  compound  with  a  higher  melting  point  than  either  of  the 
components;  rapid  heating  would,  in  this  case,  arrest  the  formation  of 
such  a  compound.  In  the  case  of  some  materials  prolonged  heating 
brings  about  volatilization  of  the  more  volatile  constituents,  such  as 
alkali  compounds,  with  a  consequent  increase  of  refractoriness.6 

The  observed  melting  point  will  also  vary  markedly  with  the  rate  of 
heating.  In  accordance  with  the  well-known  principle  of  the  increase  in 
the  rate  of  reaction  with  the  rise  in  temperature,  the  speed  of  a  vitrifica- 
tion is  accelerated  by  raising  the  temperature;  consequently,  the  faster 
the  rise  in  temperature,  the  smaller  is  the  total  amount  of  sintering  or 
vitrification.  At  the  same  time,  by  rapid  heating  the  solution  of  the 
components  and  the  formation  of  eutectics  may  be  arrested  considerably ; 
thus  some  of  the  factors  that  can  cause  the  material  to  flow  are  largely 
diminished  in  effectiveness. 

It  takes  a  long  time  for  some  pure  refractory  compounds  to  melt; 
thus  the  melting  temperature  will  vary  with  the  rate  of  heating,  for  the 
extent  of  superheating  while  melting  will  be  different  for  every  rate  of 

6J.  W.  Mellor:  loc.  cit. 


272  MELTING    POINT    OP   REFRACTORY    MATERIALS 

heating.  In  practically  all  instances  of  impure  refractory  mixtures  or 
compounds,  the  melting  range  will  depend  on  the  rate  of  heating  also 
because  of  the  time  effect  in  melting.  It  is  believed  that  the  effect  of  a 
change  of  rate  of  heating  is  more  marked,  the  closer  one  approaches  the 
melting  point. 

As  a  general  rule,  and  within  certain  limits,  the  faster  the  rise  in  tem- 
perature the  higher  is  the  apparent  melting  point.  No  better  illustration 
of  this  can  be  found  than  with  Seger  cones,  where  the  softening  tempera- 
tures can  be  easily  varied  by  50°  C .  or  more  by  changing  the  rate  of  heating. 7 
At  the  Bureau  of  Standards,  no  difference  in  melting  point  was  found  in  the 
case  of  a  firebrick  heated  to  the  melting  point  in  1  hr.  and  one  heated  for 
•5  hr.  In  the  case  of  very  rapid  rates  of  heating,  the  large  temperature 
gradient  in  the  sample  may  play  a  part  in  causing  a  high  value  for  the 
melting  point. 

Nature  of  the  Surroundings. — Several  possible  external  conditions 
affect  the  melting  point.  The  pressure  of  the  atmosphere,  per  se,  will 
have  practically  no  effect;  that  is,  it  would  take  a  pressure  of  many 
atmospheres  to  change  the  melting  point  even  slightly.  In  an  indirect 
manner,  however,  the  melting  point  may  be  changed  considerably  in  a 
vacuum;  namely,  the  more  volatile  and  fusible  components,  such  as  alkali 
and  alkali  earth  compounds,  may  distill  or  sublime,  thus  causing  a  rise 
in  melting  point,  and  vice  versa,  those  substances  that  go  off  at  atmos- 
pheric pressure  may  not  do  so  at  higher  pressures. 

Due  to  chemical  reaction  with  the  gases  in  the  atmosphere  surrounding 
the  refractory,  its  melting  point  can  be  altered  considerably.  In  the 
case  of  some  materials,  coal  ash,  for  example,  the  nature  of  the  atmosphere 
is  the  factor  exercising  the  greatest  influence  on  the  melting  point.8  The 
terms  reducing,  oxidizing,  and  neutral  atmospheres  are  not  sufficiently 
definite  and,  when  considering  the  nature  of  the  atmosphere,  the  gases 
present  should  be  indicated.  For  example,  in  a  reducing  atmosphere 
either  carbon  vapor  and  carbon  monoxide  or  hydrogen  and  water  vapor 
may  predominate;  and  the  effect  of  one  atmosphere  may  be  totally  dif- 
ferent from  that  of  the  other. 

In  a  carbon  and  carbon-monoxide  reducing  atmosphere,  many  refrac- 
tories are  very  strongly  attacked,  the  extent  being  dependent  on  the 
chemical  composition,  the  pressure,  and  the  temperature.  Under  some 
reducing  conditions,  ferric  oxides  in  fireclay  substances  or  other  refrac- 
tories are  reduced  to  the  ferrous  state  and  combine  to  form  low-melting- 
point  silicates,  which  very  materially  increase  the  fusibility.  In  very 
strongly  reducing  carbon  atmospheres,  all  the  iron  oxides  may  be  reduced 


7  R.  B.  Sosman :  The  Physical  Chemistry  of  Seger  Cones.     Trans.   Amer.  Cer. 
Soc.  (1913)  16,  482. 

8  Fieldner,  Hall  and  Field :  loc.  cit.  » 


LEO    I.    DANA  273 

to  metallic  iron,  thus  preventing  reactions  with  the  silicates.  At  high 
temperatures,  silica  and  silicates  are  reduced  by  carbon  forming,  under 
certain  conditions,  various  compounds  of  silicon,  carbon,  and  oxygen. 
On  the  other  hand,  in  oxidizing  atmospheres,  some  substances  may  be 
oxidized,  allowing  or  preventing  them  from  reacting  with  the  refractory 
and  bringing  about  a  change  in  the  melting  point. 

Conditions  in  Use  Affecting  Apparent  Melting  Point. — Substances  com- 
ing in  contact  with  the  refractory,  such  as  molten  metals,  slags,  fluxes,  and 
flue  dust,  often  attack  the  refractory  and  may  lower  its  melting  point  con- 
siderably. Since  the  temperature  at  which  a  refractory  begins  to  flow  is 
related  to  the  degree  of  viscosity  the  material  has  attained,  the  applica- 
tion of  a  load  will  make  the  material  deform  faster  and  at  a  lower  tem- 
perature. On  account  of  the  more  intimate  contact  of  the  particles,  the 
application  of  a  load  will  allow  a  refractory  to  sinter  at  a  lower  temperature 
and  the  continued  application  of  the  force  will  result  in  the  material 
softening  or  melting  at  a  lower  temperature.  It  also  appears  to  be  true 
that  the  larger  the  load  applied,  the  lower  is  the  temperature  at  which  the 
material  will  soften  and  collapse.  For  instance,  the  softening  point  of  a 
fireclay  brick  with  no  load  was  1730°  C.  while  with  a  load  of  50  Ib.  per 
sq.  in.  it  was  1200°  C.9  A  fireclay  with  a  softening  point  of  1650°  C. 
gave  a  softening  point  of  1435°  C.  with  a  load  of  54  Ib.  per  sq.  in.;  and  one 
of  1380°  C.  with  a  load  of  72  Ib.  per  sq.  in.10 

In  general,  so  large  a  number  of  complex  physico-chemical  phenom- 
ena enter  into  the  melting  of  a  refractory  material  that  it  becomes  impos- 
sible to  predict  in  most  cases  in  which  direction  the  melting  point  will 
change  by  changing  the  factors  and  conditions  under  which  the  material 
is  heated. 

PRACTICAL  DETERMINATION  OF  THE  MELTING  POINT 

The  ideal  method  of  determining  the  melting  point  of  a  refractory 
material  would  be  to  observe  it  under  the  actual  conditions  of  use;  in 
most  cases,  however,  this  is  practically  impossible.  What  actually  has 
to  be  done  is  to  compromise  between  duplicating  the  conditions  of  use, 
on  the  one  hand,  and  substituting  feasible  methods  afforded  by  labora- 
tory facilities,  on  the  other;  unfortunately  one  usually  is  compelled  to 
decide  almost  wholly  in  favor  of  the  latter.  To  determine  the  melting 
point  under  conditions  approximating  those  of  use  or  in  such  a  mannep  as 
to  form  definite  and  specific  correlations  between  the  melting  point  and 
other  properties  in  use  very  often  demands,  in  the  case  of  each  material, 
extended  and  elaborate  investigation  (as,  for  example,  the  relation  of  the 

9  Bleininger  and  Brown:  U.  S.  Bureau  of  Standards  Tech.  Paper  7  (1911). 
10  J.  W.  Mellor:  loc.cit. 
18 


274  MELTING   POINT   OP  REFRACTORY   MATERIALS 

melting  point  of  coal  ash  and  the  degree  of  clinkering).  Because  of  the 
great  variation  in  the  properties  and  characteristics  of  refractory  mate- 
rials, these  conditions  of  test  cannot  be  made  the  optimum  for  all 
materials;  accordingly  they  cannot  be  made  as  detailed  and  special  as 
if  we  were  dealing  with  one  type  of  refractory.  As  far  as  possible  they 
should  be  logical,  simple,  and  easily  reproducible  so  that  a  standard  and 
practical  method  for  the  determination  of  the  melting  point  of  refrac- 
tories may  be  established. 

The  fact  that  conditions  in  use,  such  as  load  and  chemical  reaction, 
may  give  an  apparent  melting  point  widely  different  from  that  observed 
in  the  laboratory  is  no  valid  reason  for  determining  the  melting  point 
very  roughly,  as  by  making  Seger  cone  pyrometric  measurements,  and 
by  varying  and  not  specifying  the  size  of  the  particles,  the  time  and  rate 
of  heating,  and  the  chemical  nature  of  the  atmosphere.  Each  of  these 
factors  introduces  a  variable  into  the  value  for  the  observed  melting 
point;  and  if  these  factors  are  not  specified  and  are  varied  from  time  to 
time,  the  observed  melting  point  will  not  have  a  definite  and  reproduci- 
ble meaning.  Since  the  melting-point  test  is  used  as  one  of  a  number  of 
tests  to  determine  whether  refractories  conform  to  specifications,  it  is 
very  essential  that  it  should  have  a  definite  meaning  to  the  extent  that  a 
melting-point  test  on  the  same  material  made  in  various  laboratories 
should  give  practically  the  same  value  and  that  the  value  should  be  re- 
producible in  the  same  laboratory.  In  order  that  this  agreement  be 
possible,  the  factors  and  conditions  of  the  melting-point  test  should  be 
practically  the  same  or,  at  least,  the  existing  factors  and  conditions  should 
not  be  sufficiently  divergent  to  produce  large  disagreements. 

Sampling,  Grinding,  and  Molding. — When  the  material  of  which  the 
melting  point  is  to  be  determined  is  a  fireclay  brick  with  comparatively 
large  pieces  of  grog  held  together  by  fireclay,  one  cannot  procure  a 
representative  sample  by  simply  breaking  off  a  piece  at  random.  In 
the  case  of  a  firebrick  or  any  other  material  in  which  there  is  not  uniformity 
of  texture  and  composition  or  the  particles  are  larger  than  30-mesh,  the 
material  should  be  carefully  sampled.  It  thus  becomes  necessary  to 
grind  it,  which  makes  molding  of  the  material  into  a  cylinder  or  cone  a 
requisite.  With  bricks  of  fine  and  uniform  texture,  however,  a  piece 
may  be  chipped  off  and  shaped  into  a  cone  or  cylinder.  Materials  in 
the  form  of  a  fine  powder  can,  after  mixing,  be  briqueted  directly. 

•As  the  size  of  the  particles  may  affect  the  melting  point,  the  degree 
of  fineness  to  which  the  material  has  been  ground  should  be  specified,  at 
least  approximately.  There  is  the  possibility  of  grinding  the  material 
so  fine  that  any  further  reduction  will  have  no  effect  on  the  melting  point; 
also  of  grinding  the  material  just  sufficiently  to  insure  uniformity  in  dis- 
tribution of  the  components  and  proper  consistency  to  allow  the  form  to 
retain  its  shape  after  briqueting.  From  the  standpoint  of  the  comparison 


LEO   I.    DANA  275 

of  the  melting  points  of  different  refractories,  the  first  is  the  better,  but 
it  involves  the  labor  and  time  of  fine  grinding;  the  second  is  the  more 
feasible  and,  for  a  large  variety  of  materials,  when  a  cylindrical  specimen 
is  used,  grinding  to  pass  an  80-mesh  screen  appears  to  be  suitable.  When 
cones  are  made,  the  material  should  be  ground  to  100-  to  200-mesh  and 
molded  with  a  binder.  A  binder  is  not  necessary  with  the  cylindrical 
form,  because,  with  the  aid  of  a  little  moisture,  the  material  can  very  con- 
veniently be  briqueted  under  pressure.  A  binder  should  not  be  used  if 
it  will  attack  the  refractory.  A  10  per  cent,  solution  of  dextrin  in 
water  is  a  satisfactory  binder.  After  the  sample  is  molded,  it  is  safest  to 
burn  off  the  dextrin  in  an  oxidizing  atmosphere  at  about  600°  C.,  and  then 
determine  the  melting  point.11 

Since  it  appears  that  there  is  no  material  difference  between  the  melt- 
ing point  of  a  cone  2.5  cm.  high  and  having  8  mm.  sides  for  a  base  and  a 
cylinder  1.2  cm.  in  diameter  and  2.5  cm.  high,  either  may  be  used.  As 
shown  above,  the  cylinder  is  the  most  convenient  to  use. 

Precautions  should  be  taken  that  in  no  case  the  melting-point  speci- 
men is  too  large  for  the  rate  of  heating  used,  because  large  temperature 
gradients  may  be  set  up  in  the  specimen.  The  smaller  the  sample,  the 
easier  it  is  to  maintain  fair  temperature  uniformity  in  the  sample  in  a 
laboratory  furnace. 

Time  and  Rate  of  Heating. — In  order  to  approximate  the  usual  condi- 
tions of  use,  the  rate  of  heating  would  have  to  be  slow  and  the  time  pro- 
longed; such  a  course  is  not  desirable  or  convenient  in  the  laboratory. 
The  time  of  heating  should  not  be  so  long  as  to  waste  time  nor  to  make 
it  tedious  to  watch  the  specimen ;  nor  should  the  rate  be  so  fast  as  to  mask 
the  melting  effect  or  to  make  the  melting  interval  too  short  to  afford 
sufficient  time  to  measure  the  temperature  at  the  melting  point. 

The  time  of  heating  from  room  temperature  to  about  1 000° C.  may  usu- 
ally be  very  short,  for  it  is  believed  that  the  rate  during  this  interval  is 
not  of  much  consequence.  For  materials  melting  around  1700°  C.,  a 
total  time  of  heating  of  not  less  than  30  min.  and  up  to  2  hr.,  and  a  rate 
of  heating  (from  about  50°  below  the  melting  point  and  during  melting) 
between  5°  and  10°  C.  per  minute  are  satisfactory.  The  following  table 
represents  a  temperature  versus  time  curve  of  heating  of  a  firebrick,  the 
melting  point  of  which  was  determined  at  the  Bureau,  with  sufficient 
approximation: 

TEMPERATURE  INTERVAL,  DEGREES  C.  MrnirrES 

Room  temperature  to  1000 20 

1000  to  1650 25 

1650  to  1700  (melting  point) '.5 

11  For  description  of  method  of  molding  into  a  cone  see  Hofman:  Trans.  (1894) 
24,  57;  (1895)  26,  10;  or  Fieldner,  Hall,  and  Field:  loc.  tit.,  29. 


276  MELTING   POINT   OF   REFEACTORY   MATERIALS 

It  should  be  emphasized  that  in  stating  the  melting  point  of  a  re- 
fractory material  the  temperature-time  curve  must  be  represented  as 
definitely  as  necessary  to  allow  the  duplication  of  practically  the  same 
value  for  the  observed  melting  point. 

Type  of  Furnace  and  Conditions  Existing  in  Furnace. — The  following 
may  be  said  to  be  essential  characteristics  of  a  furnace  for  determining  the 
melting  point  of  refractories:  It  should  be  capable  of  easily  reaching 
a  temperature  of  1800°  C.,  since  most  refractories  melt  below  1800°C.; 
for  those  materials  melting  over  1800°  C.,  special  procedure  and  technique 
are  usually  required.  The  atmosphere  in  the  furnace  should  not  react 
chemically  with  the  specimen  to  any  appreciable  extent.  Facilities 
should  be  provided  for  making  temperature  measurements  with  an 
optical  pyrometer.  Good  control  of  the  rate  of  heating  should  be  possible . 
The  type  of  furnace  chosen  depends,  to  some  extent,  on  the  original  cost, 
the  cost  of  operation,  and  the  number  of  melting-point  tests  to  be  made. 

The  two  general  types  of  furnaces  in  use  are  the  electric-resistance 
furnaces  and  the  fuel-fired  furnaces.  They  may  be  classified  as  follows: 

Electric  Furnaces. — Some  form  of  carbon  as  resistor;  such  as  graphite 
tube  in  air,  crushed  carbon  or  Kryptol,  carbon  plate  resistor,  graphite 
resistance  vacuum  furnace.  Metal  as  resistor;  such  as  iridium  tube  or 
wire  and  tungsten  or  molybdenum  wire  or  tube. 

Fuel-fired  Furnaces. — Coal,  coke,  or  oil,  and  gas-air  or  gas-air- 
oxygen. 

Graphite  or  carbon-tube  furnaces,  with  or  without  water-cooled 
electrodes,  have  been  constructed  in  many  forms.  When  operated  under 
atmospheric  pressure,  the  tubes  do  not  last  long  on  account  of  oxidation ; 
they  are  somewhat  protected  from  oxidation  by  passing  a  neutral  or  re- 
ducing gas  through  or  around  them.  This  gas  may  serve  to  carry  away 
smoke  so  that  optical  temperature  measurements  may  be  made.  With 
the  graphite  resistor,  the  atmosphere  in  the  furnace  is  strongly  reducing; 
many  refractories  are  greatly  attacked  in  a  carbon  and  carbon-monoxide 
reducing  atmosphere.12 

By  simple  inspection  of  the  melted  sample,  it  is  usually  not  possible 
to  tell  whether  the  reduction  has  reached  appreciable  proportions,  there- 
by introducing  great  uncertainty  into  the  melting-point  determination. 
In  many  instances,  the  surface  of  the  sample  is  attacked  and  a  shell  of 
higher  melting  point  than  the  inner  portion  is  formed.  Thus,  while  the 
inner  material  may  have  been  melted,  no  outward  evidence  of  this  fact 
is  shown.  Whenever  possible,  it  is  much  safer  and  more  desirable  to 
protect  the  sample  from  the  strongly  reducing  atmosphere  (provided  this 
type  of  atmosphere  does  not  exist  in  actual  use  of  the  material)  with  a 

1S  The  gases  in  the  atmosphere  of  a  carbon-resistance  furnace  at  high  temperatures 
are  principally  carbon  monoxide,  nitrogen,  and  carbon  vapor. 


LEO   I.    DANA  277 

refractory  tube  of  low  porosity;  and  a  slight  current  of  air  through  this 
tube  will  serve  to  oxidize  the  reducing  gases  and  drive  off  smoke.  Porce- 
lain tubes  of  Marquardt,  or  those  approximating  the  composition  of  silli- 
manite  Al2O3.SiO2  (melting  point,  1810°  C.)  may  be  used  for  this  purpose 
up  to  temperatures  as  high  as  1800°  C.,  although  they  become  soft  and  do 
not  last  long  at  this  temperature.  Well-sintered  tubes  or  crucibles  of 
alumina  or  magnesia  or  mixtures  of  the  two  may  be  used  if  they  are  thick 
enough.  Unfortunately,  protection  tubes  for  use  at  higher  temperatures 
are  not  readily  obtainable,  although  they  undoubtedly  could  be  made. 

Crushed-carbon,  graphite,  or  kryptol,  and  carbon-plate  resistor 
furnaces,  in  addition  to  the  above  difficulties,  do  not  allow  very  accurate 
regulation.  Also,  some  forms  make  it  difficult  to  take  optical  tempera- 
ture measurements.  While  a  graphite-resistance  vacuum  furnace  pre- 
sents the  difficulties  of  greater  initial  cost,  of  producing  a  vacuum,  and 
of  opening  and  closing  large  vacuum-tight  joints,  the  atmosphere  is 
kept  free  from  smoke  and  good  temperature  uniformity  can  be  main- 
tained, thus  facilitating  optical  temperature  measurements.  Higher 
temperatures  can  be  more  conveniently  reached  in  the  vacuum  furnace. 

In  order  that  the  carbon  atmosphere  in  the  vacuum  furnace  may  have 
no  appreciable  chemical  effect  on  the  specimen,  it  is  usually  necessary  to 
keep  the  pressure  below  1  or  2  mm.  Even  then  it  is  safer  to  pro- 
tect the  sample  with  a  refractory  porcelain  tube  closed  at  one  end  and 
suspended  from  the  cool  part  of  the  furnace.  This  prevents  the  carbon 
vapor  or  particles  shooting  off  from  the  resistor  from  gaining  access  to 
the  specimen.  Practically  all  the  gases  in  the  tube  must  come  from  the 
cool  part  of  the  furnace;  and  these  gases  consist  of  nitrogen  and  carbon 
monoxide  in  low  concentration.  It  is  also  possible  that  the  tube  largely 
diminishes  the  convention  of  the  reducing  gases  past  the  specimen  and 
consequently  the  reducing  action  is  not  so  great  as  without  the  tube.  To 
prevent  the  sample  or  its  container  from  sticking  to  the  tube,  they  may  be 
separated  by  a  layer  of  powdered  alundum. 

For  a  laboratory  in  which  a  large  number  of  melting-point  determina- 
tions are  made,  a  graphite-resistance  vacuum  furnace  of  the  Arsem  type 
is  the  most  desirable  electric  furnace ;  the  lasting  operation  of  the  resistor, 
the  clarity  of  the  atmosphere,  the  rapidity  of  action  and  excellence  of 
control  and  the  good  black-body  conditions  afford,  with  suitable  protec- 
tion for  the  sample,  the  most  convenient  and  precise  method  of  melting 
refractory  materials  in  an  electric  furnace,13  see  Fig.  1. 

Several  furnaces  with  metals  as  the  resistor  and  capable  of  attaining 
temperatures  of  1800° C.  or  higher  have  been  constructed.  Iridiumin  the 
form  of  tubes  or  wire  has  been  used  by  a  number  of  investigators;  but 

13  For  description  of  use  of  Arsem  furnace  in  melting  .refractories  see  C.  W.  Kanolt: 
U.  S.  Bureau  of  Standards  Tech.  Paper  10  and  Sci.  Paper  212. 


278 


MELTING   POINT    OF   REFRACTORY   MATERIALS 


this  metal  is  so  expensive  and  volatilizes  so  readily  that  it  is  ordinarily 
outside  the  range  of  metals  accessible  for  a  practical  furnace.     Tungsten 


Fio.  1. — CROSS-SECTION  OF  ARSEM  GRAPHITE  RESISTANCE  VACUUM  FURNACE.     A  is 

COMPRESSED  CYLINDRICAL  SAMPLE.       D  IS  SILLIMANITE  PROTECTION  TUBE. 

and  molybdenum  have  been  used  considerably  in  the  form  of  wire  wound 
on  refractory  tubes.     For  lasting  operation,  it  is  requisite  that  the  wire 


LEO    I.    DANA  279 

be  protected  from  oxidation  by  surrounding  it  with  a  reducing  atmosphere, 
preferably  with  hydrogen.  Obstacles  in  the  way  of  general  adoption  of 
these  furnaces  are  the  danger  of  hydrogen  explosions  and  the  difficulty 
of  obtaining  hydrogen  in  sufficient  quantity.  Even  in  the  reducing 
atmospheres,  the  resistors  do  not  have  a  long  life,  as  they  become 
brittle. 

Furnaces  with  tubes  of  tungsten,  molybdenum,  or  similar  high  melting 
point  metals  are  still  in  the  experimental  stage.  They  should  be  run  in 
a  vacuum  or  hydrogen  and,  apart  from  the  absence  of  a  carbon  or  carbon 
monoxide  reducing  atmosphere,  they  are  not  so  advantageous  as  graphite 
resistance  vacuum  furnaces.  Coal,  coke,  or  oil-fired  furnaces  have  been 
used  for  melting  refractories,  but  they  are  unsuitable  for  laboratory  fur- 
naces in  which  small  masses  are  placed  and  for  which  accurate  control  is 
an  essential.  They  are  too  large  and  are  inconvenient  on  account  of 
"dirt,  smoke,  and  possibly  a  strongly  reducing  atmosphere;  besides,  it  is 
not  easy  to  obtain  the  requisite  high  temperature. 

Gas  furnaces  offer,  besides  the  electric  furnace,  the  only  other  prac- 
tical means  of  attaining  high  temperatures.  In  the  case  of  furnaces 
using  illuminating  gas  made  from  coal  and  air  at  2  or  3  Ib.  (0.9  or  1.4  kg.) 
pressure,  the  maximum  temperature  reached  in  an  ordinary  furnace  is 
about  1400°  C.;  when  burning  natural  gas,  temperatures  100°  or  200° 
higher  may  be  attained.  With  air  pressures  at  10  Ib.  (4.5  kg.)  and  over 
and  illuminating  gas,  it  is  possible  to  obtain  temperatures  as  high  as 
1650°  C.;  and  in  the  case  of  natural  gas,  as  high  as  1800°  C.  By  recuper- 
ating the  waste  heat  or  by  preheating  the  gases,  these  temperatures  could 
be  raised  considerably;  also,  by  the  addition  of  oxygen,  temperatures  over 
2000°  C.  have  been  maintained.  These  statements  are  very  general  and 
may  not  hold  in  the  case  of  a  number  of  furnaces;  they  are  not  intended  as 
a  complete  statement  of  the  problem  of  attaining  high  temperatures  in 
gas  furnaces. 

It  is  quite  a  different  matter  to  obtain  a  temperature,  say,  of  1750°  C. 
uniformly  over  a  space  of  several  cubic  inches  than  it  is  to  reach  this 
temperature  in  a  small  spot.  For  melting  refractories  one  should  be  able 
to  obtain  a  uniform,  high  temperature  over  a  sufficient  volume  to  procure 
reliable  results.  In  order  to  do  this,  it  is  usually  necessary  to  enclose  the 
specimen  in  a  refractory  crucible  or  muffle.  At  present,  one  of  the  diffi- 
culties is  that  of  refractories  for  the  lining  and  crucibles  to  contain  the 
specimen — a  problem  that  is,  perhaps,  more  difficult  of  solution  for  gas  fur- 
naces than  for  electric  furnaces. 

The  Bureau  of  Standards  has  been  working  on  the  problem  of  design- 
ing a  gas  furnace  suitable  for  the  determination  of  the  melting  point  of 
refractories  but  has  not  yet  perfected  it.  Gas  furnaces  can  be  made  in 
convenient  laboratory  form  and,  on  account  of  their  simplicity,  low  initial 
cost  and  cost  of  operation,  and  the  comparative  ease  with  which  high 


280  MELTING    POINT    OF   REFRACTORY    MATERIALS 

temperatures  are  obtained,  probably  will  offer,  after  considerable  develop- 
ment, the  best  solution  of  the  problem. 

The  question  of  the  atmosphere  in  the  furnace  is  intimately  bound  up 
with  the  type  of  furnace  used.  Probably  most  refractories  used  in  indus- 
try are  heated  in  a  reducing  atmosphere.  But  it  is  not  possible  to  repro- 
duce exactly  these  conditions  in  the  laboratory.  Carbon-resistance 
furnaces  are  very  liable  to  have  a  strongly  reducing  atmosphere;  means  for 
avoiding  this  have  been  described  before.  Gas  furnaces,  on  the  other 
hand,  which  are  intended  for  use  at  the  higher  temperatures  usually 
have  oxidizing  atmospheres.  Since  a  refractory  would  ordinarily  have 
its  lowest  melting  point  in  a  slightly  reducing  atmosphere,  such  a  one  may 
be  preferable.  At  the  present  stage  of  development  of  furnaces,  how- 
ever, the  question  of  the  best  type  of  atmosphere  must  be  left  unsettled. 

Temperature  Measurements  and  Observation  of  Melting.- — For  a  very 
rough  measurement  of  the  melting  temperature  of  a  refractory,  the  tem- 
perature as  indicated  by  the  melting  of  a  material  the  melting  point  of 
which  is  known — by  a  pyrometric  cone — has  often  been  taken.  The  re- 
fractory is  then  said  to  have  a  softening  temperature  corresponding  to  a 
certain  cone  number.  The  reason  given  in  justification  of  this  process 
is  that  cones  are  used  in  furnaces,  or  kilns,  in  which  ceramic  products 
are  fired  to  measure  temperatures,  or  at  least  heat  effects.  But  the  cones 
are  used  in  the  ceramic  industry  under  quite  different  conditions  from 
those  when  softening  points  are  determined.  First,  the  rate  of  heat- 
ing of  the  cone  is  much  slower  in  a  kiln  than  in  a  laboratory  furnace. 
Second,  the  cone  is  heated  in  the  kiln  over  a  short  range  of  tempera- 
ture. The  cone  serves  in  such  a  case  as  a  sort  of  integrator  of  the  time 
versus  temperature  curve  of  heating.  When  a  small  range  of  cones  is 
hfeated  under  the  same  circumstances  the  practical  ceramist  can  draw 
conclusions  from  the  deformation  of  those  cones,  which  serve  to  indicate 
the  proper  degree  of  firing  of  his  products. 

But  in  the  laboratory,  where  a  rather  large  range  of  cones  is  used 
and  where  the  rate  of  heating  is  comparatively  rapid  and  may  vary 
from  time  to  time,  the  determination  of  the  cone  softening  point  is  not 
of  much  significance  and  serves  no  useful  purpose  in  practice.  What 
should  be  done  is  to  determine  the  point  at  which  the  refractory  softens 
on  a  uniform  and  reproducible  temperature  scale  under  properly  specified 
conditions.  This,  we  believe,  reduces  the  number  of  variables  in  the 
determination  to  a  minimum  and  affords  more  reliable  results. 

It  has  repeatedly  been  shown  and  emphasized  that  the  softening 
point  of  Seger  cones  depends  on  a  number  of  factors  and  conditions  simi- 
lar to  those  affecting  the  melting  of  other  refractories.  That  is  to  say, 
the  softening  point  depends,  to  some  extent,  on  the  nature  of  the  atmos- 
phere but,  most  important  of  all,  on  the  tune  and  rate  of  heating.  Usu- 
ally the  more  rapid  the  heating,  the  higher  is  the  softening  temperature; 


LEO   I.    DANA  281 

50°  to  75°  C.  difference  in  softening  temperature  easily  results  from  vary- 
ing the  rate  of  heating.  Cones  do  not  even  measure  heat  effects  with 
any  semblance  of  precision  when  employed  under  different  conditions; 
it  is  possible  to  subject  a  cone  to  different  conditions  of  heating  and  still 
produce  the  same  amount  of  deformation.  As  far  as  an  approximate 
measure  of  a  reproducible  temperature  or  heat  effect  is  concerned,  the 
indication  of  a  Seger  cone  is  unreliable  even  for  commercial  precision, 
unless  the  nature  of  the  atmosphere  in  which  the  cone  is  heated  and  the 
temperature  versus  time  curve  of  heating  are  specified.  Even  though 
these  conditions  are  specified,  there  is  no  way  of  accurately  deducing  a 
common  basis  of  comparison  of  temperature  or  heat  effect  when  the  cones 
are  used  under  different  conditions. 

To  measure  the  temperature  of  melting  with  sufficient  accuracy,  it  is 
necessary  to  use  some  form  of  optical  or  radiation  pyrometer.  With 
relatively  slow  heating,  it  is  possible  to  use  a  Wanner  or  a  Fe"ry  optical 
pyrometer  by  alternately  watching  the  specimen  melt  and  sighting 
through  the  pyrometer  to  measure  the  temperature.  A  more  rapid  and 
precise  instrument,  however,  and  one  which  permits  the  observation  of 
the  specimen  simultaneously  with  the  measurement  of  its  temperature, 
is  the  Holborn-Kurlbaum  type  of  Morse  optical  pyrometer.14  In  deter- 
mining the  temperature  with  any  form  of  optical  pyrometer,  it  is  essen- 
tial that  sufficiently  good  black-body  conditions  exist  in  the  furnace  as  it 
is  impossible  to  apply  any  reliable  emissivity  correction.  With  electric 
furnaces  and  with  gas  furnaces  containing  muffles  or  crucibles  in  which 
the  specimen  is  heated,  it  is  not  difficult  to  obtain  good  black-body  con- 
ditions; even  a  slight  departure  from  a  perfect  black  body  will  enable 
one  to  discern  the  specimen.  To  see  the  specimen  with  sufficient  clarity, 
it  is  usually  necessary  to  keep  the  eye  constantly  fixed  on  it  and  its 
surroundings. 

In  sighting  on  a  melting  specimen,  it  is  sometimes  important  to  sight 
on  a  surface  that  is  rather  oblique  to  the  line  of  vision,  for  the  surfaces 
that  are  more  or  less  perpendicular  to  the  line  of  vision  may  appear 
darkened  in  the  field  because  they  do  not  reflect  any  light  from  the  sur- 
roundings into  the  pyrometer  and  because  their  emissivity  is  low.  In 
this  connection,  one  should  guard  against  the  condition  of  sighting  on  a 
surface  reflecting  light  from  a  hotter  spot  for  in  such  a  case  the  measured 
temperature  would  be  higher  than  the  true  temperature.  The  atmos- 
phere between  the  pyrometer  and  the  sample  must  be  perfectly  clear, 
that  is,  free  from  smoke  or  flames,  because  the  latter  act  as  an  absorption 
screen,  causing  errors  of  an  uncertain  magnitude. 

As  a  rule  the  flow  of  the  sample  can  be  observed  a  great  deal  more  dis- 

14  This  pyrometer  is  manufactured  in  this  country  by  The  Leeds  &  Northrup  Co., 
Philadelphia  Pa. 


282  MELTING   POINT    OF   REFRACTORY   MATERIALS 

tinctly  when  sighted  on  sidewise  rather  than  from  above;  that  is,  in  the 
former  case  the  change  in  linear  dimensions  and  in  position  appears 
greater  to  the  eye.  In  the  Arsem  vacuum  furnace  the  sample  is  viewed 
from  above,  consequently  the  point  at  which  a  marked  flow  of  the  sample 
begins  is  rather  difficult  to  observe,  being  subject  to  the  personal  equa- 
tion of  the  observer.  We  see,  then,  that  to  measure  the  temperature  of 
melting  of  a  refractory  material  it  is  advisable  to  use  a  Holborn-Kurl- 
baum  type  of  the  Morse  optical  pyrometer;  to  be  sure  that  sufficiently 
good  black-body  conditions  are  maintained;  to  sight  on  the  proper  part 
Of  the  specimen;  and  finally,  to  be  sure  that  the  absorption  of  the  atmos- 
phere is  negligible. 

DISCUSSION 

J.  S.  UNGER,*  Pittsburgh,  Pa.  (written  discussionf). — Firebricks 
intended  for  the  same  purpose,  but  supplied  by  different  manufacturers, 
may  be  of  entirely  different  clays,  contain  different  proportions  of  flint, 
calcined  and  plastic  clays;  the  particles  may  vary  widely  in  size;  the  water 
used  to  make  the  brick  will  vary;  the  pressure  in  molding  will  vary,  depend- 
ing on  whether  the  brick  is  hand  made  or  power  pressed;  and  the  degree 
of  burning  in  the  same  kiln  will  differ.  These  variables  affect  the  melting 
point  and  the  strength  of  the  brick  when  heated. 

An  important  property  of  a  firebrick  is  its  ability  to  resist  heat  and, 
at  the  same  time,  weight  or  load  without  serious  deformation.  The  soft' 
ening  point  and  the  melting  point  of  a  firebrick  may  be  several  hundred 
degrees  apart,  and  two  bricks  may  show  considerable  difference  in  their 
softening  points,  but  the  melting  points  may  be  approximately  the  same. 
Bricks  are  not  usually  employed  at  temperatures  close  to  their  melting 
points.  If  they  must  withstand  very  high  temperatures,  the  firebrick 
is  discarded  and  a  brick  of  more  refractory  material  is  used.  Under  these 
conditions  it  is  doubtful  whether  the  determination  of  the  melting  point 
of  a  brick  has  much  practical  value. 

If  the  determination  of  the  melting  point  is  necessary,  the  test  should 
be  made  on  a  portion  of  the  original  brick  and  not  on  a  specially  prepared 
sample.  A  small  triangular  pyramid  with  a  base  about  1%  in.  and  3  in. 
high  can  be  sawed  from  the  corner  of  the  brick  with  a  thin-bladed  car- 
borundum wheel,  without  injuring  or  destroying  the  size  of  particles, 
the  bond,  or  degree  of  burning  of  the  original  brick.  This  specimen  can 
then  be  compared  with  standard  Seger  cones  or  tested  by  any  other 
method  desired. 

P.  D.  FOOTE,  Washington,  D.  C. — Probably  more  "melting-point" 
tests  of  firebrick  have  been  made  at  the  Bureau  than  all  other  high-tem- 
perature tests  combined  and  I  feel  sure  that  the  consensus  of  opinion 


*  Manager.  Central  Research  Bureau.  t  Received  Sept.  25,  1919. 


DISCUSSION  283 

among  manufacturers  is  that  the  melting-point  determination  furnishes 
some  desirable  check.  Where  possible,  we  cut  pyramids  and  cones  from 
firebrick,  but  frequently  the  bricks  are  of  such  coarse  texture  that  this 
method  would  not  give  a  fair  sample.  In  such  cases  we  make  the  little 
cones  from  the  ground  material,  realizing,  of  course,  that  the  method 
influences  the  results  obtained.  Obviously  the  melting-point  determina- 
tions are  not  failure  tests.  The  Bureau  of  Standards  makes  failure  tests 
but  these  were  not  considered  in  this  paper.  Mr.  Dana  has  carefully  de- 
fined what  is  meant  by  "melting  point"  and  on  the  basis  of  this  definition 
the  paper  is  consistent  and  the  data  obtained  are  reliable  and  conclusive. 

R.  C.  PURDY,  Worcester,  Mass.— We  should  keep  in  mind  that 
the  fusion  of  clay  is  only  a  progressive  proposition.  When  the  bricks 
are  burned,  that  is,  formed  and  burned  in  the  kilns,  a  certain  amount  of 
fusion  has  taken  place,  permitting  the  bricks  to  attain  a  mechanical 
strength.  The  melting  point,  erroneously  so  called,  is  only  the  carrying 
of  fusion  to  the  stage  where  the  cone  will  not  sustain  its  own  shape,  that 
is,  without  a  load.  The  load  test  or  the  softening  point,  as  referred  to 
erroneously  again,  if  you  will,  is  a  measure  of  the  degree  to  which  the 
material  has  been  softened  by  fusion  sufficient  to  be  unable  to  sustain 
the  standard  load.  Both  of  them  are  measuring  the  same  thing  except 
that  the  softening  test  is  a  measure  of  resistance  to  pressure  of  the  soft- 
ened mass.  At  the  temperature  of  the  load  test,  the  clay  is  more  or  less 
molten.  The  melting-point  test  or,  more  properly  speaking,  the  fusion 
test,  gives  the  temperature  or  heat  treatment  required  to  so  soften  the 
clay  that  it  will  deform  or  flow  under  pressure  of  gravity. 

A  protection  shield  made  of  fused  bauxite  will  have  a  great  deal  of 
strength.  A  piece  12  in.  by  1  in.  by  ^  in.  suspended  between  knife-edge 
supports  10  in.  apart  will  not  be  deflected  by  a  3-lb.  load  applied  midway 
between  supports  and  heated  for  several  hours  at  1450°  C. 

The  same  thought  can  be  carried  to  firebrick.  A  firebrick  made  of 
fused  bauxite  will  sustain  loads  well  up  to  the  fusion  point  of  the  crys- 
tallized alumina  whereas  bricks  made  of  fireclay  will  be  deformed  under 
load  at  temperatures  and  heat  treatments  much  lower  than  is  required 
when  no  load  is  applied. 

There  is  no  such  thing  as  a  melting  point  of  clay;  it  is  a  progressive 
fusion,  and  to  speak  of  melting  point  or  softening  point  is  not  accurate. 

LEO  I.  DANA  (author's  reply  to  discussion*). — I  believe  the  deter- 
mination of  the  melting  point  of  a  firebrick  is  of  great  practical  value. 
It  serves  to  show  the  upper  limit  of  temperature  beyond  which  the 
brick  must  not  be  heated  under  any  conditions  of  use,  under  no  load  in- 
cluded. Of  course  the  conclusions  to  be  drawn  from  this  test  should  be 
correlated  with  the  results  of  a  number  of  other  tests  made  on  the  re- 

*  Received  Jan.  17,  1920. 


284  MELTING    POINT    OF   REFRACTORY   MATERIALS 

fractory,  such  as  the  softening  point  under  load,  the  chemical  com- 
position, etc.  At  any  rate,  the  melting-point  test  has  been  included 
as  a  desirable  test  by  the  American  Society  for  Testing  Materials. 

In  order  that  this  test  may  have  the  same  meaning  and  give  the 
same  results  in  various  laboratories,  it  is  necessary  that  it  be  standardized 
in  such  a  way  that  the  least  number  of  variable  factors  possible  enter 
into  the  test.  This  I  have  attempted  to  do;  and  the  reasons  for  using  a 
specially  prepared  sample,  and  for  not  employing  Seger  cones,  are  fully 
discussed  in  the  paper. 

The  fact  that  the  ordinary  refractory  materials  melt,  soften,  or 
fuse  over  a  range  of  temperature  and  time  has  been  pointed  out  in  the 
paper.  In  attempting  to  fix  this  phenomenon,  the  upper  and  lower 
limits  of  the  range  or  some  one  particular  point  in  it  must  be  defined. 
Because  the  phenomenon  is  one  of  plastic  law,  a  certain  degree  of  in- 
definiteness  is  inherently  attached  to  the  upper,  lower,  or  any  other 
point  of  the  melting  range.  As  a  result  of  melting-point  determina- 
tions of  a  large  number  of  various  refractory  materials  in  the  Bureau  of 
Standards'  laboratories,  it  appeared  that  the  simplest  and;  most  exact 
method  of  fixing  the  phenomenon  so  as  to  be  reproducible  was  to  define 
it  as  the  temperature  at  which  a  marked  flow  of  the  sample  began,  other 
controlling  conditions,  such  as  time  and  rate  of  heating,  being  specifically 
stated.  In  the  light  of  this  definition,  the  melting,  softening,  and  fusing 
points  are  synonymous.  The  term  "melting  point"  was  chosen,  be- 
cause it  is  the  most  general  phrase  describing  the  phenomenon. 


HIGH-TEMPERATURE    SCALE   AND    ITS   APPLICATION  285 


High -temperature   Scale  and   its  Application  in  the  Measurement  of 
True,  Brightness,  and  Color  Temperatures 

BY     EDWARD    P.    HYDE,*    CLEVELAND,    OHIO 
(Chicago  Meeting,  September,  1919) 

AT  the  basis  of  optical  pyrometry  lie  the  theoretical  and  experi- 
mental data  of  the  so-called  black  body.  The  black  body  is  essentially 
a  theoretical  conception,  with  certain  simple  properties  attributed  to  it 
— fundamentally,  the  property  that  its  radiation  at  any  temperature 
is  a  unique  function  of  the  temperature  and  does  not  involve  any  other 
variables.  As  a  theoretical  conception,  an  equation  has  been  developed 
by  Boltzmann  connecting  the  total  radiant  power  with  the  temperature, 
and  another  equation  by  Planck  connecting  the  distribution  of  radiant 
power  throughout  the  spectrum  with  the  temperature.  The  integral 
of  the  latter  with  respect  to  wave-length  conduces  to  the  former  and  per- 
mits the  evaluation  of  the  constant  c\  of  the  Planck  equation,  as  given  in 
the  form 

^ClX-'-.--  (1) 

e^-l 

assuming  that  the  constant  a  of  the  Stefan-Boltzmann  law 

E  =  a  (T74  -  T*o) 

is  known.  The  other  constant  cz  is  arrived  at  by  other  methods,  which 
will  be  discussed  later.  This  equation  for  the  black  body  lies  at  the 
foundation  of  optical  pyrometry. 

To  make  the  discussion  complete,  it  should  be  stated  that  the  theo- 
retical black  body  is  quite  closely  realized  by  a  hollow  enclosure  with  walls 
at  a  uniform  temperature  and  radiating  through  a  small  orifice.  The  elec- 
trically heated  black  body,  first  introduced  by  Lummer  and  Kurlbaum, 
is  an  essential  part  of  the  equipment  of  a  pyrometric  research  or  testing 
laboratory.  Numerous  investigations  have  shown  that  its  radiation 
conforms  to  that  of  the  theoretical  black  body  within  experimental  errors. 
Hence  by  operating  the  electrically  heated  black  body  at  the  melting 
point  of  some  pure  metal  for  which  the  temperature  of  melting  has  been 
determined  on  the  gas  scale,  a  standard  is  obtained  for  comparison  with 
the  radiation  from  other  substances;  and  starting  with  this,  other  high 
temperatures  of  the  black  body  may  be  obtained  by  the  use  of  Planck's 
equation. 

*  Director,  Nela  Research  Laboratory. 


286  HIGH-TEMPERATURE   SCALE   AND   ITS  APPLICATION 

To  establish  a  correct  high-temperature  scale  it  is  necessary  to  adopt 
the  melting  point  of  some  convenient  metal,  to  determine  with  accuracy 
the  value  of  the  constant  c2,  and  to  select  some  method,  free  from  error, 
by  which  a  monochromatic  radiation  may  be  secured,  if  the  temperature 
scale  is  to  be  extended  in  the  usual  way. 

Before  discussing  these  various  points  it  is  well  to  note  that  the  Planck 
equation  may  be  reduced,  for  all  ordinary  temperatures  and  for  wave- 
lengths within  the  visible  spectrum,  to  the  simpler  form  of  the  Wien 
equation 

J  =  d  X~5  e~?T  (2) 

without  introducing  any  appreciable  error.  The  equation  in  this  form 
is  used  in  most  practical  work;  and  for  temperatures  not  in  excess  of 
3000°  K.  and  for  wave-lengths  not  in  excess  of  0.7/x,  it  is  correct  to  within 
an  error  of  0.1  per  cent. 

It  is  necessary,  in  practice,  first  to  connect  the  experimental  black- 
body  scale  with  the  gas  scale  by  the  adoption  of  the  melting  points  of 
one  or  more  pure  metals,  the  temperatures  of  melting  of  which  have 
already  been  determined.  There  are  a  number  of  these  to  select  from 
as  a  result  of  the  classical  work  of  Day  and  Sosman,1  who  carried  the  gas 
scale  up  to  the  melting  point  of  palladium,  including  in  their  measure- 
ments the  melting  points  of  gold,  copper,  tin,  etc.  Of  these  various 
metals,  gold  and  palladium  have  probably  been  used  most  frequently. 
Gold  is  given  by  them  as  melting  at  1335.6°  K.  (273°  +  1062.6°  C.)  and 
palladium  at  1822.5°  K.,  and  until  recently  both  of  these  melting  points 
have  frequently  been  adopted  as  two  points  on  the  high-temperature 
scale.  Within  the  last  few  years,  however,  it  has  been  found,  as  the 
result  of  researches  at  Nela  Research  Laboratory  and  at  the  Reichsan- 
stalt,  that  the  two  values  are  not  quite  consistent  if  the  most  probable 
value  of  c2  is  taken  and  if  precautions  as  to  monochromatic  radiation  are 
followed.  It  has  therefore  been  proposed  in  this  country  that  the  melting 
point  of  gold,  as  given  by  Day  and  Sosman,  be  adopted  as  the  standard 
temperature,  since  this  is  several  hundred  degrees  lower  than  that  of 
palladium,  and  hence  quite  probably  more  nearly  correct.  On  this  pro- 
posed scale,  the  melting  point  of  palladium  is  found  to  be  1828°  K.,  and  the 
radiation  from  the  black  body  at  the  melting  point  of  palladium  with 
this  new  value  is  frequently  used  as  a  secondary  standard  in  pyrometrical 
calibrations  and  measurements. 

The  constant  c2  has  been  assigned  various  values  as  investigation  has 
proceeded,  starting  with  the  value  c2  =  14,500/i  deg.  as  found  in  the 
early  work  of  Lummer  and  Pringsheim.  As  the  result  of  a  number  of 
recent  investigations,  including  the  extended  work  of  Coblentz  and  the 

1  Am.  JnL.  Sci.  [4]  (1910)  29,  93. 


EDWARD   P.    HYDE  287 

careful  evaluation  by  Millikan  from  data  obtained  on  the  photoelectric 
effect  and  interpreted  in  terms  of  the  quantum  theory,  the  most  probable 
value  of  c2,  in  the  light  of  all  data  available,  is  taken  as  14,350/i  deg. 
Recently  the  laboratories  of  the  General  Electric  Co.2 adopted,  as  the  most 
probable  high-temperature  scale,  that  based  on  the  melting  point  of 
gold  as  1336°  K.  and  the  value  c2  =  14,350/i  deg. 

As  stated,  the  only  other  obstacle  in  the  way  of  experimentally  ex- 
tending the  optical  scale  from  the  accepted  gold  point  lay  in  the  difficulty 
of  properly  securing  monochromatic  radiation,  or,  what  amounts  to  the 
same  thing,  of  determining  the  effective  wave-length  of  transmission  of 
the  so-called  monochromatic  red  glass  filters  commonly  used  in  the  eye- 
pieces of  optical  pyrometers.  Since  this  question  will  be  discussed  in 
another  paper  to  be  presented  at  this  symposium,  it  need  only  be  men- 
tioned here. 

TRUE  AND  BLACK-BODY  TEMPERATURES 

With  a  properly  calibrated  optical  pyrometer,  it  is  a  simple  matter 
to  measure  directly  the  true  temperature  of  any  radiator  that  has  the 
properties  of  a  black  body.  But  it  is  well  known  that  no  material  sub- 
stance possesses  the  radiating  properties  of  a  black  body.  At  any  given 
temperature  not  only  is  the  radiant  flux3  less  than  that  of  a  black  body 
at  the  same  temperature  because  its  emissive  power4  is  less  than  unity, 
but  also  the  distribution  of  radiant  flux  in  the  spectrum  is,  in  general, 
different  from  that  of  the  black  body  at  the  same  temperature  owing 
to  the  non-uniformity  of  the  emissive  power  of  the  substance  throughout 
the  spectrum.  For  platinum  and  tungsten,  and  possibly  for  one  or  two 
other  substances,  the  true  temperatures  have  been  determined  and  the 
characteristics  just  described  have  been  verified.  Particularly  in  the 

*Gen.  Eke.  Rev.  (1917)  20,  811. 

3  Radiant  flux  has  been  defined,  in  the  1918  report  of  the  Committee  on  Nomen- 
clature and  Standards  of  the  Illuminating  Engineering  Society,  as  "the  rate  of  flow 
of  radiation  evaluated  with  reference  to  energy  and    .  .  .  expressed  in  ergs  per  second 
or  in  watts."     In  the  present  paper,  it  is  also  used  to  express  the  rate  of  flow  of  radia- 
tion per  unit  wave-length,  as  well  as  the  integral  rate  of  flow  for  the  entire  spectrum. 
The  context  will  show,  in  each  case  in  which  the  expression  is  used,  which  significance 
is  intended. 

4  By  the  emissive  power  of  any  radiating  body  at  a  given  wave-length  and  for  a 
given  temperature  is  meant  the  ratio  of  the  radiant  flux  per  unit  area  emitted  by  the 
body  at  the  given  wave-length  and  for  the  given  temperature  to  that  emitted  by  a 
black   body  at   the  same  wave-length  and  for  the  same  temperature.     The  term 
emissivity  has  been  used  by  some  authors  to  express  this  same  quantity,  but  others 
prefer  to  use  this  term  to  signify  the  radiant  flux  emitted  per  unit  area,  measured  in 
absolute  units.     According  to  these  authors  the  term  relative  emissivity  would  replace 
the  term  emissive  power  employed  in  the  present  paper.     It  is  to  be  understood  that 
unless  some  spectral  limitation  is  imposed  the  emissive  power  refers  to  the  total  radiant 
flux  for  the  complete  spectrum. 


288  HIGH-TEMPERATURE   SCALE   AND   ITS   APPLICATION 

case  of  tungsten  elaborate  studies  of  its  radiating  properties  and  an  accu- 
rate determination  of  its  true  temperatures  have  been  made. 

For  practically  all  substances  other  than  the  two  or  three  mentioned, 
it  is  necessary  to  content  oneself  with  apparent  temperatures.  It  is 
for  this  reason  that  the  term  black-body  temperature  has  come  into  use; 
but  whereas  it  has  been  employed  in  a  unique  way,  there  are  now  recog- 
nized at  least  two  essentially  different  kinds  of  apparent  black-body 
temperature  in  optical  pyrometry,  and  still  a  third  kind  if  total  radiation 
pyrometry  is  included.  Hence  it  becomes  necessary  to  be  more  explicit 
and  to  adopt  a  nomenclature  that  will  adequately  discriminate  between 
them. 

As  formerly  used,  the  term  black-body  temperature  of  a  hot,  radiating 
substance  signified  the  temperature  of  a  black  body  at  which  its  radiant 
flux  at  a  given  wave-length,  usually  in  the  neighborhood  of  0.65/z,  was 
the  same  as  that  of  the  radiating  substance  the  temperature  of  which  was 
desired.  This  determined  temperature  was  not  that  of  the  substance, 
because  the  emissive  properties  of  the  latter  are  known  to  differ  by  an 
unknown  amount  from  those  of  a  black  body.  But  by  determining  the 
temperature  of  the  black  body  at  which  it  would  have  the  same  brightness 
in  some  specified  spectral  region  as  that  of  the  substance  under  investiga- 
tion, an  empirical  scale  of  apparent  temperature  would  be  established 
for  the  hot  substance  and  so  furnish  a  means  of  reproducing  any  given 
hot  condition  and  of  comparing  the  properties  of  the  radiating  matter  al 
various  conditions  of  apparent  temperature. 

Since  recently  there  has  come  into  fairly  common  use  in  optical 
pyrometry  a  second  kind  of  apparent  black-body  temperature,  viz., 
black-body  color  temperature,  to  which  reference  will  be  made  shortly, 
it  is  necessary  to  further  qualify  the  general  term  black-body  temperature 
in  its  older  significance.  This  quantity,  in  its  use  in  Nela  Research 
Laboratory,  has  come  to  be  designated  as  black-body  brightness  tempera- 
ture, or,  in  brief,  brightness  temperature,  and  the  author  suggests  here 
the  acceptance  of  this  term,  and  furthermore  the  acceptance  of  the 
symbol  £X,°K.  to  represent  it.6  The  subscript  is  intended  to  show  the 
wave-length  at  which  the  brightness  equality  has  been  obtained,  and 
°K.,  consistent  with  much  good  practice,  is  added  to  show  that  the  tem- 
perature is  expressed  on  the  so-called  Kelvin  scale,  that  is,  in  degrees 
centigrade  +273°  on  the  gas  scale. 

With  respect  to  the  second  kind  of  apparent  black-body  temperature 
in  use  in  optical  pyrometry,  attention  is  recalled  to  the  previous  discussion 
of  the  various  ways  in  which  the  radiating  properties  of  a  substance  may 
differ  from  those  of  a  black  body.  Not  only  is  the  emissive  power  of  a 
substance  always  different  (less)  from  that  of  a  black  body,  but  in  general 


6  See  footnote  6. 


EDWARD    P.    HYDE  289 

the  emissive  power  of  a  material  substance  will  vary  from  wave-length 
to  wave-length  throughout  the  spectrum.  Hence  one  may  establish  the 
hot  condition  of  a  substance  not  only  by  determining  its  black-body 
brightness  temperature  but  also  by  determining  the  temperature  of  a 
black  body  at  which  the  ratio  of  the  radiant  flux  at  some  two  arbitrarily 
chosen  wave-lengths  Xi  and  X2  is  the  same  as  that  for  the  substance  under 
investigation.  This  method  of  obtaining  an  apparent  temperature  is 
based  on  the  distribution  of  radiant  flux,  rather  than  on  the  absolute 
amount  of  it  in  any  one  wave-length.  It  frequently -happens  in  the  case 
of  radiating  metals  that  the  relative  distributions  of  radiant  flux,  taken 
the  same  for  the  radiating  substance  and  for  the  black  body  at  0.7/*  and 
at  0.5ju,  i.e.,  near  the  two  ends  of  the  visible  spectrum,  are  found  by  experi- 
ment to  be  essentially  the  same  throughout  the  visible  spectrum.  In 
such  cases  the  color  of  the  light  from  the  substance  and  from  the  black 
body  would  be  identical. 

Since  the  application  of  this  method  of  determining  an  apparent  black- 
body  temperature  by  comparison  of  the  distribution  of  the  spectral 
radiant  flux  of  a  substance  with  that  of  a  black  body  was  made  first  by 
use  of  the  integral  color,  this  apparent  black-body  temperature  is  called 
the  black-body  color  temperature,  or,  briefly,  the  color  temperature.  In 
its  more  general  use,  it  refers  to  the  relative  radiant  flux  in  two  wave- 
lengths, and  the  following  symbol  for  its  designation  is  proposed,6 
^ctxiXj)0  K.  When  the  measurement  is  made  by  means  of  the  integral 
light  the  term  in  parentheses  may  be  omitted. 

As  stated,  there  is  a  third  kind  of  apparent  black-body  temperature 
if  total  radiation  pyrometry  is  included.  Thus  the  determination  of  the 
temperature  of  a  black  body  at  which  its  integral  radiant  flux  equals  that 
of  the  given  radiating  substance  at  the  given  unknown  temperature 
furnishes  an  empirical  measure  of  its  temperature.  This  method  of 
pyrometry  is  in  practical  use.  The  apparent  temperature  obtained  by 
this  method  should  be  designated  as  the  black-body  total  radiation 
temperature,  more  briefly,  the  total  radiation  temperature,  or  still  more 
briefly  in  practice,  the  radiation  temperature.  As  a  symbol  TR°  K.  is 
suggested. 

RELATION  BETWEEN  TRUE,  BRIGHTNESS,  AND  COLOR  TEMPERATURES 

It  would  be  well  if  in  every  case  one  could  arrive  at  the  true  tempera- 
ture of  a  hot  substance,  even  though  in  many  cases  any  means  of  repro- 
ducing definite  temperature  conditions  are  satisfactory.  But  generally 


6  If  Tjs  the  symbol  chosen  to  represent  temperature  it  would  be  more  consistent 
to  express  brightness  temperature  by  TB(\\)°  K.  rather  than  by  S\t  °  K.  as  proposed. 
Since,  however,  the  latter  term  is  so  generally  used,  the  author  hesitates  to  suggest 
a  change  that  might  cause  confusion. 

19 


290  HIGH-TEMPERATURE   SCALE  AND   ITS   APPLICATION 

it  is  not  feasible  to  measure  the  true  temperature.  The  best  that  can  be 
done  is  to  measure  an  apparent  temperature  such  as  the  brightness  or 
color  temperature.  In  cases  where  an  approximation  to  true  temperature 
is  for  some  reason  particularly  desirable,  it  is  helpful  to  learn  what  is 
known  regarding  the  relations,  if  any,  that  exist  between  true,  color,  and 
brightness  temperatures. 

There  is  one  relation  that  exists  without  exception,  viz.,  that  the 
brightness  temperature  of  a  radiating  substance  is  always  less  than  the 
true  temperature.  This  then  gives  a  limit  in  one  direction,  though  it 
throws  no  light  on  the  difference  between  true  and  brightness  temperature 
for  any  particular  substance.  Another  relation  has  been  found  to  hold 
for  tungsten  and  platinum,  and  quite  probably  holds  for  many  other 
metals  but  is  not  universally  true,  viz.,  that  the  color  temperature  for 
these  metals  at  high  temperatures  is  always  higher  than  the  true  tempera- 
ture, thus  giving  an  upper  limit  to  the  true  temperature.  Moreover,  for 
tungsten  and  not  improbably  for  many  other  metals,  the  color  tempera- 
ture, though  higher,  is  much  nearer  the  true  temperature  than  the  lower 
limit  given  by  the  brightness  temperature. 

The  two  methods  of  determining  apparent  black-body  temperatures 
have  their  respective  merits  as  applied  to  different  cases,  and  in  many 
cases  both  may  be  used  advantageously.  It  is  not  the  place  in  this  paper, 
however,  to  enter  upon  a  discussion  of  these  matters,  or  to  introduce  a 
discussion  of  accuracy  attainable  or  of  special  experimental  difficulties. 


I 

THEORY   AND   ACCURACY   IN   OPTICAL   PYROMETRY  291 


Theory  and  Accuracy  in  Optical  Pyrometry  with  Particular  Reference 
to  the  Disappearing-filament  Type 

BY   W.   E.   FORSYTHE,*   CLEVELAND,    OHIO 
(Chicago  Meeting,  September,  1919) 

WHEN  measuring  ordinary  temperatures,  the  instrument  is  generally 
placed  in  very  close  contact  with  the  body  the  temperature  of  which  is 
desired.  However,  if  the  temperature  of  the  source  is  continually  raised, 
a  point  is  soon  reached  where  no  known  substance  will,  in  general,  remain 
constant  in  any  of  its  temperature-measuring  properties  if  placed  in 
direct  contact  with  the  source.  Also,  it  is  occasionally  necessary  to  meas- 
ure the  temperature  of  a  source  that  is  so  small  or  so  situated  that  it 
would  be  very  hard  to  bring  the  measuring  instrument  into  direct  contact 
with  the  source.  When  these  conditions  exist,  advantage  is  taken  of  the 
well-known  fact  that  all  bodies,  when  at  sufficiently  high  temperatures, 
send  out  radiation  in  amounts  readily  measurable.  This  radiation  has 
been  found  to  be  related  to  the  temperature.  The  temperatures  of  very 
hot  bodies  have  probably  always  been  judged  by  the  color,  or  the  bright- 
ness, of  the  light,  given  off.  With  practice,  one  can  estimate  probably 
within  50°  to  100°  C.  of  the  correct  value.  However,  if  judgment  is 
left  to  the  eye  alone,  very  much  larger  errors  are  sometimes  made,  due  to 
the  use  that  has  been  made  of  the  eye  just  previous  to  the  time  of  esti- 
mating. To  secure  accurate  estimates  by  eye,  a  comparison  source  is 
necessary. 

The  introduction  of  a  comparison  source  is  the  first  step  toward  an 
optical  pyrometer,  which  consists  of  a  comparison  source  and  some  con- 
venient arrangement  for  matching  this  source,  either  in  brightness  or  in 
color,  against  the  source  studied.  In  Fig.  1  is  shown  diagrammatically 
the  arrangement  used  in  one  form  of  the  Le  Chatelier  optical  pyrometer. 
The  light  from  the  comparison  source  at  C  is  reflected  into  the  eyepiece 
by  a  mirror  E  so  arranged  that  one-half  of  the  field  is  illuminated  by 
light  from  the  comparison  source  and  the  other  by  light  from  the  source 
studied.  The  match  is  obtained  either  by  varying  the  intensity  of  the 
comparison  source  or  by  varying  the  size  of  the  opening  before  the  ob- 
jective lens  D.  In  the  Wanner  optical  pyrometer,  the  beams  of  light 
from  the  comparison  source  and  the  source  studied  are  so  arranged  that, 
by  the  use  of  a  polarizing  device,  the  two  beams,  as  viewed  through  the 
eyepiece,  are  polarized  in  a  plane  at  right  angles  to  each  other.  By  rotat- 


Physicist,  Nela  Research  Laboratory. 


292 


THEORY   AND   ACCURACY   IN   OPTICAL   PYROMETRY 


ing  another  nicol  located  in  the  eyepiece,  the  two  sources  can  be  made  to 
appear  the  same  in  brightness. 

The  disappearing-filament  type  of  optical  pyrometer  is  very  simple 
in  its  construction,  being  practically  a  telescope  in  appearance.  It  differs 
from  a  telescope  in  that  it  contains,  for  use  as  a  comparison  source,  a 
lamp  filament,  called  the  pyrometer  filament,  which  is  located  at  the  focus 
of  the  objective  lens.  In  series  with  this  filament  is  a  small  battery,  a 
resistance,  and  an  ammeter.  To  measure  the  temperature  of  any  hot 
body  with  this  pyrometer,  the  instrument  is  first  sighted  upon  the  hot 
body,  which  is  done  as  easily  and  in  much  the  same  manner  as  the  fo- 
cusing of  an  opera  glass.  When  looked  at,  the  hot  object  is  seen  with  the 
pyrometer  filament  crossing  it,  so  that  the  filament  appears  much  the  same 
as  the  cross-hairs  in  an  ordinary  telescope.  By  varying  the  current,  the 


FIG.  1. — ARRANGEMENTS  USED  IN  ONE  FORM  OF  LE  CHATELIER  OPTICAL,  PYROMETER. 

brightness  of  the  filament  will  change  and  it  can  be  made  to  disappear 
against  the  image  of  the  source  studied.  By  mounting  a  piece  of  red 
glass  in  the  eyepiece,  it  is  much  easier  to  tell  when  the  brightness  of  the 
object  sighted  upon  and  the  comparison  source  are  the  same. 

When  working  with  optical  pyrometers,  certain  precautions  are  neces- 
sary if  errors  are  to  be  avoided.  In  this  paper  an  attempt  is  made  to 
discuss  fully  some  of  these  precautions  and  also  some  of  the  principles 
governing  the  accuracy  and  use,  in  particular  as  applied  to  the  dis- 
appearing-filament type  of  optical  pyrometer.  There  are  also  included 
the  results  of  several  experiments  with  this  type  of  pyrometer,  and 
several  reasons  why  it  has  been  used  quite  extensively  in  the  research 
laboratories  of  this  country  and  is  being  used  more  and  more  in  industrial 
work.  These  experiments  show  that  different  observers  working  with 
the  same  pyrometer  will  obtain  almost  exactly  the  same  readings  and 


W.    E.    FORSYTHE  293 

that  different  pyrometers  of  this  type,  calibrated  in  different  laboratories 
by  different  observers,  agree  very  well  in  their  readings. 

Definition  of  Black  Body. — Kirchhoff  has  defined  the  black  body  as  one 
that  will  absorb  all  the  radiation  that  it  receives;  that  is,  it  will  neither 
reflect  nor  transmit  any  of  the  incident  radiation.  He  also  showed  that 
the  radiation  from  such  a  body  is  a  function  of  the  temperature  alone. 
There  is  no  known  substance  that  has  exactly  this  property,  the  nearest 
approach  is  probably  some  form  of  untreated  carbon.  As  a  cavity  with 
walls  at  a  uniform  temperature  possesses  the  properties  of  a  black  body, 
the  radiation  issuing  from  a  hole  made  in  the  wall  of  the  cavity  will  obey 
the  laws  of  black-body  radiation. 

Many  attempts  have  been  made  to  realize  this  uniformly  heated 
cavity  with  an  opening  through  which  the  radiation  can  be  studied. 
The  one  most  commonly  used  for  temperatures  above  1000°  K.  consists 
of  a  refractory  tube  more  or  less  uniformly  wound  with  platinum  ribbon, 
and  containing  a  central  and  other  diaphragms  with  small  holes  in  them. 
For  temperatures  as  high  as  the  melting  point  of  palladium  (1828°  K.), 
it  is  necessary  to  have  a  second  refractory  tube  outside  of  the  first;  this 
tube  also  is  wound  with  platinum  ribbon  but  the  windings  are  spaced 
farther  apart  at  the  center  than  at  the  ends,  in  order  to  correct  for  the 
end  cooling.  Such  a  furnace  with  the  space  between  these  two  heater 
tubes  filled  with  very  pure  aluminum  oxide  and  with  two  tubes  slipped 
inside  of  another  tube  that  has  been  packed  with  a  good  heat-insulating 
material  has  been  found  to  work  very  well.  If  this  outside  tube  and  its 
packing  extend  well  beyond  the  ends  of  the  heater  tubes,  the  end  cooling 
will  be  very  much  reduced.  Such  a  platinum-wound  black-body  furnace 
has  been  heated  to  the  temperature  of  melting  palladium  a  dozen  times 
and  held  at  this  high  temperature  for  several  hours  each  time,  and  is  still 
in  working  order. 

In  any  form  of  optical  pyrometry  where  the  temperature  of  a  black 
body  is  determined  by  a  measurement  of  the  brightness  for  a  particular 
wave-length  interval,  the  temperature  is  calculated  from  the  brightness 

-5     _  -*- 

measurement  by  means  of  Wien's  equation  £'x  =  CiX  e  xr.  One 
of  the  advantages  of  optical  pyrometers  can  be  seen  by  an  investigation 
of  this  equation.  For  a  particular  wave-length  interval  (0.665/i),  the 
brightness  varies  about  twelve  times  as  fast  as  the  temperature  for  a 
temperature  of  about  2000°  K.  Wien's  equation,  which  is  hardly  more 
than  an  empirical  law,  has  been  found  experimentally  to  represent  the 
facts  quite  accurately  for  such  temperatures  and  wave-lengths  for  which 
the  product  \T  is  less  than  3000.  Thus  for  optical  pyrometry  where  the 
longest  wave-length  used  is  less  than  0.7ju,  the  equation  is  valid  and  an 
instrument  thus  calibrated  may  be  used  throughout  a  wide  range  of 
temperature. 


294  THEORY   AND   ACCURACY   IN   OPTICAL   PYROMETRY 

It  is  not  absolute  measurements  of  brightness  that  are  made  with 
the  optical  pyrometer,  but  rather  comparisons  of  brightness,  that  is, 
the  brightness  of  the  unknown  source  is  compared  with  that  of  a  standard 
black  body  at  a  particular  temperature.  Using  Wien's  equation  in  this 
way  requires  but  the  one  constant  c2.  The  value  of  c2  that  fits  best  all 
the  different  experimental  data1  is  14,350/i  deg.  High-temperature  scales 
are  based  on  the  brightness  of  a  black  body  at  the  temperature  of  the 
melting  point  of  some  one  or  more  selected  metals.  Owing  to  their  con- 
venience, both  as  to  their  freedom  from  oxidization  and  as  to  the  tem- 
peratures of  their  melting  points,  gold  and  palladium  are  generally  chosen. 
The  value  generally  accepted  for  the  melting  point  of  gold  is  1336°  K., 
which  is  the  value  found  by  Day  and  Sosman.2  This  value  for  the 
gold  point  together  with  the  above  value  of  c2  leads  to  1828°  K.  as  the 
palladium  point. 

THE    DlSAPPEARING-FILAMENT    PYROMETER 

The  disappearing-filament  type  of  optical  pyrometer  is  known  in  this 
country  as  the  Morse  or  the  Holborn-Kurlbaum  pyrometer.  An  early 


-i 

FIG.  2. — ARRANGEMENT  OF  MORSE  PYROMETER.  A,  BACKGROUND;  B,  OBJEC- 
TIVE LENS;  C,  ENTRANCE  CONE  DIAPHRAGM;  D,  PYROMETER  FILAMENT;  E,  EYEPIECE 
DIAPHRAGM;  F,  EYEPIECE;  G,  MONOCHROMATIC  FILTER.  FOR  THE  SET  UP  AS  USUALLY 

USED,   THE   DIMENSIONS   ARE   AS  FOLLOWS:   AB   =  40  CM.;  CD  =  55  CM.;  DE   —  60  CM. 

DIAMETER  OF  OPENING  IN  DIAPHRAGM  AT  C  AND  E  ARE  RESPECTIVELY  20  MM.  AND  9  MM. 

form  of  this  pyrometer  was  patented  by  Morse  in  1902.  In  the  instru- 
ment as  now  used,  a  small  lamp  is  mounted  at  the  focus  of  a  telescope, 
as  shown  in  Fig.  2.  An  image  of  the  source  whose  temperature  is  to  be 
measured  is  brought  to  focus  at  the  same  point  by  means  of  the  objective 
lens  A.  Theoretically,3  the  image  of  any  source  as  observed  through  a 
particular  telescope  will  not  vary  in  brightness  with  a  change  in  distance 
from  the  source  (except,  of  course,  differences  due  to  air  absorption,  etc.), 
providing  a  certain  solid  angle  is  always  filled  with  radiation  from  the 
source  and  this  angle  is  of  such  size  that  the  cone  of  rays  entering  the 
eye  is  constant.  This  angle  is  generally  determined  by  having  the  eye- 
piece at  a  fixed  distance  from  the  pyrometer  lamp  and  having  before  the 
eyepiece  a  limiting  diaphragm  of  such  size  that  it  is  always  filled  with 
light  from  the  objective  lens.  It  is  also  necessary  to  have  a  fixed  dia- 
phragm between  the  objective  lens  and  the  pyrometer  lamp. 

1  Coblentz:  U.  S.  Bureau  of  Standards  Bull.  13,  470. 

2  Am.  Jnl.  Sci.  (1910)  29,  93. 

3  Schuster's  "Theory  of  Optics,"  2d  edition,  152 


W.    E.    PORSYTHE 


295 


A  form  of  this  pyrometer  that  has  been  used  quite  extensively  in 
Nela  Research  Laboratory  is  shown  in  Fig.  3.  The  objective  lens 
is  a  very  high-grade  lens  made  by  Bausch  &  Lomb,  diameter  about  6.2 
cm.,  focal  length  about  30  cm.  With  a  lens  of  this  aperture,  it  is  possible 
to  use  very  large  magnifications  and  still  have  the  cone  of  rays  large 
enough  to  work  well.  The  working  parts  are  mounted  on  a  very  sub- 
stantial optical  bench.  The  telescope,  used  as  an  eyepiece,  is  per- 
manently fastened  to  one  end  of  the  bench  and  the  pyrometer  lamp, 
diaphragm,  and  lens  are  so  mounted  that  each  is  movable  in  any  direction 
by  means  of  slow-motion  screws.  This  is  necessary  since  for  very  accurate 


FIG.  3. — LABORATORY  FORM  OP  OPTICAL  PYROMETER.     LENGTH  FROM  EYEPIECE  TO 

OBJECTIVE   LENS,   225  CM. 

work  the  different  parts  of  the  instrument  must  be  very  closely  in  line.4 
By  the  use  of  caps  with  very  small  holes  in  the  center  in  front  of  the  lenses 
and  a  very  small  hole  in  the  diaphragm,  the  different  parts  are  brought 
into  very  good  alinement.  This  pyrometer  is  not  portable  but  is  useful 
in  the  laboratory  when  the  temperature  of  small  filaments  is  to  be  mer  3- 
ured;  with  this  set  up  the  temperature  of  a  2-mil  (0.05  mm.)  filament 
has  been  measured. 

MONOCHROMATIC  SCREEN 

In  working  with  an  optical  pyrometer,  it  is  generally  sufficient  to 
use  a  so-called  monochromatic  screen  between  the  eye  and  the  pyrometer 
filament,  or  other  comparison  source,  in  order  that  brightness  comparisons 
can  be  made  without  trouble  due  to  color  differences.  For  the  most  part, 
red-glass  screens  have  been  used  rather  than  blue  screens,  or  those  having 
a  transmission  band  near  the  central  part  of  the  visible  spectrum,  be- 
cause for  sources  at  low  temperatures  it  is  the  red  radiation  that  first 
becomes  visible.  Besides,  better  red-glass  screens  than  green  or  blue 
screens  may  be  obtained.  Colored  glass  to  be  suitable  for  a  mono- 

<  Phys.  Rev.  [2]  (1914)  4,  163. 


296 


THEORY   AND    ACCURACY   IN    OPTICAL    PYROMETRY 


chromatic  screen  must  have  a  rather  narrow  transmission  band,  in  order 
that  there  will  not  be  enough  color  difference  between  the  source  studied 
and  the  comparison  source  to  prevent  accurate  comparisons  from  being 
made.  As  there  may  be  more  than  1000°  difference  in  temperature 
between  the  sources  compared,  this  is  very  important. 

In  Fig.  4  are  shown  the  spectral  transmissions  of  several  red  glasses 
that  are  nearly  enough  monochromatic  for  use  under  various  conditions. 
The  glass  having  the  spectral  transmission  shown  by  curve  A  does  very 
well  for  a  commercial  pyrometer  for  low-temperature  ranges  because 
the  amount  of  light  transmitted  is  so  great.  The  glass  having  the  trans- 


0.60      0.62      0.64      0.66      0.68      0.70      0.72      0.74      0.76 


Wave  -  length  in  /J. 

FIG.  4. — SPECTRAL  TRANSMISSION  OP  VARIOUS  RED  GLASSES.  CURVE  C  FOR 
JENA  RED  No.  4512,  2.93  MM.  THICK.  CURVE  E  FOR  JENA  RED  No.  2745,  3.2  MM. 
THICK.  CURVE  A  FOR  CORNING  HIGH  TRANSMISSION  RED,  MARKED  150  PER  CENT.,  5  MM. 
THICK.  CURVE  B  FOR  CORNING  HIGH  TRANSMISSION  RED,  MARKED  50  PER  CENT.,  5  MM. 
THICK.  CURVE  D  FOR  CORNING  HIGH  TRANSMISSION  RED,  MARKED  28  PER  CENT.,  6 
MM.  THICK. 

mission  shown  by  curve  E  was  formerly  used  for  this  purpose.  As  the 
effective  wave-length  for  this  glass  varies  about  twice  as  much  as  for 
the  other  glasses,  it  is  not  as  satisfactory.  The  glasses  having  the  trans- 
mission shown  by  curves  B,  C,  and  D,  are  suitable  for  the  most  accurate 
work.  To  test  the  constancy  of  the  spectral  transmission  of  the  red 
glass,  a  piece  of  the  Corning  red  50-per  cent,  glass,  curve  B,  the  trans- 
mission of  which  had  been  carefully  measured,  was  placed  on  the  roof 
of  the  laboratory,  where  it  was  exposed  to  the  direct  rays  of  the  sun,  for  a 
little  more  than  a  year;  when  it  was  brought  in  no  change  was  found  in 
its  spectral  trans-mission.  Evidently  then,  the  spectral  transmission 
of  such  glass  is  constant  for  all  ordinary  uses. 


W.    E.    FORSYTHE  297 

Effective  Wave-length  of  Monochromatic  Screen.  —  An  optical  pyrometer 
can  be  so  calibrated  and  so  used  as  to  make  unnecessary  a  knowledge  of 
the  extent  to  which  the  screen  is  monochromatic.  To  do  this  requires 
a  black-body  furnace  that  can  be  operated  at  various  temperatures  up 
to  the  highest  temperature  for  which  the  pyrometer  is  to  be  used.  How- 
ever, to  use  Wien's  equation  to  extend  the  temperature  scale  either  above 
or  below  that  of  the  standard  furnace  by  the  use  of  rotating  sector  disks 
or  absorbing  glass,  that  is,  to  find  the  temperature  of  a  black  body  having 
a  brightness  of,  say,  ten  times  (assuming  a  sector  or  absorbing  glass  trans- 
mission of  one-tenth)  that  of  a  black  body  whose  temperature  can  be 
measured  directly,  a  knowledge  of  what  wave-length  to  use,  or  the  effect- 
ive wave-length,5  is  necessary.  The  effective  wave-length  also  must  be 
known  if  the  pyrometer  is  used  to  measure  the  temperature  of  non- 
black  bodies.  In  using  the  pyrometer,  it  is  the  integral  luminosities 
through  the  red  glass  that  are  compared,  for  which  reason  the  effective 
wave-length  of  the  red-glass  screen  for  a  certain  temperature  interval 
has  been  defined  as  the  wave-length  for  the  definite  temperature  interval 
for  a  black  body,  such  that  the  ratio  of  its  radiation  intensities  equals 
the  ratio  of  the  integral  luminosities  through  the  screen  used. 

Knowing  the  spectral  transmission  of  the  red  glass,  it1  is  possible  to 
calculate  the  effective  wave-length  X,  for  any  temperature  interval  by 
means  of  the  following  equation: 

'x 
' 


where  J(^T)  d\  is  the  energy,  as  given  by  Wien's  equation,  for  the  wave- 
length interval  from  X  to  X  +  d\;  t'R  is  the  spectral  transmission  of  the 
red  glass;  and  Fx  is  the  visibility.  These  integrals  can  be  computed  by 
the  step-by-step  method  with  sufficient  accuracy  for  this  purpose.  Us- 
ing equation  1,  the  effective  wave-length  was  calculated  for  the  red  glass 
having  the  spectral  transmission  shown  by  curve  B,  Fig.  4,  for  a  number 
of  temperature  intervals  and  plotted,  as  shown  in  Fig.  5.  By  connecting 
the  points  where  the  curve  for  the  effective  wave-length  from  any 
particular  temperature  crosses  the  same  temperature  ordinate,  a  curve 
is  obtained,  E,  Fig.  5,  that  gives  the  limiting  effective  wave-length  for 
a  particular  temperature. 

To  show  how  these  curves  may  be  used,  the  effective  wave-length  for 
a  couple  of  temperature  intervals  will  be  found.  The  effective  wave- 
length between  1800°  and  2900°  K.  is  given  by  the  ordinate  of  the  point 
where  the  1800°  K.  curve  crosses  the  2900°  K.  ordinate,  that  is,  it  is  0.6587^. 
For  the  range  between  2100°  and  2900°  K.  the  effective  wave-length  is 
likewise  given  by  the  point  where  the  2100°  K.  curve  would  cross  the 

8  Astrophys.  Jnl  (1915)  42,  294. 


298 


THEORY  AND   ACCURACY  IN   OPTICAL   PYROMETRY 


2900°  K.  ordinate.  The  2100°  K  curve  is  not  drawn,  but  will  have  to 
be  imagined  as  being  drawn  parallel  to  the  1800°  K.,  one  point  of  its 
position  being  determined  by  where  the  curve  E  crosses  the  2100°  K. 
ordinate.  The  effective  wave-length  for  this  interval  is  0.6584^*.  It 
can  be  seen  from  the  figure  that  the  effective  wave-length  for  any  tem- 
perature interval  is  given  quite  closely  by  the  mean  of  the  limiting 
effective  wave-length  for  the  two  temperatures. 

To  determine  the  effective  wave-length  for  a  particular  red  glass  re- 
quires considerable  work  both  in  determining  the  spectral  transmission 
and  in  calculating  the  final  values.  To  determine  the  effective  wave- 


0.60 


0.59 


•5068 


0.57 


3600 


1300       1600  2000  2400  2800  3200 

Temperature  in  Degrees  Kelvin 

FIG.  5. — EFFECTIVE  WAVE-LENGTHS  FOR  CORNING  RED  GLASS.  SPECTRAL  TRANS- 
MISSION SHOWN  BY  CURVE  B,  FlG.  4.  CURVE  A,  EFFECTIVE  WAVE-LENGTHS  FROM 
1300°  TO  OTHER  TEMPERATURE.  CURVE  B,  EFFECTIVE  WAVE-LENGTHS  FROM  1800° 
TO  OTHER  TEMPERATURE.  CURVE  C,  EFFECTIVE  WAVE-LENGTHS  FROM  2400°  TO  OTHER 
TEMPERATURE.  CURVE  D,  EFFECTIVE  WAVE-LENGTHS  FROM  3600°  TO  OTHER  TEMPERA- 
TURE. CURVE  E,  LIMITING.  EFFECTIVE  WAVE-LENGTH. 

length  for  every  glass  used  in  optical  pyrometry  would  be  laborious.  It 
is  fortunately  possible  to  obtain  the  effective  wave-length  of  an  unknown 
piece  of  glass  in  terms  of  a  standard  red  glass  with  very  few  measurements. 
If  the  relative  brightness  of  a  black  body  for  two  temperatures  is 
measured  both  with  the  standard  red  glass  and  the  glass  being  investi- 
gated, it  is  possible  to  compute  the  effective  wave-length  of  the  unknown 
glass  from  that  of  the  standard  red  glass  and  the  ratios  of  the  black-body 
brightness  thus  found.  From  the  definition  of  the  effective  wave-length 
and  Wien's  equation,  the  following  equation  is  found  connecting  the 
effective  wave-length  and  the  ratio  of  the  black-body  brightness: 


(2) 


(Xe)z      log  Ba 


W.    E.    FORSYTHE  299 

where  (\e)a  and  (\e)x  are  the  effective  wave-lengths  for  the  glasses  a 
and  x  for  the  range  studied  and  Ba  and  Bx  the  ratio  of  the  brightness  of 
the  black  body  for  the  two  temperatures  for  red  glasses  a  and  x.  It 
is  often  impossible  to  get  a  black  body  that  can  be  operated  over  the 
temperature  range  necessary  for  an  accurate  determination  of  the  ratio 
of  brightness.  However,  it  is  not  necessary  to  use  a  black  body  in  this 
determination,  providing  a  calibrated  lamp  filament  is  available.  The 
tungsten  filament  can  be  color  matched6  against  a  black  body.  Further, 
the  relation  between  the  brightness  temperature  and  the  color  tempera- 
ture has  been  worked  out.  If  a  tungsten  filament  is  used  and  the  ratio 
of  its  brightness  is  measured  between  two  particular  color  temperatures, 
as  described,  the  effective  wave-length  can  be  calculated,  using  the  color 
temperature  in  place  of  the  true  temperature  of  the  black  body. 

The  tungsten  filament  does  not  show  the  same  increase  in  brightness7 
for  the  range  between  color  temperatures  Tci  and  Tc2  as  does  a  black  body 
for  the  same  range  of  temperature  where  for  the  black  body  Tci  and  TcZ 
are  the  true  temperatures.  This  difference  is  quite  small  and  for  most 
work  would  be  negligible  but  for  the  highest  accuracy  it  must  be  taken 
into  account.  If  the  brightness  of  a  tungsten  lamp  is  measured  between 
the  two  color  temperatures  Tc\  and  TcZ  for  both  the  standard  and  the 
unknown  glass,  the  effective  wave-length  for  the  unknown  glass  can  be 
calculated  accurately  by  means  of  the  equation  : 

(3) 


n 

Because  the  ratio  -^  for  tungsten   is   only  slightly  different  from  the 
*>x 

ratio  for  a  black  body  between  the  same  color  temperatures,  equation  3, 
with  a  very  slight  error,  may  be  reduced  to  equation  2. 

Experimentally  it  has  been  shown  that  if  the  effective  wave-lengths 
are  determined  for  two  red  glasses  that  have  somewhat  the  same  spectral 
transmission,  the  relation  between  the  effective  wave-length  is  given 
quite  approximately  by  the  equation: 

(Xe)a  =  0&)x  +  constant 

This  equation  holds  for  the  glasses  investigated  to  all  the  accuracy  needed 
for  temperature  measurements. 

Effect  of  Change  of  Temperature  of  Red  Glass  on  its  Spectral  Transmis- 
sion. —  In  connection  with  the  investigation  of  the  effective  wave-lengths 
it  was  observed  that  the  transmission  of  the  red  pyrometer  glass,  presum- 
ably dependent  for  the  color  on  a  colloidal  solution,  is  subject  to  a  large 
change  with  temperature.  This  has  not  been  investigated  thoroughly, 
but  observations  were  made  at  two  temperatures,  20°  and  80°  C.*  by 

6  Hyde:  See  page  285,  this  volume.        7  Hyde:  Astrophys.  Jnl  (1912)  36,  89. 


300 


THEORY    AND    ACCURACY   IN    OPTICAL    PYROMETRY 


immersing  the  glass  in  water  heated  to  these  temperatures;  the  results 
are  given  in  Fig.  6.  Curve  A  is  the  transmission  of  the  glass  at  the  lower 
temperature  20°  C.,  and  curve  B  the  corresponding  curve  at  the  higher 
temperature  80°  C.  The  transmission  is  shown  to  decrease  with  increase 
in  temperature,  the  coefficient  of  change  of  temperature  being  greatest  in 
the  shorter  wave-lengths.  The  change  is  such  as  to  make  the  trans- 
mission band  appear  to  shift  to  longer  wave-lengths  as  the  temperature 
is  increased. 

A  test  was  made  of  the  effect  of  this  temperature-shift  of  the  trans- 
mission band  on  temperature  measurements  when  the  red  glass  was 
used  as  a  screen  before  the  eyepiece  of  the  pyrometer.  The  temperature 
of  a  broad  carbon-filament  lamp  operated  at  a  brightness  temperature 


0.60     0.62     0.64    0.66     0.68    0.70    0.72 
Wave-length,  in   M 

FIG.  6. — SPECTRAL  TRANSMISSION  OF  A  SINGLE  THICKNESS  OF  GLASS  F-4512:  A  AT 

20°  C.;  B  AT  80°  C. 

of  1900°  K.  was  measured  with  the  red  glass  at  room  temperature  20° 
and  at  80°  C.,  using  a  sectored  disk  with  a  2°  opening  as  this  gives  a  larger 
effect  than  a  sectored  disk  with  greater  transmission.  It  was  found  that 
there  was  a  decrease  of  about  5°  C.  in  the  temperature  obtained  when  the 
glass  was  heated  to  80°  C.  over  that  obtained  with  the  glass  at  room  tem- 
perature. This  shows  that  for  all  ordinary  temperature  changes,  the 
effect  is  negligible. 


•An  optical  pyrometer  of  the  disappearing-filament  type  is  calibrated 
by  finding  the  current  through  the  pyrometer  filament  for  an  apparent 
brightness  match  between  the  pyrometer  filament  and  a  black  body 


W.    E.    FORSYTHE  301 

at  a  particular  temperature.  The  direct  way  is  to  have  a  black  body 
that  can  be  operated  over  the  entire  range  for  which  the  pyrometer  is  to 
be  calibrated.  This  quite  often  is  impracticable.  An  optical  pyrome- 
ter can  be  calibrated  from  a  black  body  held  at  a  particular  tempera- 
ture.8 To  do  this  it  is  necessary  to  have  some  means,  such  as  a  sector 
disk  or  an  absorbing  glass  of  known  transmission,  for  cutting  down  the 
apparent  brightness  of  the  incident  radiation.  If  readings  of  the  current 
through  the  pyrometer  filament  for  an  apparent  brightness  match  with 
the  rotating  sector  between  the  pyrometer  lamp  and  the  standard  black 
body  are  taken,  there  will  be  obtained  a  measure,  in  terms  of  a  current 
through  the  pyrometer  filament,  of  a  brightness  that  is  some  known 
fraction  of  that  of  the  standard  black  body  at  the  standard  temperature. 
If  monochromatic  radiation  is  used,  it  is  easy  to  calculate  the  temperature 
TZ  of  the  black  body  corresponding  to  this  current  through  the  pyrometer 
filament,  that  is,  to  this  measured  brightness,  from  T\,  the  standard 
temperature  by  the  following  formula  derived  from  Wien's  equation: 

1       1  _  Mogg  ,4x 

T!       T*       c,  log  e 

where  R  is  the  transmission  of  the  sector  and  X  the  wave-length  used.  If 
the  measurements  are  made  with  a  red  glass  in  the  eyepiece,  the  temper- 
ature that  would  correspond  to  this  fraction  of  the  brightness  of  the 
standard  black  body  can  be  calculated  just  as  before,  except  that  in  this 
case  the  effective  wave-length  of  the  red  glass  for  this  temperature  interval 
is  to  be  used. 

If  a  potentiometer  is  used  to  measure  the  current  through  the  pyrome- 
ter filament,  a  great  deal  of  time  is  wasted  when  a  number  of  readings 
are  taken  on  the  same  point.  A  very  good  method,  which  is  at  the  same 
time  quite  accurate,  is  to  use  the  deflection  potentiometer  principle.  The 
regular  Leeds  &  Northrup  potentiometer  lends  itself  quite  readily  to 
such  an  adaptation.  By  connecting  a  millivoltmeter  in  series  with  the 
standard  resistance  and  between  the  binding  post  marked  jBr-and  a 
traveling  plug  inserted  in  the  proper  place  on  the  dial,  currents  can  be 
read  to  one  part  in  three  or  four  thousand  very  easily.  The  readings 
can  thus  be  made  very  rapidly  and  at  the  end  of  the  set  these  same  read- 
ings can  be  checked  on  the  potentiometer.  This  makes  the  current 
readings  entirely  independent  of  the  constancy  of  any  deflection  instru- 
ment. With  a  switch  in  this  millivoltmeter  line,  the  potentiometer  is 
left  free  to  check  any  other  current,  such  as  the  one  through  the  lamp 
that  is  being  investigated.  . . 

If  a  rotating  sector  or  an  absorbing  glass  of  known  transmission  is 
used  between  the  source  being  investigated  and  a  calibrated  pyrometer 
lamp,  the  calibration  of  this  pyrometer  with  the  sector  can  be  extended 

•  C.  E.  Mendenhall:  Phys.  Rev.  (1911)  33,  74. 


302  THEORY   AND   ACCURACY   IN    OPTICAL    PYROMETRY 

above  the  standard  black  body.  In  this  case  the  temperature  is  to  be 
calculated  from  the  temperature  corresponding  to  the  current  through 
the  pyrometer  filament  and  the  transmission  of  the  sector  or  absorbing 
glass  used  by  means  of  equation  3,  excepting  in  case  R  is  the  reciprocal  of 
.the  transmission  of  the  sector  or  glass  and  then  772  will  come  out  greater 
than  TI.  A  very  convenient  method  is  to  work  out  such  extrapolated 
temperatures  for  the  various  sectors  and  absorbing  glasses  that  are  to  be 
used  and  plot  the  extrapolated  temperatures  against  the  temperatures 
as  determined  from  the  pyrometer  reading.  Such  curves  can  then  be 
used  with  any  pyrometer  using  the  same  red  glass,  providing  the  same 
sectors  or  absorbing  glasses  are  used. 

It  is  very  troublesome  to  operate  a  standard  black  body  every  time 
it  is  necessary  to  calibrate  an  optical  pyrometer.  Much  time  can  be 
saved  if  a  tungsten  lamp  with  a  filament  of  a  suitable  size  is  standardized 
so  as  to  have  the  same  brightness,  as  observed  with  the  optical  pyrometer, 
as  the  standard  black-body  furnace  for  a  particular  temperature.  The 
lamp  may  also  be  standardized  for  other  temperatures  and  thus,  by  its 
use,  the  pyrometer  can  be  calibrated  very  easily. 

For  the  highest  accuracy,  the  tungsten  lamp  that  is  to  be  used  for 
calibration  purposes  should  be  standardized  with  an  optical  pyrometer, 
using  a  red  glass  that  is  the  same  as  that  on  the  pyrometer  to  be  com- 
pared, or  corrections  should  be  made  for  the  difference.  If  the  effective 
wave-lengths  are  known,  this  correction  can  easily  be  made  by  the 
method  outlined  below.  For  practical  purposes,  however,  if  similar  red 
glasses  are  used,  the  error  will  be  quite  small,  see  Table  7. 

Tungsten  filaments  have  been  found  to  depart  very  markedly  from 
Lambert's  cosine  law  in  their  radiation.9  To  avoid  error  due  to  this 
cause  care  must  always  be  taken  to  determine  the  temperature  of  circular 
filaments  by  measuring  the  brightness  of  the  central  part  of  the  filament. 
For  this  reason  the  pyrometer  filament  should  always  be  parallel  to  the 
background  filament.  This  of  course  requires  that  the  pyrometer 
filament  be  much  smaller  than  the  image  of  the  background  filament. 

PYROMETER  FILAMENTS 

Some  care  is  required  in  the  selection  of  the  pyrometer  filament. 
Carbon  filaments  are  quite  satisfactory  for  low  temperatures  but  they 
will  not  have  a  very  long  life  if  operated  at  a  very  high  temperature. 
Each  kind  of  pyrometer  filament  has  its  own  particular  field.10  For  some 
conditions  a  carbon  filament  may  be  the  better,  but  for  most  work  a 
tungsten  filament  should  be  used.  Tungsten  pyrometer  filaments  are 
just  as  good  as  carbon  filaments  and  have  a  long  life  if  not  oper- 
ated at  a  brightness  above  that  necessary  to  match  a  black  body  at 

» Worthing:  Astrophys.  JnL.  (1912)  36,  345.          "  Phys.  Rev.  [2]  (1914)  4,  165. 


W.    E.    FORSYTHE  303 

the  temperature  of  melting  palladium  (1828° JK.).  They  are  often 
constructed  with  a  small  bend  at  the  exact  point  where  the  filament  is  to 
be  observed.  A  small  pointer  is  also  sometimes  used  to  help  locate 
the  exact  point.  The  2^-mil  (0.063  mm.)  filaments  require  about 
0.46  amp.  to  apparently  match  in  brightness  the  black  body  at  the  tem- 
perature of  melting  palladium.  One  lamp  that  has  been  in  use  almost 
every  day  for  about  2  years,  when  first  calibrated  required  0.4573  amp. 
to  apparently  match  the  black  body  at  the  temperature  of  melting  palla- 
dium; after  about  2  years  of  use,  it  required  0.4578  amp.  This  difference 
in  current  corresponds  to  less  than  1.5°  C. 

The  statement  is  often  made  that  carbon-filament  pyrometer  lamps 
require  a  much  larger  change  in  current  for  a  given  change  in  brightness 
than  do  tungsten-filament  lamps.  To  test  this,  it  was  decided  to 
use  some  data  obtained  for  another  purpose.  The  ratio  of  the  currents 
through  a  number  of  carbon  and  tungsten  pyrometer  lamps  were  found 
when  these  pyrometer  filaments  had  been  matched  in  an  optical  pyrome- 
ter against  a  black  body,  first,  at  the  palladium  point,  and,  second,  at  the 
gold  point.  Several  different  carbon-filament  pyrometer  lamps  had  been 
calibrated  by  the  author  in  the  University  of  Wisconsin  and  several 
tungsten  lamps  had  been  calibrated  in  this  Laboratory.  The  ratio  of 
these  two  currents  for  all  the  tungsten  pyrometer  lamps  was  about  the 
same,  the  average  being  about  1.780.  The  carbon  lamps  used  were  from 
two  lots  obtained  at  different  times.  The  average  of  the  ratio  of  these  two 
currents  for  the  first  lot  was  the  same  as  the  ratio  for  the  tungsten  lamps; 
the  average  ratio  for  the  other  lot  was  1.857.  This  ratio  of  currents  was 
recently  tested  for  an  untreated  carbon  and  was  found  to  be  about  5  per 
cent,  greater.  The  cooling  due  to  the  conduction  at  the  ends  is  the  cause 
of  the  larger  ratio  for  the  carbon  at  very  low  temperatures  for 
short  filaments.  Whatever  advantage  the  carbon  filament  possesses 
on  account  of  a  larger  current  variation  for  a  given  brightness 
variation  disappears  if  sufficiently  sensitive  current-measuring  in- 
struments are  used,  is  small  in  any  case,  and  is  usually  far  outweighed 
by  other  advantages  possessed  by  the  tungsten  filament. 

A  criticism  that  has  been  made  concerning  the  disappearing-filament 
pyrometer  is  that  there  is  such  a  large  time-lag  between  the  current  and 
the  temperature  of  the  pyrometer  filament.  It  has  been  shown11  that 
for  a  2.5-mil  filament  it  would  require  somewhat  less  than  2  sec.  for  a 
cold  filament  to  reach  to  within  1°  of  full  brightness  after  the  current  is 
turned  on  for  a  maximum  brightness  corresponding  to  a  temperature  of 
1828°  K.  If,  however,  the  pyrometer  filament  is  already  heated  to 
within  about  50°  of  its  maximum  temperature  it  will  require  only  about 
0.6  sec.  to  reach  within  1°  of  the  maximum.  (For  a  lower  temperature, 

11  Some  unpublished  work  of  A.  G.  Worthing  of  this  Laboratory. 


304  THEORY   AND   ACCURACY   IN   OPTICAL   PYROMETRY 

this  lag  will  be  somewhat  greater.)     For  the  greater  part  of  the  work, 
this  small  time-lag  is  negligible. 

TEMPERATURE  OP  NON-BLACK  BODIES 

The  value  for  the  temperature  of  any  source  obtained  with  an  optical 
pyrometer  must  be  calculated  from  the  observed  brightness  for  a  par- 
ticular wave-length  interval.  The  only  body  for  which  this  relation  is 
accurately  known  is  the  black  body,  and  for  this  it  is  given  by  Wien's 
equation.  From  the  definition  of  a  black  body  and  Kirchhoff  s  law,  all 
other  bodies  that  owe  their  brightness  to  thermal  causes  alone  are  less 
bright  when  at  a  particular  temperature  than  a  black  body  at  the  same 
temperature.  Thus,  if  the  temperature  of  any  hot  non-black  body  is  cal- 
culated from  a  measurement  of  its  brightness,  as  though  it  were  a  black 
body,  values  that  are  lower  than  the  true  temperature  will  be  obtained. 
The  temperature  obtained,  however,  is  the  temperature  that  a  black 
body  must  have  in  order  to  have  the  same  brightness  for  the  particular 
wave-length  interval  as  the  body  being  investigated.  As  the  brightness 
thus  measured  corresponds  to  some  particular  wave-length  interval,  the 
temperature  obtained  corresponds  to  the  particular  wave-length.  The 
difference  between  the  true  temperature  and  the  temperature  thus 
obtained  varies  from  a  few  degrees,  for  such  a  substance  as  untreated 
carbon,  to  more  than  200°  C.  for  such  a  metal  as  polished  platinum  at  its 
melting  point. 

Wave-length  to  Which  Brightness  Temperature  Should  be  Ascribed. — 
If  a  screen  that  is  absolutely  monochromatic  is  used  before  the  eyepiece 
it  is  at  once  evident  to  what  wave-length  the  temperature  of  a  non- 
black  body  thus  measured  should  be  ascribed.  However,  if  a  red  glass 
is  used,  such  as  those  having  the  transmission  shown  in  Fig.  4,  some 
consideration  is  necessary. 

When  the  black-body-brightness  temperature  of  a  source  is  deter- 
mined with  an  optical  pyrometer  with  a  so-called  monochromatic  screen 
before  the  eyepiece,  what  is  really  measured  is  the  brightness  of  the 
source  through  the  screen.  The  value  of  the  brightness  thus  obtained 
would  correspond  to  a  certain  temperature  T  if  it  were  obtained  from 
measurements  of  a  black  body.  Therefore,  the  temperature  of  the 
source  is  to  be  called  a  brightness  temperature  S,  where  S  =  T. 

The  color  temperature  of  a  particular  source  has  been  defined  as  the 
temperature  of  a  black  body  that  has  the  same  distribution  of  energy  in 
the  visible  spectrum  as  the  source  under  consideration.  It  has  been 
found  experimentally  that  most  metals  when  heated  radiate  in  such  a 
manner  that  they  can  be  color-matched  against  a  black  body;  these 
color  matches  are  very  easily  and  accurately  made  with  an  ordinary 
photometer.  When  two  bodies  have  the  same  color  temperature,  it  is 
not  necessary  that  they  have  the  same  brightness  for  any  particular 
wave-length  interval. 


W.    E.    FORSYTHE  305 

The  brightness  temperature  S  must  be  ascribed  to  a  wave-length 
such  that  the  energy  emitted  by  a  black  body  per  unit  area  at  tempera- 
ture T  (  =  S)  ,  for  this  wave-length  will  equal  that  emitted  per  unit  area 
by  the  source  for  the  same  wave-length.  Thus  there  are  two  sources 
with  different  spectral  distributions  that  have  the  same  brightness  when 
observed  through  the  red  screen,  a  black  body  at  temperature  T  and  the 
source  being  studied  which  is  at  a  brightness  temperature  S.  Call  the 
color  temperature  of  the  source  studied  Tc.  As  these  two  distributions 
are  different  and  yet  the  sources  have  the  same  brightness,  the  curves 
representing  these  distributions  must  cross  if  they  are  plotted  with 
energy  emitted  per  unit  area  against  wave-length.  The  point  at  which 
these  two  curves  cross  evidently  gives  the  wave-length  to  which  the 
brightness  temperature  S  is  to  be  ascribed. 

This  brightness  temperature  is  to  be  ascribed  to  the  effective  wave- 
length of  the  red  glass  for  black-body  radiation  for  the  temperature 
interval  T  to  Tc.  The  effective  wave-length  \e  for  the  temperature 
interval  is  so  denned  that 


BT 


where  BT  and  BTc  represent  the  brightness  for  the  black  body  at  tempera- 
tures T  and  TV  through  the  screen  used  and  J(X!T)  and  J(\TC)  represent 
•the  energy  emitted  by  a  black  body  at  temperatures  T  and  Tc  for  the 
wave-length  interval  whose  center  is  at  \g. 

If  the  source  studied  is  considered,  it  will  be  seen,  from  the  definition 
of  color  temperature,  that  its  distribution  of  energy  corresponds  to  that 
of  a  black  body  at  Tc,  the  difference  being  that  each  ordinate  of  the 
curve  representing  the  black-body  distribution  at  temperature  Tc  bears 
a  constant  ratio  K  to  the  corresponding  ordinate  for  the  source  studied. 
Thus  the  actual  energy  distribution  of  the  source  being  investigated 

J(\TC} 
is  given  by  —  jr^~"     As  stated,  this  curve  and  the  one  representing  the 

distribution  of  a  black  body  at  the  temperature  T  will  cross  at  the  wave- 

J(\TC) 

length  where       „  e-  =  J(\T).     As  each  ordinate  of  the  curve  represent- 
j\. 

ing   the   distribution   of  energy  from  the  source  studied  is  a  certain 

fraction  ^  of  that  for  a  black-body  at  temperature  Tc,  the  brightness  will 
A 

be  reduced  the  same  amount.     Thus,  if  Bs  is  the  brightness  of  the  source 
studied, 


°r 


Ufrr.) 


L 

20 


(6) 


306  THEORY   AND   ACCURACY  IN   OPTICAL   PYROMETRY 

J(\TC) 
Thus  /(XT7)  and  —  j^-  are  equal  for  the  wave-length  \e,  in  other  words 

the  curves  representing  these  two  distributions  cross  at  this  point.  From 
this  it  follows  that  the  brightness  temperature  S  is  to  be  ascribed  to  the 
effective  wave-length  for  the  screen  used  for  the  temperature  interval 
of  a  black  body  from  T(=  S}  to  Te. 

Corrections  of  Brightness  Temperatures  to  a  Constant  Effective  Wave- 
length. —  As  the  brightness  temperatures  of  a  source  are  measured  using 
a  particular  screen  before  the  eyepiece,  there  will  be  a  variation  in  the 
wave-length  to  which  these  temperatures  are  to  be  ascribed.  .Sometimes 
it  is  desirable  to  know  the  brightness  temperature  over  quite  a  range  of 
temperatures  for  the  same  wave-length.  If  the  color  temperature  of 
the  source  is  known,  the  brightness  temperature  can  be  calculated  for 
any  wave-length  when  it  is  known  for  one  wave-length.  Thus  for  a 
source  at  a  color  temperature  Tc,  using  Wien's  equation  and  the  conditions 
that  hold  for  color  match,  the  following  relation  between  two  brightness 
temperatures  (Si  and  S2)  for  two  wave-lengths  (Xi  and  X2)  can  be  derived. 

* 


If  a  double  thickness  (6.8  mm.)  of  the  red  glass  known  as  Jena  Rotfilter 
No.  4512  (spectral  transmission  shown  by  curve  C,  Fig.  4),  is  used  before 
the  eyepiece  of  the  pyrometer,  this  correction  when  applied  to  the  bright-. 
ness  temperature  of  tungsten  will  be  small.  It  has  been  shown  that  the12 
effective  wave-length  of  this  red  glass  changes  from  0.6657/i,  for  the  range 
between  brightness  and  color  temperature  of  tungsten  at  a  brightness 
temperature  of  1600°  K.,  to  0.6626/i  for  this  range  for  a  brightness 
temperature  of  3000°  K.  If  the  brightness  temperatures  are  corrected 
to  a  wave-length  0.6657/i,  this  correction  will  amount  to  about  —  2°  K. 
at  a  brightness  temperature  of  3000°  K.;  for  most  work  when  using  -this 
screen,  this  correction  will  be  negligible. 

Objections  have  often  been  made  to  the  use  of  red-glass  screens  on 
the  ground  that  as  the  range  of  wave-length  transmitted  was  so  large, 
there  was  no  method  of  knowing  to  what  wave-length  the  resulting 
temperature  was  to  be  assigned.  If  the  effective  wave-length  of  the 
red  glass  used  is  known  for  different  temperature  ranges,  the  results 
can  be  treated  just  as  definitely  as  if  an  absolutely  monochromatic 
screen  were  used;  in  addition,  the  red  glass  has  the  added  advantage  of 
transmitting  enough  light  to  enable  very  accurate  brightness  comparisons 
to  be  made. 


If  an  optical  pyrometer  of  the  disappearing-filament  type  is  con- 
structed without  a  limiting  diaphragm  between  the  objective  lens  and  the 

12Loc.  cit. 


W.    E.    FORSYTHE  307 

pyrometer  lamp,  an  error  will  be  made  if  the  position  of  the  objective 
lens  is  changed,  even  though  there  is  a  limiting  diaphragm  between  the 
pyrometer  lamp  and  the  eyepiece.  In  other  words,  the  current  required 
through  the  pyrometer  filament  for  an  apparent  brightness  match  with  a 
particular  source  is  a  function  of  the  angle  that  the  cone  of  rays  from 
the  objective  lens  makes  at  the  pyrometer  filament.  This  has  been  found 
to  be  due  to  light  from  the  source  being  diffracted  around 13  the  pyrometer 
filament.  If  from  the  central  part  of  the  aperture  C,  Fig.  2,  a  region 
is  blocked  out  such  that,  from  a  consideration  of  geometrical  optics  only, 
none  of  the  light  from  the  background  in  the  immediate  neighborhood  of 
the  place  where  the  pyrometer  filament  is  seen  projected  can  enter  the 
aperture  at  E,  and  if  no  current  is  passed  through  the  pyrometer  filament, 
this  filament  can  be  seen  through  the  eyepiece  to  be  apparently  glowing 
where  it  crosses  the  background  image.  In  case  the  resolving  power  of 
the  eyepiece  is  sufficiently  great,  this  apparent  brightness  of  the  pyro- 
meter filament  is  seen  to  consist  of  two  bright  streaks  along  the  edge. 
Whether  or  not  the  axes  of  these  bright  streaks  lie  within  or  without  the 
boundaries  of  the  pyrometer  filaments  is  very  difficult  to  determine. 

It  has  often  been  assumed  that  the  pyrometer  filament  and  the  back- 
ground source  were  at  the  same  brightness  (except  for  a  small  difference 
due  to  lens  absorption,  etc.)  when  there  was  an  apparent  brightness 
match  between  them.  This  variation  in  brightness  has  been  studied14 
by  varying  the  angle  made  at  the  pyrometer  filament  by  the  cone  of 
rays  from  the  objective  lens,  having  at  the  same  time  a  definite  fixed  cone 
of  rays  from  both  the  pyrometer  filament  and  the  background  source  en- 
tering the  eyepiece.  For  very  small  values  of  these  angles  (about  0.006 
radian)  it  was  found  that  the  pyrometer  filament  would  be  actually  much 
brighter  than  the  background  when  there  was  an  apparent  brightness 
match.  This  effect  depended  on  the  size  of  the  pyrometer  filament,  being 
greater  for  a  small  filament.  For  somewhat  larger  angles  (0.02  radian)  a 
2^-mil  (0.063-mm.)  pyrometer  filament  is  actually  only  about  95  per 
cent,  as  bright  as  the  image  of  the  background  located  at  this  same 
point  for  an  apparent  brightness  match  as  observed  in  the  pyrometer. 

A  test  of  this  effect  was  recently  made  with  the  pyrometer  shown  in 
Fig.  3.  Two  test  pyrometer  lamps  were  used,  one  having  a  10-mil 
(0.25-mm.)  tungsten  wire  filament  and  the  other  a  tungsten  ribbon  fila- 
ment 13^  mm.  wide.  The  angle  that  the  cone  of  rays  made  at  the  pyrom- 
eter filament  was  such  as  would  be  subtended  by  a  diaphragm  with  a 
circular  opening  2  cm.  in  diameter  at  a  distance  of  50  cm.  These  pyro- 
meter filaments  were  matched  against  a  wide  tungsten  ribbon  background 
and  the  current  required  noted.  The  exact  point  on  each  pyrometer 
filament  and  on  the  background  was  indicated  by  means  of  a  small 


"Phys.  Rev.  [2]  (1914)  4,  163.  "Loc.  tit. 


308  THEORY    AND    ACCURACY    IN    OPTICAL    PYROMETRY 

pointer,  or  otherwise.  These  pyrometer  lamps  were  then  replaced  by  the 
regular  pyrometer  lamp  and  the  brightness  of  the  two  filaments  that  had 
been  used  as  pyrometer  filaments,  as  well  as  that  of  the  background,  was 
measured.  After  correcting  for  the  transmission  of  the  lamp  bulbs  and 
the  projecting  lens,  the  10-mil  tungsten  filament  was  found  to  be  only 
about  84  per  cent,  as  bright  as  the  background  and  the  ribbon  filament 
only  about  88  per  cent,  as  bright  as  the  background,  notwithstanding  the 
fact  that,  as  observed  through  the  pyrometer,  they  were  at  a  brightness 
match.  When  wire  as  large  as  10  mil  is  used  for  pyrometer  filaments, 
there  is  some  question  concerning  the  disappearance  due  to  the  variations 
from  Lambert's  .cosine  law.  There  is  not  much  doubt  that  the 
observer  matches  the  edges  of  such  a  filament  against  the  background 
without  any  great  consideration  of  the  central  brightness.  When  the 
brightness  of  the  filament  is  measured  in  the  regular  manner,  observa- 
tions are  generally  made  on  the  central  part.  There  may,  therefore,  be  2 
or  3  per  cent,  to  be  added  to  the  brightness  measured  for  the  wire  filament. 
These  results,  considering  the  difficulties,  agree  quite  well  with  the 
results  that  were  previously  found.  All  of  these  errors  can  be  avoided 
by  keeping  the  cone  of  rays  from  the  objective  lens  to  the  pyrometer 
filament  as  well  as  the  cone  of  rays  entering  the  eyepiece,  fixed  for  any 
particular  set  of  measurements.  This  can  readily  be  done  by  having 
fixed  limiting  diaphragms  between  both  the  objective  lens  and  the  pyrom- 
eter filament  and  between  the  pyrometer  filament  and  the  eyepiece. 

ACCURACY  TESTS 

To  test  out  the  accuracy  that  might  be  expected  in  the  use  of  the 
disappearing-filament  type  of  optical  pyrometer,  different  experiments 
were  made.  In  the  first  experiment, 15  readings  were  made  by  a  nunrber  of 
observers  with  no  experience  in  this  kind  of  work;  in  the  second  experi- 
ment readings  were  made  by  experienced  observers.  The  instrument 
used  in  each  case  was  the  laboratory  form  of  pyrometer  shown  in  Fig.  3. 
The  resistance  that  controls  the  current  through  the  pyrometer  filament 
was  so  chosen  that  the  sliding  contact  had  to  be  moved  quite  a  distance 
in  order  to  change  the  apparent  brightness  of  the  filament  by  an  appre- 
ciable amount.  The  current  was  measured  by  means  of  a  potentiometer. 

In  Table  1  are  given  the  results  of  the  first  experiment.  Observers 
1  and  2  were  high-school  graduates  with  several  months'  experience  as 
laboratory  assistants.  Observer  3  was  a  ntan  with  several  years'  experi- 
ence in  shop  work.  Observer  4  was  a  man  with  several  years'  experience 
in  a  lamp  factory.  Observers  5  and  6  were  girls*  from  the  lamp  factory ; 
No.  5  had  no  experience  with  this  kind  of  work  while  No.  6  had  had  ex- 
perience with  the  photometer. 

"  Gen.  Elec.  Rev.  (1917)  20,  749. 


W.    E.    FORSYTHE 


309 


TABLE    1. — Results   Obtained    With   a   Disappearing-filament    Type   of 
Pyrometer  By  Inexperienced  Observers 


Value  Obtained  for  Temperature, 
Observer                                    as  an  Average  of  Six  Readings, 
Degrees  K. 

Variation  of  Single 
Readings  from  Mean, 
Degrees  K. 

Standards  

.  1438 
1439 
1438 
1439 
1436 
1436 
1436 

1643 
1643 
1642 
1642 
1644 
1636 
1640 

4 
3 
2 
3 
5 
2 

1.  L.  C  

2.  H.  W  

3.  F.  G  

4.  E.  H  

5.  E.  W  

6.  L.  R.. 

The  table  shows  that  but  a  single  observer  made  an  error  greater  than 
3°  K.  in  the  temperature  as  obtained  from  the  average  of  six  readings. 
In  no  instance  was  a  value  of  temperature  obtained  from  a  single  reading 
that  differed  more  than  5°  from  the  mean  of  the  set  of  readings.  These 
results  are  thought  to  be  very  good  and  to  indicate  the  character  of  results 
that  could  be  obtained  with  this  form  of  pyrometer  in  industrial  works. 
The  pyrometer  used  probably  enabled  the  observers  to  make  much  more 
accurate  observations  than  is  possible  with  a  commercial  form  of  the 
instrument.  However,  even  a  commercial  instrument  could  be  so 
constructed  that  very  good  observations  could  be  obtained.  In  this 
work,  as  in  almost  all  work  depending  on  eye  observations,  a  small  amount 
of  training  makes  a  very  great  improvement  in  the  accurac}'  of  the  results. 

The  second  experiment16  was  more  extended  and  had  as  an  object  to 
test  out  the  constancy  of  readings  of  different  observers  when  using  a 
sectored  disk  of  low  transmission  or  a  dense  absorbing  glass  to  cut  down 
the  apparent  brightness  of  a  source  studied.  The  observers  who  made 
these  readings  obtained  very  nearly  the  same  readings  when  using  the 
pyrometer  directly;  that  is,  with  no  sector  or  absorbing  glass. 

In  this  experiment,  first  with  two  pieces  of  Jena  red  glass  No.  4512 
(spectral  transmission  shown  by  curve  C,  Fig.  4)  and  second  with  two 
pieces  of  Corning  red  50  per  cent,  (spectral  transmission  shown  by  curve 
B,  Fig.  4)  in  the  eyepiece  of  the  pyrometer,  readings  were  made  on  the 
apparent  brightness  of  a  particular  source  as  observed  through  a  rotating 
sector  with  two  1°  openings,  the  no vi weld  absorbing  glass  having  the 
spectral  transmission  shown  by  curve  C,  Fig.  7,  and  through  two  pieces  of 
the  Jena  absorbing  glass  having  the  spectral  transmission  shown  by  curve 
B,  Fig.  7.  The  source  used  was  a  15-mil  tungsten  lamp  operated  at  a 
color  temperature  of  2610°  K.  The  brightness  was  measured  in  terms  of 
the  current  through  the  pyrometer  filament  for  an  apparent  brightness 
match.  Four  observers  made  the  measurements,  three  of  whom  had  had 

16  Astrophys.  Jnl.  (1919)  237. 


310 


THEORY   AND   ACCURACY   IN    OPTICAL    PYROMETRY 


considerable  experience  with  this  kind  of  work.  Values  of  the  current 
through  the  pyrometer  filament  thus  obtained  are  given  in  Table  2. 
The  maximum  range  with  the  two  glasses  occurs  for  K.H.M.  and  W.E.F. 
for  the  noviweld  glass  when  the  Jena  red  No.  4512  was  used.  This 
amounted  to  about  1  per  cent,  in  brightness  and  to  less  than  5°  in  tempera- 
ture at  about  2500°  K. 


0.48        0.52       0.56        0.60        0.64       0.68        0.72       0.76 
Wave-length  =  IJ. 

FIG.  7. — SPECTRAL  TRANSMISSION  OF  VARIOUS  ABSORBING  GLASSES.  CURVE  B, 
JENA  ABSORBING  GLASS  1.5  MM.  THICK.  CURVE'  C,  NOVIWELD  OBTAINED  FROM 
CORNING  GLASS  WORKS;  SHADE  ABOUT  6.  CURVE  D,  LEEDS  &  NORTHRUP  ABSORBING 

GLASS  MADE  OF  PURPLE  AND  GREEN  GLASS. 

TABLE  2. — Results  Obtained  By  Experienced  Observers  Using  Different 
Red  Glasses  and  Different  Absorbing  Glasses 


Observer 

Red  Glass  Used 

Direct 

Current  Through  Pyrometer  Filament  for 
Apparent  Brightness  Match  With 

2°  Sector 

Noviweld 
Absorbing 
Glass 

Two  Jena 
Absorbing 
Glass 

I.A.V  

Jena  No.  4512 
Jena  No.  4512 
Jena  No.  4512 
Jena  No.  4512 
Corning  red 
Corning  red 
Corning  red 
Corning  red 

0.4343 
0.4343 
0.4344 
0.4343 

0.3358 
0.3361 
0.3361 
0.3358 
0.3380 
0.3380 
0.3380 
0.3378 

0.3804 

0.3807 
0.3803 
0.3805 
0.3784 
0.3785 
0.3783 
0.3784 

0.3547 
0.3546 
0.3546 
0.3547 

K.H.M    

W.E.F  

A.G.W  

I.A.V  

K.H.M  

W.E.F  

A.G.W  

The  visibility  curves  of  the  four  observers  are  quite  different.  Two  of 
the  observers  (I.A.V.  and  K.H.M.)  are  quite  blue  sensitive,  one  (W.E.F.) 
is  somewhat  red-sensitive,  and  the  other  (A.G.W.)  is  very  red  sensitive. 
The  values  of  the  visibility  extend  toward  the  red  and  only  to  wave- 
length 0.66/i.  In  this  work  the  visibility  much  beyond  this  point  must 
be  taken  into  consideration.  It  is  not  the  visibility  in  the  blue  end  of  the 


W.    E.    FORSYTHE  311 

spectrum  that  is  important  but  rather  the  relative  shapes  of  the  differ- 
ent visibility  curves  in  the  red  end.  In  some  other  work,  it  was  shown 
that  though  there  was  a  great  variation  in  the  values  given  by  the  indi- 
vidual observers  to  the  brightness  in  the  extreme  red  in  comparison  with 
those  in  the  central  part  of  the  spectrum,  the  relative  values  in  the  red 
end  do  not  vary  so  widely.  From  this,  it  is  to  be  expected  that  different 
observers  will  get  very  closely  the  same  values  of  brightness  if  they  are 
limited  to  the  extreme  red.  For  the  currents  given  in  Table  2,  a  change 
of  0.0005  amp.  corresponds  to  a  change  of  about  1  per  cent,  in  brightness 
of  the  background.  This  same  change  in  current  through  the  pyrometer 
filament  corresponds  to  about  3°  K.  in  temperature  at  about  2500°  K. 

Another  experiment17  shows  what  results  are  to  be  expected  with  the 
optical  pyrometer.  The  temperatures  of  several  tungsten  lamps  were 
very  carefully  measured  in  this  Laboratory  for  different  currents  and 
sent  to  the  Bureau  of  Standards,  the  Physical  Laboratory  of  the  University 
of  Wisconsin,  and  the  Research  Laboratory  of  the  General  Electric  Co., 
at  Schenectady,  where  the  temperatures  were  measured  for  the  same 
currents.  The  lamps  were  then  returned  to  this  Laboratory  for  a  second 
check  on  the  temperatures.  This  gave  an  intercomparison  of  the  tem- 
perature scales  that  are  in  use  in  the  different  laboratories. 

In  each  of  the  laboratories,  the  temperatures  were  measured  by  means 
of  a  disappearing-filament  optical  pyrometer  using  red  glass  as  the  mono- 
chromatic screen.  As  the  different  laboratories  used  a  red  glass  having  a 
slightly  different  effective  wave-length,  a  small  correction  was  necessary 
to  reduce  the  temperature  to  the  same  wave-length.  Such  a  correction 
would  have  been  unnecessary  if  the  sources  whose  temperatures  were 
measured  had  been  black  bodies.  A  summary  of  the  results,  using  the 
data  on  but  three  of  the  six  lamps,  is  given  in  Table  3;  the  data  obtained 
on  the  other  three  lamps  are  about  the  same. 

Of  the  lamps  used,  all  except  one  had  flat  filaments  about  3  cm.  long 
and  about  1^  mm.  wide.  The  exact  point  at  which  it  was  desired  to 
have  the  temperature  measured  was  indicated  either  by  a  pointer,  a  notch 
in  the  supporting  lead,  or  a  small  notch  in  the  filament  itself.  Three 
of  the  lamps  were  gas-filled  and  two  were  of  the  vacuum  type;  the  other 
lamp,  which  was  gas-filled,  had  a  20-mil  (0.5-mm.)  filament  in  the  shape 
of  a  hairpin  loop.  As  the  loop  was  rather  sharp,  the  exact  point  at  which 
it  was  desired  to  have  the  temperature  measured  was  easily  indicated. 
This  Laboratory  is  planning  to  send  similar  lamps  to  several  other  labora- 
tories for  like  comparisons  in  the  near  future. 

In  Fig.  8  is  shown  a  picture  of  three  of  the  lamps  used  in  this  tempera- 
ture intercomparison.  The  flat-filament  lamps  have  been  quite  useful 
in  intercomparing  the  calibration  of  pyrometers  and  in  calibrating  a 
pyrometer.  Such  a  lamp  has  been  used  as  a  source  for  work  with  the 

17  Phys.  Rev.  [2\  (1918)  11,  139. 


312 


THEORY    AND   ACCURACY   IN    OPTICAL   PYROMETRY 


FIG.  8. — LAMPS   USED   IN   INTERCOMPARISON   OP  TEMPERATURE   SCALES. 
TABLE  3. — Results  of  Inter  comparison  of  Temperature  Scales 


-  14350/1  deg.| 


0.665M 


Melting  Point  of  Au  =  1336° 
(Pd  =  1828°  K.) 


Lamp      R£ 

Labor 
Oct.  1 
Degr 

i                       Research 
.arrJi               Laborator 
General 
7    unfi           Electric  C 
^sK           (Schenectac 
Degrees  K 

Nela 
Research 
i..            Laboratory 
'°;          Nov.  28,  191 
Degrees  K. 

Bureau 
of 
3          Standards 

Nela 
Research 
Laboratory 
Apr.  2,  1917 
Degrees  K. 

T-1QB 

1429 

1431 

1427 

T-30C               1813                  1813 

1813 

1814 

1813 

T-30C 

..    .                   2307 

2304 

2302 

2303 

T-30C              27 

56                  2752 

2752 

2762 

2752 

Lamp 

Nela  Research 
Laboratory 
Mar.  27,  1916 
Degrees  K. 

Physical  La 
University  of 

C.  E.  M. 
Degrees  K. 

joratory 
Wisconsin 

G.  R.  G. 
Degrees  K. 

Nela  Research 
Laboratory 
July  14.  1917 
Degrees  K. 

T-17-c 

1810 

I 

1813 

1816 

1810 

2193 

2197 

2202 

2196 

2499 

2506 

2516 

2497 

W.    E.    FORSYTHE  313 

spectroscope.  The  high  intensity  attainable,  together  with  its  size  and 
the  fact  that  it  will  remain  so  constant,  makes  it  a  very  good  source  for 
this  kind  of  work. 

The  table  shows  that  the  results  by  the  Bureau  of  Standards,  Re- 
search Laboratory  of  the  General  Electric  Co.,  Schenectady,  and  Nela 
Research  Laboratory  agree  very  well  for  the  entire  range  studied.  The 
results  obtained  in  the  Physical  Laboratory  of  the  University  of  Wisconsin 
do  not  agree  quite  so  well,  but  even  here  the  differences  are  quite  small. 

ABSORBING  SCREENS  FOR  OPTICAL  PYROMETRY 

A  sector  that  has  been  very  carefully  calibrated,  if  used  properly,  is 
without  doubt  the  best  means  that  can  be  used  for  cutting  down  the  ap- 
parent intensity  of  a  source  that  is  being  studied.  In  a  research  or  a 
standardizing  laboratory,  the  rotating  sector  is  thus  a  very  valuable 
instrument.  However,  for  commercial  work  where  a  portable  instru- 
ment is  desired,  a  rotating  sector  adds  to  its  size  and  makes  necessary 
another  source  of  power  to  drive  the  sector,  so  for  a  commercial  pyrome- 
ter absorbing  glasses  are  generally  used,  and  when  properly  calibrated 
and  properly  used  are  very  satisfactory. 

When  it  is  necessary  to  use  glass  absorbing  screens  to  reduce  the 
apparent  brightness  of  the  source  studied,  the  main  requirement  is  to 
have  a  screen  that  approximates  a  neutral-tint  screen  sufficiently  well  to 
enable  comparisons  in  brightness  to  be  made  by  different  observers  with 
the  same  results.  The  degree  to  which  it  is  necessary  for  the  absorbing 
screen  to  have  a  spectral  transmission  independent  of  the  wave-length, 
depends  on  the  so-called  monochromatic  glass  used  in  the  eyepiece.  It 
is  quite  evident  that  if  this  eyepiece  glass  is  absolutely  monochromatic, 
any  absorbing  glass  will  answer. 

In  Fig.  7  are  shown  the  spectral  transmissions  of  a  piece  of  noviweld 
(curve  C)  and  of  a  piece  of  a  Jena  absorbing  glass  (curve  B).  Either  of 
these  glasses  is  nearly  enough  neutral  tint  for  use  with  the  red  glasses 
having  transmission  curves  shown  by  B,  C,  and  D  in  Fig.  4.  The  novi- 
weld absorbing  glass  and  the  samples  of  Corning  high-transmission  red 
glass  were  obtained  from  Mr.  F.  P.  Gage  of  the  Corning  Glass  Works, 
Corning,  New  York.  This  absorbing  glass  is  made  in  different  shades  with 
transmissions,  when  used  in  connection  with  red  glass,  ranging  from  less 
than  1  per  cent,  to  several  per  cent. 

If  a  red  glass  is  used  in  the  eyepiece,  by  total  transmission  for  a  par- 
ticular temperature  is  meant  the  ratio  of  the  brightness  of  the  source 
observed  through  both  the  red  glass  and  the  black  glass,  to  the  brightness 
of  the  same  source  observed  through  the  red  glass  alone.  Without  a  red 
glass,  using  the  entire  visible  spectrum,  it  is  generally  very  hard  to  make 
such  measurements  owing  to  the  color  differences  introduced  by  even  the 


314 


THEORY   AND    ACCURACY    IN    OPTICAL    PYROMETRY 


best  absorbing  glasses,  but  with  a  good  red  glass  in  the  eyepiece,  such 
transmission  measurements  can  be  made  easily. 

The  total  transmission  of  the  absorbing  glass,  when  used  with  a  red 
glass,  can  be  calculated  for  any  black-body  distribution  by  the  following 
formula,  taken  from  Preston's  "Theory  of  Light": 


(8) 


where  Jxd\  =  black-body  energy  for  interval  X  to  X  +  d\.  Fx  =  visi- 
bility, t'B  and  t'B  =  spectral  transmission  of  red  and  absorbing  glasses, 
respectively.  It  is  very  evident  that  if  the  spectral  transmission  of 


1500 


3000 


2000  2500 

Temperature  in  degrees  K 

FIG.  9. — TOTAL  TRANSMISSION  OF  ABSORBING  GLASSES,  AS  A  FUNCTION  OF  TEMPERA- 
TURE WHEN  USED  WITH  RED  GLASS  No.  4512 5.8  MM.  THICK.       CURVE  A,  TWO  PIECES 

JENA  ABSORBING  GLASS.    B,  ONE  PIECE  JENA  ABSORBING  GLASS.     C,  NOVIWELD  GLASS 
FROM  CORNING  GLASS  WORKS.     CURVES  DRAWN  THROUGH  POINTS  CALCULATED  FROM 


„ 

EQUATION  TB   = 


d\  • 


CROSSES  REPRESENT  VALUES  OF  TRANSMISSION  OB- 


TAINED WITH  OPTICAL  PYROMETER. 

the  absorbing  glass  is  different  for  different  wave-lengths,  the  total 
transmission  will  be  a  function  of  the  temperature  of  the  source  under 
investigation. 

In  Fig.  9  is  shown,  as  a  function  of  the  temperature  of  the  source,  the 
total  transmission  for  red  light  of  the  absorbing  glasses  having  the  spec- 
tral transmission  given  by  curves  B  and  C,  Fig.  7.  The  measured  points 
were  determined  by  the  author  and  the  calculated  values  were  obtained 
by  means  of  equation  8  by  using  an  average  visibility  curve18  for  this 
spectral  region.  Transmission  values  were  also  calculated,  using  the 
writer's  visibility  curve.  Values  thus  obtained,  using  the  two  different 

"  Astropkys.  Jnl.  (1918)  48,  87. 


W.    E.    FORSYTHE  315 

visibility  curves,  differ  from  each  other  by  only  a  small  fraction  of  a  per 
cent. 

There  has  been  some  question19  as  to  the  effect  on  the  effective  wave- 
length of  a  red  glass  for  a  certain  temperature  interval  due  to  using  with 
it  an  absorbing  glass  that  is  not  neutral  tint.  It  has  been  stated  several 
times  that  the  effective  wave-length  that  should  be  used  is  the  effective 
wave-length  that  would  be  obtained  by  substituting  in  equation  for  the 
spectral  transmission  of  the  red  glass  the  product  of  the  spectral  trans- 
mission of  the  red  glass  and  the  absorbing  glass,  that  is,  the  spectral  trans- 
mission of  the  two  glasses  together.  In  what  follows,  it  is  shown  that 
such  is  not  the  case  but  that  the  same  effective  wave-length  is  to  be  used 
with  an  absorbing  glass  as  is  used  with  a  sector  of  the  same  transmission. 

Suppose  that  using  the  same  red  glass  in  both  cases,  a  sector  with  a 
transmission  Ts  were  found  such  that  the  brightness  observed  through 
the  black  glass  would  equal  that  observed  through  the  sector  (i.e.,  sector 
and  glass  have  same  transmission).  Then 


VJRtrsd\  =  ts    AF/ftdX  (9) 

Since  the  brightness  is  measured  in  terms  of  the  current  through  the 
pyrometer  filament,  this  current  will  be  the  same  in  the  two  cases.  This 
means  that  in  both  cases  the  temperature  Tz  that  is  being  determined 
must  be  calculated  from  the  same  initial  temperature. 

The  question  to  be  considered  is:  What  effective  wave-length  is  to 
be  used  in  calculating  the  temperature  of  the  source  having  the  bright- 
ness thus  measured?  When  the  brightness  is  measured  by  using  the 
rotating  sector,  the  temperature  TZ  is  calculated  from  the  transmission 
of  the  sector  and  T\  the  temperature  corresponding  to  the  pyrometer 
reading  when  no  sector  is  used.  For  this  calculation,  as  was  previously 
pointed  out,  the  following  formula  derived  from  Wien's  equation  is  used. 

JL^        1    =  Xe  log  Tt 
Tz      T\        c2loge 

In  this  expression  Xe  is  the  ordinary  effective  wave-length  for  the  red 
glass  for  the  temperature  interval  TI  to  T2,  and  is  defined  as  the  wave- 
length such  that  the  ratio  of  the  radiation  intensities  for  the  temperature 
interval  for  this  wave-length  shall  equal  the  ratio  of  the  integral  lumi- 
nosities through  the  screen  used. 

When  the  brightness  is  measured  by  using  an  absorbing  glass,  the 
temperature  T2  must  be  calculated,  using  the  same  formula,  from  the  trans- 
mission of  the  absorbing  glass,  and  Tl}  the  temperature  corresponding  to 
the  pyrometer  current  when  no  absorbing  glass  is  used.  As  the  trans- 
mission of  the  absorbing  glass  is  equal  to  that  of  the  sector  and  the  two 


19  Foote:  U.  S.  Bureau  of  Standards  Bull.  12  (1915-16)  483. 


316  THEORY   AND   ACCURACY   IN    OPTICAL   PYROMETRY 

temperatures  are  the  same  in  both  cases,  the  other  unknown,  that  is  the 
effective  wave-length,  must  be  the  same.     That  is,  since  the  transmission 
of  the  absorbing  glass  given  by  equation  8  is  the  same  as  that  obtained 
experimentally  by  comparing  its  transmission  with  that  of  a  sector,  the 
same  effective  wave-length  of  the  red  glass  is  to  be  used  with  both  the 
absorbing  glass  and  a  sector  having  the  same  transmission.     Thus,  to 
calculate  the  extrapolated  brightness  temperature  of  a  source  whose 
brightness  temperature  is  measured,  using  an  absorbing  glass,  it  is  neces- 
sary to  know  the  transmission  of  the  glass  as  a  function  of  the  temperature 
of  the  source  studied,  and  also  the  ordinary  effective  wave-length  for 
the  red  glass  used.     In  calculating  the  extrapolated  temperature  when 
using  a  sector  disk,  it  is  necessary  to  know  this  temperature  approximately 
in  order  to  find  the  effective  wave-length  for  the  interval.     The  calcula- 
tion is,  therefore,  one  of  successive  approximation.     When  a  so-called 
neutral-tint  glass  is  used,  additional  care  is  required  because  both  the  effect- 
ive wave-length  and  the  transmission  depend  on  the  temperature  reached. 
Attempts  have  been  made  to  obtain  an  absorbing  glass  that  is  strictly 
neutral  tint  or  even  one  that  has  such  a  transmission  as  to  correct  for 
the  change  in  effective  wave-length  of  the  red  glass  used.     Such  a  glass 
would  probably  be  very  nice  but  it  is  not  necessary.     What  is  wanted  is 
a  glass  that  will  permit  comparisons  of  brightnesses  to  be  made  by  dif- 
ferent observers  with  the  same  result.     If  a  good  red  glass  is  used,  suitable 
absorbing  glasses  can  easily  be  found.     The  same  thing  might  be  said 
about  the  red  glasses.     Many  attempts  have  been  made  to  obtain  abso- 
lutely monochromatic  screens  for  optical  pyrometry.     This  is  very  nice 
for  some  purposes  but  is  not  necessary,  in  general,  and  such  screens  have 
the  disadvantage  of  not  transmitting  enough  light  to  permit  of  accurate 
brightness  comparisons  at  low  temperatures.     A  good  red  glass  can 
easily  be  obtained  that  transmits  enough  light  to  permit  brightness 
comparisons  at  low  temperatures  and  at  the  same  time  is  sufficiently 
monochromatic  to  enable  different  observers  to  obtain  the  same  results, 
even  under  the  unfavorable  conditions  existing  when  the  comparison 
source  and  the  source  studied  are  quite  different  in  temperature.     In 
addition  to  this,  if  the  effective  wave-length  of  the  red  glass  is  known,  all 
results  can,  in  general,  be  readily  reduced  to  the  condition  for  a  common 
wave-length. 

Effect  of  Change  in  Temperature  of  Absorbing  Glass  on  its  Transmission. 
— As  the  spectral  transmission  of  the  red  glass  showed  such  a  marked 
change  with  a  change  in  its  temperature,  it  was  thought  worth  while  to 
investigate  the  transmission  of  the  absorbing  glass  as  a  function  of 
temperature.  Accordingly  a  heater  was  built  and  so  mounted  that 
different  absorbing  glasses  could  be  heated  in  position  to  a  temperature 
of  about  200°  C.  Their  transmissions  were  then  measured  as  recorded 
in  Table  4.  These  transmissions  correspond  to  a  color  temperature  of 


W.    E.    FORSYTHE  317 

TABLE  4. — Transmission  of  Absorbing  Glasses  at  Different  Temperatures 
When  Used  with  Red  Glass  No,  4512,  5.8  mm.  Thick 


Temperature,  Degrees  C. 

Noviweld  Absorbing  Glass,                   Jena  Absorbing  Glass, 
Per  Cent.                                              Per  Cent. 

20 
102 
200 

1.70 
1.55 
1.39 

8.96 
8  90 
8.87 

the  source  of  2380°  K.  The  Jena  glass  has  the  spectral  transmission 
shown  by  curve  B,  Fig.  7.  The  noviweld  is  a  piece  of  shade  5,  which 
was  obtained  somewhat  later  than  that  used  in  the  preceding  work. 
These  two  pieces  of  noviweld  probably  have  somewhat  the  same  spectral 
transmission.  The  Jena  absorbing  glass  shows  but  a  very  small  change 
in  transmission  due  to  a  change  in  its  temperature.  If  the  spectral 
transmission  of  this  glass  behaves  in  the  same  manner  as  that  of  the 
red  glass  discussed  above,  this  is  what  would  be  expected  from  the 
shape  of  the  spectral  transmission  curve. 

The  change  in  transmission  of  the  noviweld  absorbing  glass  is  such  as 
to  indicate  that  the  spectral  transmission  curve  has  shifted  in  the  same 
direction  as  that  shown  in  Fig.  6  for  the  red  glass.  No  attempt  has  been 
made  to  investigate  the  effect  of  this  temperature  shift  on  the  transmission 
of  the  glass  as  a  function  of  the  temperature  of  the  source  studied.  The 
relative  changes  would  probably  be  about  the  same  as  those  shown  in  the 
table.  The  changes  in  transmission  in  the  Jena  glass  are  so  small  as  to  be 
negligible  for  any  temperature  change  met  with  in  practice.  The  changes 
in  the  noviweld,  however,  are  enough  to  cause  a  small  error  due  to  the 
temperature  changes  met  with  in  practice.  For  a  glass  of  this  kind 
calibrated  at  a  temperature  of  20°  C.  and  used  at  a  temperature  of  30°  C. 
in  extrapolating  from  1800°  K.  to  2400°  K.  and  3000°  K.,  the  errors  would 
be  respectively  +7.5°  K.  and  +11.5°  K. 

GENERAL  NOTES 

If  an  optical  pyrometer,  as  shown  in  Fig.  3,  is  constructed  so  as  to 
transmit  sufficient  light  to  enable  temperatures  to  be  measured  as  low 
as  1000°  K.,  this  pyrometer  will  transmit  too  much  light  for  comfort  at 
high  temperatures.  The  diaphragm  before  the  eyepiece  telescope  at  E 
can  be  constructed  as  shown  so  as  to  have  several  openings  of  various 
sizes.  For  a  low  temperature,  the  larger  opening  is  to  be  used,  thus 
transmitting  more  light,  while  for  a  higher  temperature,  a  smaller  opening 
should  be"  used.  In  this  manner  the  same  instrument  can  be  used  over  a 
wide  range  without  discomfort. 

If  too  large  an  opening  is  used  before  the  telescope  eyepiece,  the 


318  THEORY    AND    ACCURACY   IN    OPTICAL    PYROMETRY 

pyrometer  filament  will  not  disappear  against  the  image  of  the  back- 
ground but  there  will  be  dark  streaks  along  the  edges  of  the  pyrometer 
filament.  If  these  dark  streaks  are  too  prominent,  it  is  impossible  to 
make  consistent  settings.  The  resolving  power  of  whatever  eyepiece  is 
used  should  be  so  adjusted  that  the  pyrometer  filament  disappears  as  a 
whole,  that  is  such  that  one  does  not  see  either  dark  or  bright  streaks 
along  the  edge  of  the  pyrometer  filament.  To  see  what  error  would 
result  due  to  such  dark  streaks,  the  diaphragms  and  the  pyrometer 
filament  were  so  chosen  as  to  give  very  marked  dark  streaks  along 
•each  edge  of  the  pyrometer  filament.  A  4-mil  (0.1-mm.)  pyrometer 
filament  was  used.  The  arrangement  of  the  different  parts  was  as  is 
shown  in  Fig.  2,  except  that  the  opening  in  the  diaphragm  before  the 
eyepiece  telescope  was  2  cm.  in  diameter.  The  results,  in  terms  of  the 
current  through  the  pyrometer  filament  for  a  brightness  match  with  a 
15-mil  (0.38-mm.)  tungsten  lamp  at  a  temperature  of  1735°  K.,  are  given 
in  Table  5.  As  everything  was  kept  constant  during  the  three  days 
these  readings  were  made  these  variations  are  errors  due  to  a  change  in 
the  criterion  for  a  brightness  match.  For  the  value  of  the  current  given 
in  Table  5,  a  change  of  about  0.0009  amp.  corresponds  to  1°  in  temperature. 

TABLE  5. — Results  Obtained  by  Experienced  Observers  Using  Pyrometer 
with  Dark  Streaks  along  Edges  of  Pyrometer  Filament 

Day  A.G.W.  K.H.M.  W.E.F. 

Current,  in  amperes,  through  pyrometer  filament 


! 

0.9162 

0.9124 

0.9152 

2 

0.9187 

0.9120 

0.9179 

3 

0.9125 

0  9169 

Average  

0  9174 

0  9123 

0  9168 

The  greatest  error  was  made  by  W.E.F.     In  this  case  the  range  was 
about  2°  K. 

It  is  thus  seen  that  for  bad  conditions,  as  to  disappearance,  the  error 
for  experienced  observers  though  small  is  much  larger  than  occurs  when 
the  conditions  are  good.  The  results  given  in  Table  2  show  that  when 
the  conditions  are  good  these  three  observers  get  practically  the  same 
reading.  However,  in  this  case  the  differences  are  quite  large.  If  for 
any  reason  it  is  necessary  to  use  a  large  resolving  power  eyepiece,  good 
disappearance  can  be  obtained  by  increasing  the  size  of  the  cone  of  rays 
that  reach  the  pyrometer  filament  from  the  objective  lens.  If  this  is 
pushed  too  far,  an  objective  lens  with  a  very  large  aperture  is  required. 
If  the  light  is  too  intense  for  comfort,  it  can  be  cut  down  by  using  one  or 
more  additional  red  glasses  before  the  eyepiece.  If  two  red  glasses  are 


W.    E.    FORSYTHE  319 

used  in  the  eyepiece,  the  addition  of  a  third  red  glass  will  reduce  the 
apparent  intensity  of  the  image  by  about  50  per  cent.  If  more  light  is 
desired  for  sources  at  lower  temperatures,  it  is  often  quite  a  help  to  re- 
move one  of  the  two  red  glasses  that  are  being  used.  If  two  red  glasses 
are  being  used  and  one  of  them  is  removed,  the  brightness  of  the  image 
observed  will  appear  about  twice  what  it  did  with  two  red  glasses.  If 
no  sector  or  absorbing  glass  is  used  with  the  pyrometer  there  will  be 
very  little  effect  on  temperature  measurements  if  the  number  of  red 
glasses  in  the  eyepiece  is  changed.  If  a  sector  or  absorbing  glass  is 
used  corrections  will  have  to  be  made  for  the  change  in  effective  wave- 
length for  the  number  of  red  glasses  that  are  used. 

Polarization. — To  test  out  the  effect  of  polarization  with  this  type  of 
pyrometer,  a  large  nicol  was  mounted  directly  in  front  of  the  pyrometer 
lamp  and  readings  made  with  the  position  of  the  nicol  varied  with 
respect  to  the  pyrometer  filament.  It  was  found  that,  with  a  red  glass 
before  the  eyepiece  of  the  pyrometer,  the .  apparent  brightness  of  the 
background  (a  black  body)  was  about  1  per  cent,  more  when  the  nicol 
was  so  set  that  the  transmitted  light  was  polarized  in  a  plane  at  right 
angles  to  the  pyrometer  filament  than  with  nicol  turned  through  90°. 
Since  the  source  is  known  to  be  free  from  polarization,  this  shows  that 
the  effect  of  polarization  is  almost  negligible  even  when  all  the  light  is 
polarized.  From  the  work  on  diffraction20  around  the  pyrometer  fila- 
ment already  referred  to,  a  small  difference  would  be  expected  even  with 
this  instrument. 

Position  of  Rotating  Sector. — If  the  rotating  sector  is  used  to  cut 
down  the  apparent  intensity  of  the  background,  care  must  be  taken  as  to 
the  location  of  the  sector.  There  is  a  very  marked  difference  in  the 
results  of  temperature  measurements,  depending  on  whether  the  sector 
is  located  near  the  objective  lens  or  as  near  as  possible  to  the  pyrometer 
lamp.  There  is  also  a  difference  depending  on  the  relative  position  of 
the  openings  in  the  sector  and  the  source,  providing  the  source  is  a  lamp 
filament.  If  a  sector  of  small  transmission  is  mounted  near  the  lens  and 
so  placed  that  the  openings  of  the  sector  are  parallel  to  the  axis  of  the 
background  filament  when  the  sector  is  passing  across  the  center  of  the 
lens,  the  definition  will  be  very  bad,  while  if  the  openings  of  the  sector 
are  turned  through  90°,  so  that  they  are  perpendicular  to  the  axis  of  the 
filament,  the  definition  will  be  quite  good,  but  not  as  good  as  if  the  sector 
is  located  near  the  pyrometer  lamp,  see  Fig.  10.  When  the  rotating  sector 
is  located  near  the  pyrometer  lamp,  the  definition  is  good  and  practically 
independent  of  the  position  of  the  opening  of  the  sector.  If  a  very 
large  source  is  used  no  such  effect  is  noted.  Using  a  pyrometer  cali- 
brated against  such  a  large  background,  and  thus  independent  of  the 
position  of  the  sector  to  measure  the  brightness  temperature  of  a  small 

™Phys.  Rev.  [2]  (1914)  4,  163. 


320 


THEORY   AND    ACCURACY   IN    OPTICAL    PYROMETRY 


tungsten  filament,  large  variations  in  temperature  were  found  when  differ- 
ent sectors  were  used  near  the  objective  lens.  No  such  differences  were 
found  when  the  sector  was  located  near  the  pyrometer  filament. 

In  Table  6  are  given  results  of  a  test  showing  the  effect  of  the  position 
of  the  sector.  A  15-mil  (0.381-mm.)  tungsten  lamp  operated  at  a  bright- 
ness temperature  of  about  2275°  K.  was  used  as  a  background  and  read- 
ings were  made  on  the  current  through  a  2^-mil  (0.063-mm.)  tungsten 
pyrometer  filament,  for  an  apparent  brightness  match  with  a  sector 
having'two  1°  openings.  From  the  table  it  can  be  seen  that  the  position 
of  a  sector  of  this  size  can  cause  an  error  of  about  14°  K.  for  this  condition 
if  care  is  not  taken  as  to  its  location.  When  a  sector  is  used,  it  should 
be  rotated  so  fast  that  no  flicker  is  noticeable.  Not  only  is  an  error  apt 
to  be  made  if  the  sector  is  not  rotating  fast  enough,  but  the  flicker  is  very 
bothersome  in  making  accurate  brightness  comparisons. 


a  b 

FIG.  10. — APPEARANCE  OP  IMAGE  OP  SPIRAL  TUNGSTEN  FILAMENT  WHEN  ROTATING 
SECTOR  WITH  TWO  1°  OPENINGS  IS  MOUNTED  NEAR  LENSJ  a,  WITH  OPENINGS  OP  SECTOR 
PARALLEL  TO  AXIS  OP  COIL,  THAT  IS,  PERPENDICULAR  TO  INDIVIDUAL  TURNS,  WHEN 
SECTOR  IS  PASSING  IN  FRONT  OP  CENTER  OF  LENSJ  b,  WITH  OPENINGS  OF  SECTOR  PER- 
PENDICULAR TO  AXIS  OF  COIL  WHEN  SECTOR  IS  PASSING  IN  FRONT  OF  CENTER^  Op'LENS. 

TABLE  6.— Errors  in  Temperature  Measurements  Due  to  Improper  Loca- 
tion of  Sector] 


Position  of  2°  Sector 

* 

Near  Lens 

Near  Pyrometer  Lamp 

Opening  of 
Sector  Parallel 
to  Background 
Filament 

Opening  of 
Sector  Per- 
pendicular to 
Background 
Filament 

Opening  of 
Sector  Parallel 
to  Background 
Filament 

Opening  of 
Sector  Per- 
pendicular to 
Background 
Filament 

Current,  in  amperes, 
through  pyrometer  fila- 
ment for  brightness 
match  

0.3332 
0.9390 

2263 

0.3354 
0.9950 

2275 

0.3357 
1.0000 

2277 

0.3357 
1.0000 

2277 

Apparent  relative  bright- 
ness   

Temperature  of  back- 
ground for  these  read- 
ings, in  degrees  K  

Errors  Due  to  Various  Causes. — In  Table  7  is  given  the  variation  in 
extrapolated  temperature  due  to  a  variation  in  initial  temperature,  in 


W.    E.    FORSYTHE 


321 


effective  wave-length,  in  transmission  of  absorbing  glass  or  transmission 
of  sector,  and  in  current  through  pyrometer  filament.  First  is  given  the 
change  in  the  temperature  due  to  one  percentage  variation  of  each  and 
then  some  other  possible  variation.  An  inspection  of  this  table  will 
show  that  in  extrapolated  temperatures  quite  an  error  is  allowed  in  the 
effective  wave-length  or  the  transmission  of  the  sector  or  of  absorbing 
glass  without  any  great  error  in  the  final  results.  However,  any  error 
in  calibrating  at  the  initial  temperature  will  cause  a  much  larger  error  in 
the  final  result. 

If  a  tungsten  lamp  is  used  as  a  background  to  standardize  pyrometer 
lamps,  for  the  highest  accuracy,  either  the  same  kind  of  red  glass  must 
be  used  in  calibrating  the  standard  as  is  used  with  the  pyrometer  being 
calibrated,  or  correction  must  be  made  for  the  difference.  The  example 
given  in  the  table  corresponds  to  the  difference  between  two  glasses 
having  the  spectral  transmissions  shown  by  curves  A  and  C,  Fig.  4. 

TABLE  7.— Changes  in  Temperature  of  2400°,  and  3000°  K.  Extrapolated 

from  1800°  K.  as  Initial  Temperature,  Using  Wien's 

Equation,  Due  toVarious  Changes 


Variation  Leading  to  Error 

Percentage  Change 

Actual  Change, 
Degrees  K. 

1 
1800 

2400       3000 

1 

1800 

2400 

3000 

Change  of  1  per  cent,  in  initial  temperature  .  . 
Change  of  3°  K.  in  initial  temperature   

1.0 

1.30 

1.70 

18.0 
3.0 

3.5 
9.0 

32:0 
5.0 

8.0 
.1.2 

2.4 

2.7 
16.0 

50.0 
8.0 

20.0 
3.0 

7.5 

4.2 
25.0 

Using  a  wave-length  that  is  1  per  cent,  in 
error  

0.30 
0.05 

0.10 

0.70 
0.10 

0.30 

0.001/i  error  in  wave-length                       .    . 

If  in  extrapolating  the  X«  of  red  glass  betwee*n 
1300°  and  1800°  K.  is  used,  see  Fig.  5  

Calibrating     pyrometer     filament     against 
tungsten    lamp    as    background  that  vjfis 
standardized  with  a  red  glass  different  from 
one  used  in  pyrometer  being  calibrated. 
Suppose  X«  to  change  from  0.665/i  to  0.650/1. 
Error  of  1  per  cent,  in  value  used  for  trans- 
mission of  sector  or  absorbing  glass  

0.11 
0.70 

0.14 
0.80 

Variation  of  1  per  cent,  in  current  through 
2/^-mil  pyrometer  filament  

0.5 

If  with  the  use  of  sectors  or  otherwise  the  pyrometer  is  calibrated  to 
give  relative  brightness  instead  of  temperature,  the  pyrometer  is  quite 
valuable  for  measuring  the  relative  brightness  of  different  sources.  The 
pyrometer,  thus  calibrated,  can  also  be  used  to  measure  the  transmission 
of  different  glasses  as,  for  instance,  the  transmission  of  the  lamp  bulb 
when  an  attempt  is  made  to  get  the  actual  temperature  of  a  filament. 


322  THEORY   AND   ACCURACY   IN    OPTICAL   PYROMETRY 

These  transmissions  and  relative  brightnesses  will  correspond  to  the 
monochromatic  glass  used  in  the  eyepiece. 

This  type  of  pyrometer  possesses  several  advantages  over  other  forms. 
In  the  first  place,  the  observer  is  able  to  see  the  object  whose  temperature 
is  being  measured  directly  through  the  pyrometer,  the  same  as  through 
a  telescope.  It  is  hard  to  overestimate  this  advantage.  Often  it  is 
very  desirable  to  measure  the  temperature  of  a  particular  point  of  an 
extended  body,  as  for  instance,  a  mass  of  molten  iron  in  the  furnace  or 
a  particular  spot  on  an  ingot  that  is  being  rolled.  This  can  be  easily 
done  with  this  pyrometer  while  it  is  very  difficult  with  most  other  forms. 
For  this  reason,  this  form  of  pyrometer  gives  the  temperature  of  a  par- 
ticular small  area  of  the  object  whose  temperature  is  being  measured 
rather  than  an  average  over  a  more  extended  area.  Also,  it  is  not  nec- 
essary to  have  an  extended  source  in  order  to  measure  its  temperature. 
Some  pyrometers  require  a  source  that  is  very  large  if  the  observer  is 
at  any  distance;  such  is  not  the  case  with  this  form. 

Another  advantage  that  is  to  be  considered  is  the  fact  that  this  form 
of  pyrometer  is  almost  free  from  any  error  due  to  polarization.  Any 
effect  due  to  this  cause  would  be  negligible  in  almost  the  worst  case 
possible. 

DISCUSSION 

• 

C.  0.  FAIRCHILD,  Washington,  D.  C.  (written  discussion*).' — Referring 
to  the  paragraph  entitled  "Effect  of  change  in  temperature  of  absorbing 
glass  on  its  transmission,"  Dr.  Foote  and  the  writer  have  been  using,  since 
June,  1916,  a  correction  for  room  temperature  with  absorbing  glasses. 
For  Jena  black  glass  No.  3815,  with  a  red-glass  eyepiece  the  effective 
transmission  increases  instead  of  decreases  with  a  rise  in  room  tempera- 
ture, corresponding  to  a  decrease  in  the  quantity  A  where 

_  \e  logTV 

A.    —        ^, 

cf  log  e 

The  change  in  A  was  found  to  be  approximately  0.02  per  cent,  per  degree. 
Also  noviweld  of  shade  No.  7,  having  a  very  low  transmission,  was  found 
to  give  a  decreasing  A  when  used  with  a  red  glass  although  the  total 
transmission  decreases  and  the  spectral  transmission  curve  shifts  toward 
the  red.  The  last  is  readily  detected  by  noting  the  change  in  the  tint 
of  the  glass  when  heated.  This  is  algo  indicated  by  a  marked  improve- 
ment in  the  color  match  (upon  heating)  when  the  glass  is  used  with  a  thin 
red-glass  eyepiece.  If  a  green  glass,  such  as  Jena  4930,  is  used  in  the 
eyepiece,  an  exceedingly  great  decrease  in  transmission  is  observed,  con- 
sistent with  the  shift  of  the  spectral  transmission  curve.  Dr.  Forsythe 
has  not  stated  whether  the  values  given  in  Table  4  are  for  a  red-glass 

*  Received  Sept.  20,  1919. 


DISCUSSION  323 

eyepiece.  It  is  readily  apparent  that  the  room- temperature  factor  is 
quite  dependent  on  the  particular  eyepiece  used,  in  cases  where  the  spec- 
tral transmission  of  the  absorbing  glass  varies  rapidly  in  the  region  of 
transmission  by  the  eyepiece.  So  there  is  considerable  interest  in  measur- 
ing the  change  in  spectral  transmission  of  absorbing  glasses,  such  as  has 
already  been  done  with  red  and  other  colored  glasses. 

W.  E.  FORSYTHE,  (author's  reply  to  discussion*). — The  transmission 
of  each  sample  of  absorbing  glass  that  has  been  .examined  in  this  labora- 
tory for  different  temperatures  of  the  glass  has  changed  in  such  a  direc- 
tion that  this  change  could  be  accounted  for  by  a  shift  of  the  transmission 
curve  to  longer  wave-lengths.  Thus,  whether  the  transmission  increased 
or  decreased  depended  on  the  shape  of  the  spectral  transmission  curve. 
Mr.  Luckiesh  of  this  laboratory  has  examined  the  transmission  for  the 
visible  radiation  for  a  number  of  glasses.21  Among  the  glasses  that  he 
studied  was  a  cobalt  glass,  the  transmission  of  which  increased  with  an 
increase  in  temperature.  This  is  what  is  to  be  expected  if  the  trans- 
mission curve  shifted  toward  the  red  end  of  the  spectrum  when  the 
glass  was  heated. 

From  the  spectral  transmission  curve  of  Jena  glass,  No.  3815,  given 
by  Doctor  Foote,22  one  would  expect  that  its  transmission  for  red  radia- 
tion would  decrease  when  it  was  heated,  if  the  spectral  transmission  of  this 
glass  changes  like  the  other  glasses  examined.  However,  not  enough 
work  has  been  done  in  this  field  to  enable  any  definite  general  conclusion 
to  be  drawn. 

The  values  given  in  Table  4  of  the  transmissions  of  different  absorbing 
glasses  are  for  red  radiation,  such  as  would  be  transmitted  by  red  glass 
No.  4512,  5.8  mm.  thick. 

*  Received  Jan.  19,  1920.  22  U.  S.  Bureau  of  Standards  Bull.  12,  489. 

21  Jnl.  Amer.  Cer.  Soc.,  2,  759. 


324  OPTICAL  AND  RADIATION  PYROMETRY 


Optical  and  Radiation  Pyrometry 

BY   PAUL,   D.    FOOTE,*   PH.    D.,    AND   C.    O.    PAIRCHILD,  f  B.    S.,    WASHINGTON,    D.    C. 
(Chicago  Meeting,  September,  1919) 

THE  temperature  of  a  material  may  be  ascertained  by  measurement  of 
the  intensity  of  the  radiant  energy  it  emits.  This  measurement  may 
refer  to  the  radiation  of  all  wave  length's  emitted  by  the  material,  or,  if 
the  material  is  glowing,  the  measurement  may  refer  to  the  visible  light 
emitted,  or  to  the  radiation  in  a  very  restricted  portion  of  the  visible 
spectrum.  In  general,  the  intensity  of  radiation  depends  not  alone  upon 
the  temperature  of  the  source,  but  also  upon  its  nature.  Thus,  glowing 
carbon  appears  to  the  eye  about  three  times  as  bright  as  glowing  platinum, 
at  the  same  temperature.  This  is  technically  expressed  by  saying  that 
the  emissive  power  or  emissivity  of  carbon  is  about  three  times  that  of 
platinum. 

A  material  having  the  highest  theoretically  possible  emissivity  is 
known  as  a  "black  body;"  it  is  customary  to  assign  a  numerical  value  of 
1  to  the  emissivity  of  a  black  body.  A  black  body  is  experimentally 
realized  by  uniformly  heating  a  hollow  enclosure  and  observing  the 
radiation  coming  from  a  small  opening  in  the  wall.  The  intensity  of 
radiation  emitted  from  this  opening  depends  only  on  the  temperature  of 
the  walls,  and  not  on  the  material  of  which  they  are  constructed.  If 
E  is  the  emissivity  of  any  non-transparent  material  and  R  is  its  reflection 
coefficient,  it  can  be  shown  that  E  +  R  =  1.  If  a  material  having  an 
emissivity  of,  say,  0.40,  and  hence  a  reflection  coefficient  of  0.60,  is 
placed  inside  a  black  body  it  becomes  indistinguishable  from  its  sur- 
roundings, because  the  total  intensity  of  radiation  leaving  the  material  is 
the  same  as  that  emitted  by  the  black  body.  While  the  material  actually 
emits  only  40  per  cent,  of  the  intensity  of  a  black  body  at  the  same  tem- 
perature, 60  per  cent,  of  the  radiation  falling  upon  it  from  the  walls  of 
the  enclosure  is  reflected.  However,  if  the  material  is  removed  from  the 
black  body  and  placed  in  the  open  air,  the  reflected  intensity  is  no  longer 
present  and  the  object  appears  but  40  per  cent,  as  bright  as  a  black  body 
at  the  same  temperature. 

Optical  and  radiation  pyrometers  are  usually  calibrated  to  read  cor- 
rectly when  sighted  upon  a  black  body.  Fortunately,  many  technical 
processes  are  carried  out  under  black-body  conditions.  Muffle  furnaces, 
many  annealing  furnaces,  etc.,  are  sufficient  approximations  to  "black 

*  Physicist,  U.  S.  Bureau  of  Standards. 

t  Associate  Physicist,  U.  S.  Bureau  of  Standards. 


PAUL  D,  FOOTE  AND  C.  O.  FAIRCHILD  325 

bodies"  to  give  practically  correct  temperature  readings  with  the  optical 
or  radiation  pyrometer.  Some  materials  are  nearly  "black"  in  the  open; 
for  example,  the  oxide  formed  on  iron  and  steel  ingots,  rails,  etc.  In 
general,  however,  corrections  must  be  applied  to  the  pyrometer  readings 
to  obtain  the  correct  temperatures  of  materials  in  the  open.  These  cor- 
rections are  very  large  in  the  case  of  clean  molten  metals.  The  presence 
of  an  oxide  film  on  the  molten  metal  greatly  reduces  the  corrections. 
The  temperature  scale  for  the  optical  pyrometer  is  based  upon 
Wien's  law  for  the  distribution,  in  the  spectrum,  of  the  energy  of  a  black 
body.  This  law  may  be  stated  by  equation  (1)  in  which  X  denotes  the 
wave  length  in  microns;  Cz  is  a  constant  =  14,350;  #  is  the  absolute 
temperature  of  the  black  body;  «7X  is  the  intensity  at  the  wave  length 
X  (i.e.,  at  a  particular*color,  such  as  red);  and  c\  is  a  constant,  the  value  of 
which  is  of  no  moment  in  pyrometry,  since,  as  will  be  seen,  it  disappears 
from  the  actual  working  equation. 

_CJ 

For  a  black  body,  Jx  =  d  X~5  e  x*  (1) 

The  intensity  of  radiation,  J\,  of  wave  length,  X,  from  a  non-black  body 
of  temperature  #  and  emissivity  E^,  is  given  by  equation  (2). 

_Ct  _   Ct 

For  a  non-black  body,  Jx'  =  a  E  x  X~5  e   ™  =  d  X  ~5  e   xsx  (2) 

In  the  third  term  of  (2)  we  define  Sx  as  the  apparent  temperature,  in 
degrees  absolute  of  the  non-black  body.  This  is  the  temperature  measured 
by  the  optical  pyrometer  and  is  less  than  the  true  temperature,  #,  for 
all  materials  except  black  bodies,  when  it  becomes  equivalent  to  #. 
From  (2)  we  have: 

1  _  1       MogA.    =  Mogffx  ,,, 

tf      &       0.4343c2  "      6232 

Thus,  knowing  X  and  Ex,  it  is  always  possible  to  compute  the  true  tem- 
perature $  from  the  observed  temperature  Sx. 

An  optical  pyrometer  is  simply  a  photometer  using  monochromatic 
light  (usually  red),  in  which  the  intensity  of  radiation  from  either  a 
standard  or  a  constant  source  (electric  lamp,  oil  flame,  etc.^is  compared 
with  that  from  the  object  of  which  the  temperature  is  desired.  Fre- 
quently the  two  intensities  are  made  to  appear  equal  by  adjusting  vari- 
ous types  of  absorbing  devices  (absorption  glasses,  iris  diaphragms,  etc.) 
interposed  either  on  the  furnace  side  or  the  standard-lamp  side  of  the 
pyrometer,  depending  upon  which  source  is  normally  the  brighter.  In 
this  process  of  comparison  the  term  CiX~5  of  equation  (1)  is  embodied  as 
one  of  the  calibration  constants  of  the  instrument. 

The  temperature  scale  for  the  radiation  pyrometer  is  based  upon  the 
Stefan-Boltzmann  law  expressing  the  relation  between  the  total  energy 
J  radiated  per  unit  lime  per  unit  area  by  a  black  body,  and  its  absolute 
temperature,  #°  abs.  as  follows: 

J  =  a(t?4  -  tV)  (4) 


OPTICAL   AND    RADIATION    PYROMETRY 

where  #0  denotes  the  absolute  temperature  of  the  surroundings  or  of  the 
measuring  instrument  receiving  the  radiation,  and  a  an  empirical  con- 
stant. In  general  tV  is  negligible  in  comparison  with  $4  so  the  above 
relation  becomes: 

J  =  a&  (5) 

For  a  non-black  body  we  have: 

J'  =  ffE&*  =  <rS*  (6) 

where  E  is  the  total  emissivity  and  S  is  the  apparent  absolute  tempera- 
ture of  the  object  sighted  upon  as  measured  by  the  radiation  pyrometer. 
From  (6)  we  obtain: 

E  =  ^  or  log  E  =  4  (log  S  -  log  0)  (7) 

Thus,  knowing  the  total  emissivity  E  of  any  material,  it  is  possible  to 
obtain  the  true  temperature  #  from  the  apparent  temperature  S  as 
measured  by  a  radiation  pyrometer. 

Equation  (1)  states  that  the  intensity  of  radiation  of  a  fixed  wave- 

_  constant 

length  from  a  black  body  is  proportional  to  e        * 

Equation  (5)  states  that  the  total  radiation  of  all  wave-lengths  emitted 
by  a  black  body  is  proportional  to  #4.  These  two  laws,  which  form  the 
basis  of  optical  and  radiation  pyrometry  respectively,  are  in  agreement 
with  the  temperature  scale  defined  by  the  gas  thermometer  up  to  1550°  C., 
the  upper  limit  at  which  a  gas  thermometer  has  been  used  satisfac- 
torily. Above  this  range,  to  2500°  C.,  the  scales  defined  by  these  two 
laws  have  been  found,  experimentally,  to  be  in  mutual  agreement,  and  it 
is  believed  that  they  correctly  represent  the  thermodynamic  scale  for 
all  temperatures. 

OPTICAL  PYROMETRY 

Fig.  1  illustrates  the  principle  of  the  Fery  optical  pyrometer.  G 
is  a  means  for  producing  a  divided  photometric  field.  In  the  later  in- 
struments a  Lummer-Brodhun  or  silver-strip  cube  is  employed.  Part 
of  the  field  of  view  is  illuminated  by  the  source  sighted  upon  and  part  by 
the  gasoline  lamp  L  which  burns  at  a  constant  brightness.  By  moving 
the  wedges  of  black  glass,  pp',  the  thickness  of  absorbing  glass  in  the 
line  of  sight  can  be  varied  until  the  part  of  the  field  illuminated  by  the 
source  has  the  same  brightness  as  that  illuminated  by  the  lamp.  A  red 
glass  screen  is  used  in  the  ocular  so  that  fairly  monochromatic  light  of 
this  color  (0.65ju  to  0.63/i)  is  compared.  The  relation  between  the  thick- 
ness of  the  wedges  x,  read  on  a  scale,  and  the  absolute  temperature  t?  is 
x  +  P  =  Q/&,  where  P  and  Q  are  constants  determinable  by  two  cali- 
bration points.  The  instrument  must  be  focused  upon  the  radiating 
source  but  no  corrections  for  sighting  distance  need  be  applied.  The 
Le  Chatelier  optical  pyrometer  is  similar  in  principle  but  is  not  of  constant 
aperture  and  important  corrections  must  be  made  with  change  of  focus. 


PAUL  D.  FOOTE  AND  C.  O.  FAIRCHILD 


327 


Directions  for  Use  ofFery  Optical  Pyrometer. — Fill  standard  lamp  about 
two-thirds  full  of  gasoline  (or  amylacetate,  if  originally  calibrated  for  this 
illuminant)  and  adjust  flame  until  tip  burns  on  a  level  with  the  top  of  the 
slit  cut  in  the  tube  encasing  the  lamp.  Focus  eye  piece  by  drawing  in  or 
out  until  the  field  illuminated  by  the  lamp  is  well  defined.  Focus  the  ob- 
jective by  thumb  screw  C  until  the  source  sighted  upon  is  clearly  out- 
lined. Turn  into  the  field  the  red  glass  screen  in  the  eye  cup  and  match 
the  two  photometric  fields  by  adjusting  the  thumb  screw  which  moves 
the  wedges.  Observe  the  reading  on  Jthe  scale.  This  reading  is  con- 
verted into  temperature  by  use  of  a  table  or  plot  of  scale  readings  fur- 
nished with  the  instrument.  For  varying  and  extending  the  range  of 


FIG.    1. — FERY  OPTICAL  PYROMETER. 


FIG.  2. — SHORE  PYROSCOPE. 


the  instrument,  two  removable  absorption  glasses  are  used,  one,  A',  on 
the  standard  lamp  side  and  one,  A,  immediately  in  front  of  the  adjust- 
able wedges.  Care  must  be  taken  in  making  observations,  to  note 
whether  these  glasses  are  in  or  out,  and  in  converting  scale  readings  to 
temperature,  to  observe  that  the  correct  table  or  plot  corresponding  to 
the  particular  combination  employed  is  used. 

• 

Shore  Pyroscope 

The  Shore  pyroscope,  Fig.  2,  operates  upon  a  principle  very  similar 
to  that  of  the  Fery  optical  pyrometer.  The  instrument  has  a  scale 
graduated  to  read  temperatures  directly,  which  is  a  material  advantage. 
The  design  of  optical  parts  is  rather  unnecessarily  complicated  and  it  is 
difficult  to  match  the  two  fields  on  account  of  color  differences.  The 
pyroscope  is  used  extensively  and  with  satisfactory  results  where  high 
precision  is  not  required. 


328 


OPTICAL   AND    RADIATION   PYROMETRY 


Directions  for  Use  of  Shore  Pyroscope. — Fill  lamp  two-thirds  full  of  kero- 
sene oil.  Adjust  the  flame  to  burn  at  a  height  of  about  %  in.  (19  mm.). 
Focus  on  source  by  turning  knurled  ring  on  end  of  telescope  tube.  Turn 
knob  by  side  of  scale  until  the  inner  and  outer  fields  match  in  brightness. 
Read  temperature  of  source  directly  from  scale  setting. 

Wanner  Pyrometer 

Fig.  3  illustrates  the  arrangement  of  the  optical  parts  in  the  Wanner 
pyrometer.  The  comparison  light  is  a  six-volt  incandescent  lamp  illu- 
minating a  glass  matt  surface  in  front  of  the  slit  S2.  The  slit  Si  is  illu- 
minated by  the  source  sighted  upon.  Light  from  each  slit  passes  through 
the  collimating  lens  Oi,  the  direct-vision  spectroscope  P,  a  Wollaston 
prism  R,  a  bi-prism  B,  the  second  collimating  lens  02,  and  is  brought 
to  a  focus  at  F.  The  Wollaston  prism  produces  two  images  of  each 
slit,  which  are  polarized  at  right  angles  to  each  other.  The  bi-prism 
again  doubles  the  number  of  images,  so  that  there  are  finally  four  images 
of  each  slit  at  F.  Six  of  these  images  are  diaphragmed  off  by  the  screen 
D.  The  two  remaining  images,  one  of  each  slit,  are  superposed  and  are 


E 


FIG.  3. — WANNER  PYROMETER. 

polarized  at  right  angles  to  each  other.  From  this  point  the  light  passes 
through  the  nicol  prism  A  and  the  ocular  lens  E.  The  direct-vision 
spectroscope  is  so  adjusted  that  only  red  light  of  wave  length  about  X  = 
0.65/t  reaches  the  eye,  the  other  colors  being  diaphragmed  off  by  the 
screen  D.  The  ocular  is  focused  on  the  dividing  edge  of  the  bi-prism  B. 
The  eye  perceives  a  circular  photometric  field  half  of  which  is  illuminated 
by  the  slit  Si  and  half  by  the  slit  S2.  The  light  from  the  two  fields  is 
plane  polarized,  the  plane  of  polarization  in  one  field  being  at  right  angles 
to  the  plane  of  polarization  in  the  other  field.  Consequently,  on  rotating 
the  nicol  prism  A,  one  field  increases  and  the  other  field  decreases  in 
intensity.  •  A  setting  is  obtained  when  the  two  fields  match. 

In  order  to  determine  the  proper  brightness  at  which  to  operate  the 
electric  lamp  illuminating  the  slit  $2,  the  pyrometer  is  sighted  on  a  source 
of  standard  brightness.  This  consists  of  an  amylacetate  lamp  with  a 
flame  gage  having  a  window  of  ground  glass,  which  illuminates  the  slit 
Si.  The  analyzer  nicol  A  is  set  at  a  specified  normal  point  or  angle 
marked  on  the  instrument.  The  current  through  the  electric  lamp  is 
then  varied  by  a  rheostat  until  the  two  fields  are  matched,  and  the  cur- 
rent is  read  from  the  ammeter.  This  process  should  be  repeated 


PAUL   D.    FOOTE    AND    C.    O.    FAIRCHILD  329 

several  times  and  a  mean  value  of  the  current  settings  obtained;  when 
using  this  instrument  the  current  is  adjusted  to  this  mean  value.  The 
electric  lamp  burns  at  a  high  temperature  and  consequently  deteriorates 
noticeably.  Hence  the  above  adjustment  of  the  normal  point  requires 
frequent  redetermination.  For  high  precision  the  adjustment  should 
be  made  both  before  and  after  a  series  of  temperature  readings.  In  the 
industrial  plant,  once  a  day  or  once  a  week  is  sufficient,  depending  upon 
the  amount  of  use. 

The  calibration  of  the  instrument  follows  the  law 


log  tan  <p  =  a  -f- 

where  <p  is  the  angular  reading  of  the  analyzer,  $  the  absolute  tempera- 
ture, and  a  and  6  empirical  constants.  Since  the  relation  between 
log  tan  tp  and  l/#  is  linear,  two  calibration  points  serve  to  determine 
a  and  6,  and  <p  may  be  plotted  against  t°  C.  (t°  =  &  —  273°).  Usually 
such  a  table  is  furnished  with  the  pyrometer,  or  the  instrument  may  be 
sent  to  the  Bureau  of  Standards  for  calibration. 

The  instrument  described  above  is  satisfactory  for  temperatures 
greater  than  900°  C.  At  temperatures  between  700°  and  900°  C.,  the 
intensity  of  light  from  the  furnace  sighted  upon  is  insufficient  to  permit 
accurate  settings.  For  such  temperatures,  the  direct-vision  spectroscope 
P  is  replaced  by  a  red  glass  screen,  or  the  objective  lens  Oi  is  made  of 
red  glass,  and  the  slits  Si  and  $2  have  much  wider  openings.  For  very 
high  temperatures,  an  absorption  glass  is  mounted  in  front  of  the  slit 
Si  which  decreases  the  light  from  the  furnace  in  a  known  ratio. 

On  account  of  stray  light,  the  Wanner  pyrometer  is  not  accurate 
at  very  small  or  very  large  angular  readings.  Moreover,  at  large  angles 
the  temperature  increases  so  fast  that  the  angles  would  have  to  be  ob- 
served with  extreme  precision  to  give  accurate  results  expressed  in  degrees 
of  temperature.  The  range  of  the  instrument  is  thus  confined  to  from 
about  10  to  80  angular  degrees. 

With  the  Wanner  pyrometer,  the  tip  of  the  flame  of  the  amylace- 
tate  lamp  should  burn  level  with  the  top  of  the  flame  gage.  The  setting 
on  the  normal  point  is  tedious  because  the  flame  flickers  over  the 
field.  A  screen  of  black  paper  placed  around  the  lamp  helps  to  re- 
duce the  flicker,  and  the  observations  should  be  made  in  a  closed  room 
free  from  drafts.  Any  error  in  the  adjustment  of  the  normal  current  is 
carried  over  to  the  final  temperature  measurements,  so  that  it  is  ex- 
ceedingly important  to  exercise  all  possible  care  in  those  preliminary 
adjustments.  Examine  the  screen  of  the  flame  gage  to  assure  that  no 
smoke  has  deposited  upon  it.  A  slight  film  of  smoke  from  the  lamp  may 
cause  an  error  of  100°  or  more.  The  amylacetate  used  in  the  lamp  need 
not  be  of  high  purity. 

From  experience  with  several  hundred  instruments  in  use  in  the  tech- 


330 


nical  industries,  it  is  evident  that  these  pyrometers  are  subjected  to  great 
abuse.  The  instrument  is  composed  of  delicate  optical  parts  and  should 
not  be  allowed  to  become  heated.  Many  of  the  parts  are  set  in  wax  and 
the  various  optical  surfaces  are  cemented  by  Canada  balsam.  The 
Wollaston  prism,  and  -the  nicol  prism  in  the  rotating  eye  piece  are  made 
of  calcite.  In  about  half  the  instruments  examined  these  parts  have 
been  deeply  cut  by  knives  or  pointed  steel  tools.  All  persons  using  this 
pyrometer -should  be  cautioned  not  to  touch  any  optical  part  except  the 
lens  in  the  eye  cup,  which  requires  occasional  cleaning.  Do  not  change 
the  setting  of  any  screw,  as  this  may  throw  the  pyrometer  out  of  the 
adjustment  and  cause  errors  of  500°.  If  the  position  of  any  screw  on  the 


FlG.    4. SdMATCO    PYROMETER. 

body  of  the  instrument  is  altered,  do  not  attempt  to  readjust  the  instru- 
ment but  return  it  to  the  maker.  Also,  do  not  take  the  instrument  apart 
to  find  out  what  is  wrong.  The  replacement  of  the  electric  lamp  will 
not  alter  the  calibration  of  the  pyrometer. 

Scimatco  Pyrometer 

Fig.  4  illustrates  the  Scimatco  pyrometer  formerly  sold  by  the  Scien- 
tific Materials  Co.  This  is  an  improved  form  of  the  Wanner  pyrometer. 
All  but  one  of  the  screws,  the  tampering  with  which  affects  the  calibra- 
tion of  the  instrument,  are  enclosed  in  a  metal  sheath.  The  instrument 


PAUL   D.    FOOTE    AND    C.    O.    FAIRCHILD 


331 


has  both  an  angular  scale  and  a  scale  graduated  directly  in  degrees  of 
temperature.  The  box  at  the  left  contains  a  6-volt  storage  battery, 
an  ammeter,  and  an  adjustable  rheostat.  For  obtaining  the  proper  set- 
ting of  the  current,  the  pyrometer  is  clamped  in  its  carrying  case.  The 
amylacetate  lamp  and  flame  gage  are  so  mounted  that  the  ground  glass 
of  the  gage  is  directly  in  contact  with  the  glass  window  of  the  pyrometer, 
opening  to  the  slit  Si  of  Fig.  3.  The  tip  of  the  flame  is  adjusted  until 
it  is  just  visible  on  looking  through  the  bottom  of  the  red  glass  window 
in  the  dial  of  the  instrument.  With  the  Wanner  or  Scimatco  pyrometer, 
the  observer  cannot  see,  through  the  instrument,  the  object  sighted 
upon.  This  may  cause  inconvenience  if  it  is  desired  to  measure  the  tem- 
perature of  a  small  crucible  in  a  furnace. 

Foote  &  Fisher  Pyrometer 

Fig.  5  illustrates  the  arrangement  of  optical  parts  in  the  Foote  & 
Fisher  pyrometer  made  by  the  Scientific  Materials  Co.  Light  from  the 
furnace  is  focused  at  the  center  of  the  silver-strip  cube  C.  This  cube 


w\ 


H 


A  B 


FIG.  5. — FOOTE  &  FISHEU  PYROMETER. 

produces  a  circular  field  divided  through  the  middle.  One  half  of  the 
field  receives  light  from  the  furnace,  and  the  other  half  from  the  ground- 
glass  screen  D  which  is  illuminated  by  the  electric  lamp  F  through  the 
condenser  lens  E.  The  ocular  containing  the  red  glass  screen  A  and  lens 
B  is  focused  on  the  dividing  edge  of  this  photometric  field.  G  and  H 
are  diaphragms  which  limit  the  cone  of  rays  employed.  The  two  fields 
are  matched  by  turning  a  thumb  screw  which  moves  the  black  glass  wedge 
W  across  the  path  of  the  light  from  the  furnace.  By  a  system  of  gears 
this  movement  is  transferred  to  a  circular  scale  on  the  dial  K  of  the  instru- 
ment. In  appearance  the  pyrometer  resembles  the  Scimatco,  and  is 
used  in  the  same  manner.  For  a  normal  point  setting,  the  pointer  is 
adjusted  to  read  the  normal  angle  and  after  removing  the  tube  carrying 
the  lens  L  the  instrument  is  clamped  in  its  case.  The  flame  gage  of  the 
amylacetate  lamp  is  so  mounted  that  its  ground-glass  window  is  adjacent 
to  the  diaphragm  H .  A  table  is  furnished  with  the  instrument  giving 
the  relation  between  the  scale  reading  in  angular  degrees  and  degrees  of 


332 


OPTICAL    AND    RADIATION    PYROMETRY 


temperature.  This  instrument  is  so  designed  that  the  object  sighted  upon 
is  clearly  imaged,  a  distinct  advantage  over  the  Wanner  pyrometer.  The 
relation  between  the  scale  reading  a  and  the  absolute  temperature 
#  is  a  +  P  =  Q/d  where  P  and  Q  are  constants  determinable  by  two 
calibration  points. 

Morse,  Holborn-Kurlbaum,  and  Leeds  &  Northrup  Optical  Pyrometers 

The  filament  of  a  small  electric  lamp  F,  Fig.  6,  is  placed  at  the  focal 
point  of  an  objective  L  and  ocular,  forming  an  ordinary  telescope  which 
superposes  upon  the  lamp  the  image  of  the  source  viewed.  Red  glass, 
such  as  Corning  "High  Transmission  Red,"  is  mounted  at  the  ocular  to 
produce  approximately  monochromatic  light.  In  making  a  setting,  the 
current  through  the  lamp  is  adjusted  by  rheostat  until  the  tip  or  some 
definite  part  of  the  filament  is  of  the  same  brightness  as  the  source  viewed. 


w\A/w 


FIG.  6. — LEEDS  &  NORTHRUP  OPTICAL  PYROMETER. 

The  outline,  or  detail,  of  this  section  of  the  filament  is  then  indistinguish- 
able from  the  surrounding  field,  as  illustrated  by  Fig.  7;  in  the  third  set- 
ting the  central  portion  of  the  filament  vanishes  against  the  background. 
The  current  is  read  on  an  ammeter  and  the  corresponding  temperature  is 
computed  from  a  plot  or  table.  The  relation  between  the  current,  i, 
through  the  lamp  and  the  temperature  t°  C.,  is  of  the  form:  i  —  a  + 
bt  +  ct2  where  a,  6,  c,  are  constants  requiring  for  their  determination 
at  least  three  standardization  points. 

The  lamps  should  not  be  operated  at  temperatures  higher  than 
1500°  C.,  on  account  of  deterioration  of  the  tungsten  filament.  If  this 
temperature  is  not  exceeded,  the  calibration  of  the  lamp  is  good  for 
hundreds  of  hours  of  ordinary  use.  For  higher  temperatures,  absorption 
glasses,  S,  Fig.  6,  are  placed  between  the  lamp  and  the  objective,  or  in 
front  of  the  objective,  to  diminish  the  observed  intensity  of  the  source. 


PAUL  D.  FOOTE  AND  C.  O.  FAIRCHILD 


333 


The  relation  between  the  temperature  of  the  source,  #°  abs.;  and  the  ob- 
served temperature,  $0°  abs.,  measured  with  the  absorption  glass  inter- 
posed, is  as  follows:  '-  —  ^  =  A,  where  A  is  for  most  practical  purposes 

v         t/o 

a  constant. 

Usually  the  instrument  is  furnished  with  a  table  showing  the  relation 
between  the  current  through  the  lamp  and  the  temperature  both  with  and 
without  the  absorption  glass.  If,  however,  this  relation  is  not  given  for 
the  use  of  the  absorption  glass,  it  may  be  readily  determined  by  measuring 
the  constant  A  in  the  above  formula.  To  do  this,  sight  without  the  ab- 


Correct 

FIG.  7. — APPEARANCE  OF  FIELD  OF  .VIEW  WHEN  ADJUSTING  THE  CURRENT  THROUGH 
THE  LAMP  OF  LEEDS  &  NoRTHRUP  PYROMETER. 

sorption  glass  on  a  muffle  or  any  uniformly  heated  furnace  at  1200°  to 
1500°  C.  and  pbserve  the  temperature  #,  in  degrees  absolute.  Then,  with 
the  absorption  glass  in  place,  match  the  filament  again  and  observe  to 
what  temperature,  $o,  in  degrees  absolute,  the  current  through  the  lamp 
corresponds.  The  difference  in  the  reciprocals  of  these  two  temperatures 
is  the  constant  A,  which  is  usually  of  the  order  of  magnitude— 0.0002. 
This  determination  should  be  repeated  several  times  and  at  several  differ- 
ent temperatures  of  the  furnace.  The  separate  values  of  A  should  not 
differ  by  more  than  1  per  cent,  and  the  mean  value  is  used  for  computing 


334 


OPTICAL   AND    RADIATION    PYEOMETRY 


the  relation  between  the  observed  absolute  temperature  with  the  absorp- 
tion glass  and  the  true  temperature*  of  the  source.  In  making  these  com- 
putations, care  must  be  exercised  that  all  temperatures  are  converted  to 
degrees  absolute.  Table  1  illustrates  the  calibration  of  a  certain  pyrometer, 
both  with  and  without  the  absorption  glass,  the  constant  of  which  has 
the  value  A  =  —  0.000280.  By  use  of  this  glass,  temperatures  as  high  as 
2730°  C.  can  be  measured,  although  the  temperature  of  the  lamp  does  not 
exceed  1360°  C. 

TABLE  1. — Example  Calibration  of  Optical  Pyrometer 


Current, 
Amp. 

Temperature,  Deg.  C. 

Without  Absorption  Glass 

With   Absorption   Glass 

0.26 

634 

943 

0.28 

765 

1190 

0.30 

860 

1386 

0.32 

936 

1555 

0.34 

1002 

1710 

0.36 

1060 

1854 

0.38 

11-13   . 

1992 

0.42 

1201 

2237 

0.46 

1281 

2478 

0.50 

1359 

2733 

It  will  be  noted  that  the  range  of  current  required  is  small,  in  general 
about  0.3  to  0.6  ampere.  Thus,  if  the  ammeter  is  designed  to  give  full- 
scale  deflection  with  0.6  ampere,  nearly  half  of  the  scale,  from  0  to  0.26 
ampere,  is  never  used.  The  Hickok  depressed-zero  ammeter,  now  fur- 
nished with  the  Leeds  &  Northrup  pyrometer,  meets  this  objection.  The 
moving-coil  system,  including  the  supports,  pivots,  and  pointer,  may  be 
adjusted  relative  to  the  magnet  by  turning  a  lever  on  the  case  of  the 
instrument  to  one  of  two  positions.  In  one  position  the  pointer  is 
adjusted  on  open  circuit  so  that  it  falls  over  the  first  graduation  on  the 
scale;  this  adjustment  is  similar  to  the  ordinary  zero  adjustment  on  any 
ammeter.  In  the  second,  or  working,  position  the  zero  is  depressed  from 
the  scale  an  amount  equivalent  to  0.26  ampere.  The  entire  scale  from 
0.26  to  0.60  ampere  is  thus  utilized  for  the  range  of  the  pyrometer  lamp. 

General  Use  of  Optical  Pyrometers 

Optical  pyrometers  and  radiation  pyrometers,  described  later,  afford 
the  only  means  yet  developed  for  measuring  temperatures  above  1500°  C. 
The  high-temperature  scale,  above  1500°  C.,  is  based  on  the  extrapola- 
tion of  Wien's  radiation  law  by  means  of  a  pyrometer  of  the  Leeds  & 
Northrup  type.  When  the  instrument  is  especially  designed  for  pre- 
cision work  it  is  possible  to  measure  a  temperature  difference  of  0.2°  C. 


PAUL   D.    FOOTE    AND    C.    O.    PAIRCHILD  335 

at  1500°  C.  The  commercial  form  of  the  instrument,  when  properly  cali- 
brated, can  be  relied  upon  to  5°  C.  With  a  well-designed  optical  pyro- 
meter there  is  a  perfect  color  match  of  the  two  fields  at  all  times.  Hence, 
contrary  to  the  general  impression,  color  is  not  matched  at  all  but  simply 
brightness  of  uniform  color.  A  color-blind  observer  will  obtain  the  same 
settings  as  a  normal  observer.  Forsythe1  has  compiled  data  observed 
with  an  optical  pyrometer  of  the  Leeds  &  Northrup  type  by  six  opera- 
tors, none  of  whom  had  ever  used  an  optical  pyrometer  before.  The 
average  variation  from  the  mean  was  3°  C.,  and  the  maximum  variation, 
5°  C. 

Although  the  optical  pyrometer  is  essential  for  the  measurement  of 
temperatures  above  1500°  C.,  its  usefulness  is  by  no  means  confined  to 
the  high-temperature  range.  To  many  processes  at  low  temperatures, 
the  thermocouple  cannot  be  adapted,  for  example,  to  measure  the  tem- 
perature of  steel  rails  as  they  pass  through  the  rolls,  of  ingots  and  forg- 
ings  in  the  open,  and  of  small  sources  such  as  a  heated  wire  or  lamp  fila- 
ment. In  such  cases,  the  temperatures  may  be  accurately  measured  by 
the  optical  pyrometer.  The  temperature  of  a  microscopic  sample  of 
any  material  can  be  measured  by  a  modified  form  of  the  Leeds  &  North- 
rup pyrometer.2  Also,  in  many  processes  a  thermocouple  is  not  so 
convenient  as  an  optical  pyrometer,  especially  when  measurements  of 
temperature  are  not  required  often  enough  to  warrant  a  permanent  in- 
stallation of  thermocouples. 

One  serious  objection  to  the  optical  pyrometer,  from  the  industrial 
point  of  view,  is  the  fact  that  it  has  not.  been  made  automatically  record- 
ing. Since  a  photometric  match  is  required  for  every  setting,  the  instru- 
ment necessitates  the  attention  of  an  observer,  although  possibly  a 
satisfactory  automatic  device  will  be  developed  eventually.  Another 
objection  is  the  introduction  of  the  human  element  into  the  readings,  thus 
affording  an  opportunity  for  dishonest  or  prejudiced  settings.  The 
observer,  if  he  is  the  operator  of  the  furnace,  should  be  taught  that  the 
instrument  is  for  his  own  assistance  and  is  not  to  be  considered  as  a 
policial  measure.  Otherwise,  the  measurements  should  be  made  by  a 
disinterested  party.  In  a  plant  operating  several  furnaces,  an  intelli- 
gent boy  can  be  profitably  employed  whose  sole  work  is  to  make  the 
rounds  of  the  various  furnaces  and  measure  and  record  the  temperatures. 

Black-body  and  Non-black-body  Conditions 

Optical  pyrometers  are  usually  calibrated  to  read  correctly  when 
sighted  on  a  black  body.  Many  furnaces  approximate  black-body  con- 
ditions quite  satisfactorily.  In  a  perfect  black  body,  the  details  of  the 

1  Gen.  Eke.  Rev.  (Sept.,  1917)  20,  753. 

2  Burgess:  U.  S.  Bureau  of  Standards  Sci.  Paper  198. 


336  OPTICAL    AND    RADIATION    PYROMETRY 

inside  of  the  furnace  vanish  and  a  piece  of  steel,  for  example,  which  is 
being  heated  cannot  be  distinguished  from  the  background.  If  the 
objects  in  the  furnace  can  be  distinguished,  b.ut  only  on  close  observation, 
and  if  much  of  the  detail. is  lost,  after  the  objects  have  been  in  the  furnace 
some  time,  it  is  not  likely  that  the  temperature  measurement  will  be  seri- 
ously in  error.  If  in  error  at  all,  the  observed  temperature  will  be  too 
high  when  the  furnace  walls  are  brighter  than  the  material  being  heat 
treated,  and  too  low  when  the  walls  are  less  bright.  This  latter  condition 
is  possible  if  the  heat  supply  is  variable,  or  if  it  is  shut  off  and  the 
furnace  is  allowed  to  cool. 

That  a  steel  ingot  placed  in  a  heated  furnace  may  appear  much  hotter 
than  it  really  is,  is  a  fact  not  always  appreciated.  The  surface  of  the 
ingot  appears  hot  because  it  reflects  the  bright  light  from  the  walls  of 
the  furnace.  Of  course,  in  comparison  with  the  much  greater  bright- 
ness of  the  walls,  the  cold  ingot  appears  black,  but  this  is  due  to  the  in- 
tense contrast.  If  the  ingot  is  viewed  alone  while  the  direct  radiation 
from  the  furnace  is  screened  from  the  eye,  it  also  is  bright.  Thus,  when 
an  optical  pyrometer  is  sighted  on  an  ingot  in  the  furnace,  part  of  the 
light  reaching  the  instrument  comes  from  the  side  walls  and  is  reflected 
by  the  surface  of  the  ingot.  Iron  oxide  reflects  about  the  least  light  of 
all  materials  met  with  in  metallurgical  practice.  Its  emissivity  is  approxi- 
mately 0.95;  hence  its  reflection  coefficient  is  0.05.  Suppose  an  iron 
ingot  at  room  temperature  were  suddenly  placed  in  a  furnace  at  1200°  C. 
Although  the  surface  of  the  ingot  is  cold,  it  reflects  5  per  cent,  of  the  light 
falling  upon  it  from  the  hot  side  walls;  this  5  per  cent,  of  reflected  radia- 
tion gives  the  ingot  the  appearance  of  an  object  at  950°  C.,  and  measure- 
ment by  the  optical  pyrometer  accordingly  would  be  950°  C.  The  greater 
the  reflecting  power  of  the  material  the  higher  is  the  observed  temperature 
under  the  above  conditions.  Thus,  cold  platinum  would  appear  to  be  at 
about  1160°,  or  at  almost  the  same  temperature  as  that  of  the  furnace. 
One  method  for  reducing  the  error  due  to  reflected  radiation  is  to  view 
the  object  through  a  large  open  door,  sighting  on  a  surface  parallel  to  the 
opening.  When  the  material  has  attained  the  temperature  of  the  side 
walls,  it  is  of  course  not  desirable  to  open  a  large  door  since  the  opening 
would  then  affect  the  black-body  conditions.  The  pyrometer  should 
be  sighted  through  a  small  peep  hole  as  soon  as  approximate  temperature 
uniformity  is  obtained.  A  more  satisfactory  method  for  reducing  the 
stray  reflections  than  by  opening  a  large  door  is  to  sight  into  a  deep  wedge- 
shaped  cavity  or  hole  made  in  the  metal  being  heat  treated.  If  this 
cavity  is  deep  enough,  very  little  radiation .  from  the  furnace  walls 
can  be  reflected  from  it.  If  such  a  hole  cannot  be  made  conveniently,  a 
length  of  iron  pipe  closed  at  one  end,  or  a  porcelain  tube,  may  be  placed 
on  the  material  and  so  aligned  that  the  pyrometer  may  be  sighted  through 
a  peep  hole  directly  into  the  bottom  of  the  tube. 


PAUL   D.    POOTE    AND    C.    O.    FAIRCHILD 


337 


The  effect  of  reflected  light  is  very  noticeable  in  an  empty  coke  oven, 
the  reflection  coefficient  of  the  brick  walls  being  comparatively  high. 
The  walls  may  therefore  appear  equally  bright,  even  though  they  differ 
considerably  in  temperature.  Frequently  a  patch  of  the  wall  on  one 
side  becomes  coated  with  a  layer  of  coke.  Since  the  coke  has  a  higher 
emissive  power  than  brick,  this  patch  appears  much  hotter;  actually 
it  is  about  the  same  temperature  as  the  surrounding  wall.  On  account 
of  reflection,  a  corresponding  bright  patch  appears  on  the  opposite  wall, 
although  this  wall  may  be  free  from  coke. 

TABLE  2. — True   Temperatures  and  Apparent   Temperatures   Measured 

by  Optical  Pyrometers  Using  Red  Light  (X  =  0.65/i)  when 

Sighted  upon  Various  Materials  in  the  Open 


Observed 
Temp. 

True  Temperature,  Degrees  C. 

Degrees 

Molten 
Copper 

Molten  * 
Iron 

Solid 
Iron 
Oxide 

Solid 
Nickel 
Oxide 

Nichrome 
or 
Chromel 

Molten 
Slagf 

Bright 
Platinum 

700 

700 

701 

702 

750 

800 

801 

802 

804 

861 

900 

902 

904 

906 

973 

950 

1088 

953 

955 

958 

1030 

1000 

1150 

1004 

1007 

1010 

1087 

1050 

1213 

1055 

1058 

1063 

1144 

1100 

1277 

1183         1106 

1110 

1116 

1202 

1150 

1341 

1239 

1158 

1162 

1170 

. 

1260 

1200 

1405          1296 

1210 

1215 

1224 

1320 

1250 

1470 

1353 

1267 

1375 

1300 

•   1536 

1410 

1320 

1435 

1400 

1525 

1455 

1555 

1500 

1641 

1565 

1675 

1600 

1758 

1670 

1700 

1876 

1780 

1750 

1935 

1830 

*  Computed  for  E\    =  0.40,  this  being  the  best  value  for  ordinary  steel  practice, 
f  Computed  for  E\    =  0.65,  an  average  value  for  liquid  slags. 

When  an  optical  pyrometer  is  sighted  on  a  glowing  material  in  the 
open,  it  reads  too  low.  Certain  materials,  important  industrially, 
have  a  high  emissivity,  so  that  the  corrections  necessary  to  add  to  the 
observed  temperatures  are  small;  for  example,  for  iron  oxide  the  correc- 
tion is  only  10°  at  1200°  C.  The  corrections  are  very  large  for  clear 
molten  metals,  but  are  smaller  for  the  oxides  which  soon  form  on  the 
molten  surface  when  exposed  to  the  air.  Table  2  shows  the  true  tempera- 
tures corresponding  to  the  temperatures  observed  when  sighting  on 
certain  materials  in  the  open.  For  temperature  control  it  is  unnecessary 
to  apply  these  corrections;  the  observed  temperatures  will  be  low  by 

22 


338 


OPTICAL    AND    RADIATION    PYROMETRY 


the  same  amount  from  time  to  time,  and  hence  will  serve  just  as  satis- 
factorily as  the  corrected  temperatures  for  reproducing  temperature 
conditions  in  any  process.  This  statement  must  be  modified  if  factors 
other  than  emissivity  of  the  material  require  consideration.  For  exam- 
ple, reproducible  results  cannot  be  expected  if  heavy  clouds  of  smoke  are 
in  the  line  of  sight  one  day  and  not  on  the  next  day.  If  the  pyrometer 
is  sighted  on  a  stream  of  molten  iron  during  pouring  or  tapping,  the  sur- 
face of  the  metal  is  usually  free  from  oxide.  If  the  stream  should  at 
any  tim'e  contain  much  slag,  the  surface  will  show  bright  patches  on 
account  of  the  higher  emissivity  of  the  slag.  To  make  the  readings 
conform  with  those  taken  on  the.  clear  stream,  one  must  sight  on  the 
darker  spaces  between  the  slag  patches  or  sight  also  upon  the  slag  and 
correct  both  sets  of  data  according  to  Table  2. 

The  above  table  was  computed  from  the  following  equation,  where 
$  is  the  true  absolute  temperature,  S  the  observed  absolute  temperature, 
and  Ex  the  emissivity  for  the  wave  length  X.     This  wave  length  has  been 
selected  as  X  =  0.65ju,  the  approximate  value  for  optical  pyrometers. 
1       1  =  X  log  ^x  =  logffx 
#      S  ~     6232       :   9588 

Table  3  gives  the  emissivity  of  various  materials  for  the  above  wave 
length.  The  change  of  emissivity  with  temperature  is  usually  small  for 
metals. 

TABLE  3. — Monochromatic  Emissivity  for  Red  Light  (X  =  about  0.65/x) 


Material 


Material 


Silver !  0.07 

Gold,  solid ..0.13 

Gold,  liquid 0.22 

Platinum,  solid j  0 . 33 

Platinum,  liquid .  . 0. 38 

Palladium,  solid 0 . 33 

Palladium,  liquid 0 . 37 

Copper,  solid 0.11 

Copper,  liquid 0.15 

Tantalum,  1100°  C 0. 60 

Tantalum,  2600°  C 0. 48 

Tungsten,  1000°  C 0  46 

Tungsten,  2000°  C 0 . 43 

Tungsten,  3000°  C 0.41 

Nichrome,  600°  C 0 . 95 

Nichrome,  900°  C 0 . 90 

Nichrome,  1200°  C 0.80 


Cuprous  oxide 

Iron  oxide,  800°  C. . . 
Iron  oxide,  1000°  C. . . 
Iron  oxide,  1200°  C... 
Nickel  oxide,  800°  C . 
Nickel  oxide,  1300°  C. 


Iron,  solid  and  liquid .  . . 
Nickel,  solid  and  liquid . 
Iridium . . 


Rhodium 

Graphite  powder  (estimated) 
Carbon . . 


0.70 
0.98 
0.95 
0.92 
0.96 
0.85 


0.37 
0.36 
0.30 

0.30 
0.95 
0.85 


Porcelain  (?  ?) 0 . 25  to  0 . 50 


PAUL  D.  FOOTE  AND  C.  O.  FAIRCHILD 


339 


Table  4  shows  the  corrections  which  must  be  added  to  the  readings 
obtained  with  an  optical  pyrometer  using  light  of  wave  length  X  =  0.65ju, 
for  various  emissivities,  in  order  to  obtain  the  true  temperatures.  These 
data  are  especially  useful  when  carefully  plotted  with  observed  tempera- 
tures as  abscissas  and  corrections  as  ordinates.  A  family  of  curves  is 
thus  obtained  corresponding  to  the  different  values  of  the  emissivity. 

TABLE  4. — Corrections  to  Observed  Temperatures  for  Pyrometer 
Using  Red  Light  (X  =  0.65/*;  c2  =  14,350) 

Add  Corrections  Below  for  the  Following  Observed  Temperatures,  °C. 


700      800        900 

1000        1100  '   1200 

1300 

1400       1600        1800 

2000 

0.30 

55 

67 

80 

95 

111 

129 

148 

168 

213 

264 

322 

0.40 

41 

50 

60 

71 

83 

96 

110 

125 

158 

195 

237 

0.50 

31 

37       45 

53 

62 

71 

82 

93 

117 

144 

175 

0.60 

22 

27 

33 

39 

45 

52 

59 

67 

85 

104 

126 

0.70 

16 

19      23        27 

31 

36 

41 

47 

59 

72 

87 

0.80 

10 

12 

14 

17 

19 

22 

25 

29 

36 

44 

54 

0.90            5 

6        7 

8            9 

10 

12 

14 

17 

21 

25 

1.00            000 

0            0 

0          0 

0 

0          0 

0 

Temperature  of  Glowing  Gauze 

An  interesting  application  of  the  optical  pyrometer  is  for  the  measure- 
ment of  the  temperature  of  gauze  electrically  or  otherwise  heated.  In 
certain  chemical  processes,  platinum  gauze  electrically  heated  is  used  as 
a  catalyzing  agent,  and  must  be  maintained  at  a  constant  temperature. 
This  is  readily  done  by  sighting  normally  on  the  surface  of  the  gauze 
with  an  optical  pyrometer.  The  observed  temperatures  may  be  thus 
exactly  reproduced  from  day  to  day.  If  it  is  required  to  convert  the 
observed  temperatures  into  exact  true  temperatures  of  the  wire  forming 
the  gauze  the  problem  is  difficult.  An  approximate  solution  satisfactory 
for  all  industrial  work  is,  however,  easily  obtained. 

We  will  assume  that  the  mesh  of  the  gauze  is  sufficiently  coarse  so  that 
multiple  reflection  between  the  separate  wires  is  negligible.  Let  A\ 
be  the  fractional  part  of  the  total  area  of  the  gauze  comprised  by  the 
wire,  and  A^  be  the  fractional  part  of  the  total  area  representing  the  space 
between  the  wires.  Let  ffx  be  the  emissivity  of  the  metal  employed,  and 
E\  be  the  effective  emissivity  of  the  gauze  as  a  whole,  that  is,  taking  into 
consideration  the  spaces  between  the  wires,  which,  of  course,  are  not 
radiating  surfaces.  The  following  equations  are  readily  apparent. 


E\  = 


-f" 


since  Al  + 


=  1 


- 

1  _  1  =  log  A  iff  x 
0       S         9588 


340 


OPTICAL   AND    RADIATION    PYROMETRY 


where  #  is  the  true  absolute  temperature  of  the  wire  of  the  gauze,  and  S 
is  the  absolute  temperature  observed  with  an  optical  pyrometer  sighted 
normal  to  the  surface. 

A  platinum  gauze  commonly  employed  is  80-mesh  (80  wires  to  the 
inch)  of  0.003-in.  (0.07  mm.)  wire;  for  this  gauze,  Al  =  0.42.  The  emis- 
sivity  of  bright  clean  platinum  is  0.33.  The  platinum  of  this  gauze  soon 
becomes  somewhat  corroded;  possibly  an  emissivity  of  0.4  is  more  nearly 
the  correct  value  under  these  conditions.  Hence  the  effective  emissivity 
of  the  gauze  is  AI  EK  =  (0.42)  X  (0.4)  =  0.17.' 

Usually  the  gauze  must  be  viewed  through  a  glass  window.  A  thin 
glass  window  (see  below)  transmits  about  90  per  cent,  of  the  light  falling 
upon  it.  Hence  the  final  effective  emissivity,  using  a  glass  window,  is 
E\  =  (0.17)  X  (0.90)  =  0.15. 

Table  5  was  computed  by  the  formula  1/tf  —  1/&  =  log  0.15/9588. 
A  similar  table  for  other  gauzes  may  be  computed  in  the  manner  outlined. 

TABLE  5. — Platinum  Gauze,  SQ-mesh,  0.003-w.  Wire. 

(Temperatures  Observed  Through  One  Window  by  Optical  Pyrometer  Sighted 
Normal  to  Surface  of  Gauze,  and  True  Temperature  of  Gauze) 


Observed  Temp., 
Deg.  C. 

True  Temp., 
Deg.  C. 

Observed  Temp., 
Deg.  C. 

True  Temp., 
Deg.  C. 

600 

'675 

850 

975 

650 

730 

900 

1035 

700 

790 

950 

1095 

750 

850 

1000 

1160 

800 

910 

1050 

1220 

Observations  through  a  Window 

It  is  frequently  necessary,  especially  in  the  laboratory,  to  sight  an 

optical  pyrometer  into  a  furnace  through  a  window,  necessitating  a 

correction  to  the  observed  temperatures.     Kanolt  has  measured  the 

transmission  coefficient  for  a  number  of  ordinary  glass  windows  at  X  = 

0.65  M,  and  obtained  a  mean  value  pf  0.904.     Hence  we  have 

1       1       i 

tf       S  ~      9588 

where  t?  is  the  true  absolute  temperature  of  the  source  and  S  is  the  ob- 
served absolute  temperature.  Table  6  is  computed  from  the  above 
formula. 

Flames  and  Smoke 

The  optical  pyrometer  cannot  be  used  satisfactorily  when  sighted 
through  flames  or  smoke.     Usually  the  presence  of  dense  flames  increases 


-  =  -0.0000046 


PAUL   D.    FOOTE    AND    C.    O.    FAIRCHILD 


341 


the  temperature  reading,  and  the  presence  of  smoke  clouds  absorbs  so 
much  radiation  that  the  pyrometer  may  read  several  hundred  degrees  too 
low.  The  optical  pyrometer  can  be  used  to  measure  the  temperature  of 
the  slag  in  an  open-hearth  furnace  but  the  flames  prove  a  serious  hindrance 
except  during  reversals,  when  observations  may  be  taken  to  advantage. 
In  a  cement  kiln  the  dust,  smoke,  and  flames  all  combine  to  make  the 
observations  very  untrustworthy.  Carbon  dioxide,  water  vapor,  and 
other  invisible  gases  produce  no  effect. 

TABLE  6. — Correction  to  Observed  Temperatures  for  Absorption 
of  Light  by  a  Single  Clean  Window 


Observed  Temp., 
Deg.  C. 

Correction  to  add, 
Deg.  C. 

Observed  Temp., 
Deg.  C. 

^Correction  to  add, 
Deg.  C. 

600 

3.5 

1600 

16.0 

800 

5.4 

1800 

20.0 

1000 

8.0 

2000 

24.0 

1200 

10.0 

2500 

36.0 

1400 

13.0 

3000 

50.0 

Method  of  Sighting  into  a  Closed  Tube 

In  many  processes  where  smoke  cannot  be  eliminated,  or  where  black- 
body  conditions  are  not  satisfactory,  a  porcelain  or  other  refractory  tube 
with  a  closed  end  is  inserted  into  the  furnace.  The  pyrometer  is  sighted 
into  this  tube  which,  if  fairly  uniformly  heated  over  a  sufficient  area, 
affords  an  excellent  black  body.  This  method  has  been  employed  also 
for  obtaining  the  true  temperature  of  molten  metals,  but  suitable  refrac- 
tory tubes  for  many  molten  metals  have  yet  to  be  developed. 

RADIATION  PYROMETRY 

An  optical  pyrometer  measures  the  intensity  of  a  narrow  spectral 
band  of  radiation  emitted  by  a  glowing  object;  the  radiation  pyrometer 
measures  the  intensity  of  all  wave  lengths,  the  light  rays  and  the  heat  rays 
combined.  Usually  the  energy  radiated  by  the  source  is  focused  in  some 
manner  upon  the  hot  junction  of  a  small  thermocouple.  The  temperature 
to  which  this  junction  rises  is  approximately  proportional  to  the  rate  at 
which  energy  falls  upon  it,  which  in  turn,  by  the  Stefan-Boltzmann  law, 
is  proportional  to  the  fourth  power  of  the  absolute  temperature  of  the 
source.  The  rise  in  temperature  of  the  hot  junction  of  the  couple  gener- 
ates a  thermoelectric  force;  hence  the  calibration  of  a  radiation  pyrometer 
consists  in  determining  the  relation  between  the  e.  m.  f .  developed  and  the 
temperature  of  the  source  sighted  upon.  This  relation  follows  the  law 
e  =  at?6  where  #  is  the  absolute  temperature  of  source,  e  is  the  e.  m.  f . 


342 


OPTICAL    AND    RADIATION    PYROMETRY 


developed  by  the  instrument,  and  a  and  b  are  empirical  constants  determi- 
nable  by  two  standardization  points.  The  e.  m.  f.  may  be  measured  by  a 
potentiometer  or  galvanometer,  or  by  any  of  the  methods  applied  to 
thermoelectric  pyrometry.  The  galvanometer  should  have  as  high  a 
resistance  as  is  consistent  with  the  requirement  of  robustness.  The  same 
type  of  instrument  is  used  with  the  radiation  pyrometer  as  with  the  ordi- 
nary thermocouple.  The  temperature  of  the  cold  junction  of  the  couple 
in  the  radiation  pyrometer  is  not  controlled;  the  hot  and  the  cold  junc- 
tion are  in  fairly  close  proximity  and  hence  are  equally  affected  by  changes 
in  room  temperature.  The  cold  junction  is  always  shaded  from  the  heat 
radiated  by  the  source  sighted  upon. 


Thwing  Radiation  Pyrometer 

Fig.  8  illustrates  the  principle  of  the  pyrometer  made  by  the  Thwing 
Instrument  Co.  Radiation  from  the  furnace  enters  the  diaphragm  A 
and  falls  upon  the  hollow  conical  mirror  K.  The  hot  junction  C  of 
a  minute  thermocouple  is  located  at  the  apex  of  the  cone,  and  the  cold 


FIG.  8. — RECEIVING  SYSTEM  OF  THWING  RADIATION  PYROMETER. 

junctions  are  at  D  and  D'.  By  multiple  reflection  along  the  sides  of  the 
conical  mirror  the  radiation  is  finally  concentrated  upon  the  hot  junction 
of  the  couple.  The  e.  m.  f.  is  measured  by  a  galvanometer  graduated  to 
read  temperature  directly.  Fig.  9  shows  the  method  of  using  this  instru- 
ment for  measuring  the  temperature  of  a  large  furnace.  Except  for  inci- 
dental errors,  which  will  be  considered  later,  the  reading  of  the  instrument 
is  independent  of  the  sighting  distance,  provided  the  diameter  of  the 
source  is  sufficient  to  fill  the  cone  of  rays  defined  by  the  geometrical 
design  of  the  receiving  tube;  this  is  shown  in  Fig.  10.  The  amount  of 
radiation  falling  upon  an  element  D  of  the  conical  mirror  is  proportional 
to  the  solid  angle  HDA ',  which  is  independent  of  the  distance  from  the 
point  D  to  the  source;  this  is  true  of  every  point  on  the  base  of  the  cone 
DD'.  Hence,  the  total  energy  entering  the  cone  is  independent  of  the  dis- 
tance from  the  pyrometer  to  the  source,  provided  the  source  is  of  suffi- 
cient size.  The  minimum  size  of  source  for  any  distance  is  determined  by 
the  lines  A"D'  and  A'D.  Thus,  for  the  distance  BA  the  diameter  of 
the  source  must  be  at  least  A' A",  and  for  the  distance  BP  the  diameter  of 
the  source  must  be  at  least  P'P."  The  Thwing  instrument  is  so  con- 


PAUL   D.    FOOTE   AND    C.    O.    FAIRCHILD 


343 


structed  that  the  source  must  have  a  diameter  at  least  one-eighth  of  the 
distance  from  the  source  to  the  receiving  tube;  thus,  at  8  ft.  (2.4  m.)  from 
a  furnace  the  opening  into  the  furnace  must  be  1  ft.  (0.3  m.)  in  diameter. 


FIG.  9. — OBSERVING  TEMPERATURE  OP  A  FURNACE  WITH  THWING  RADIATION 

PYROMETER. 

For  permanent  installations,  the  tube  is  ventilated  and  has  several 
extra  diaphragms  to  prevent  local  heating  of  the  instrument  and  re- 


Fia.  10. — CONE  TYPE  OP  FIXED-FOCUS  RADIATION  PYROMETER. 

radiation  to  the  couple.  Fig.  11  shows  a  permanently  installed  Thwing 
radiation  pyrometer  sighted  into  the  bottom  of  a  closed  metal  tube  which 
projects  into  the  furnace. 


344 


OPTICAL    AND    RADIATION    PYROMETRY 


Foster  Radiation  Pyrometer 

Fig.  12  illustrates  the  principle  of  the  Foster  radiation  pyrometer,  made 
by  the  Taylor  Instrument  Companies.  The  thermocouple  6  and  a  front 
diaphragm  B  are  located  at  the  conjugate  foci  of  a  concave  mirror  DD'. 
As  in  the  case  of  the  Thwing  pyrometer,  the  source  must  be  large  enough 
to  fill  the  cone  of  rays  defined  by  the  angle  a,  or  the  lines  A'CA".  The 
position  of  the  point  C  is  marked  by  a  wing  nut  on  the  telescope  tube. 


'Fia.  11. — THWING  RADIATION  PYROMETER  FOR  FIXED  INSTALLATION  ON  FURNACE 

WALL. 

The  angle  a  is  made  such  that  the  diameter  of  the  source  sighted  upon 
must  be  at  least  one-tenth  the  distance  from  the  source  to  the  wing  nut. 
The  Brown  Instrument  Co.  makes  a  radiation  pyrometer  similar  in 
principle  to  the  Foster  pyrometer,  but  its  receiving  tube  is  collapsible  for 
convenience  in  carrying. 


FIG.  12. — MIRROR  TYPE  OF  FIXED-FOCUS  RADIATION  PYROMETER. 

Fery  Radiation  Pyrometer 

Fig.  13  is  a  cross-section  of  the  Fery  pyrometer,  made  by  the  Taylor 
Instrument  Companies.  Radiation  from  the  source  sighted  upon  is  con- 
centrated by  the  concave  mirror  of  speculum  metal  or  gold,  upon  the  hot 
junction  of  a  minute  thermocouple.  Unlike  other  types  of  radiation 


PAUL  D.  FOOTE  AND  C.  O.  FAIRCHILD 


345 


pyrometer,  this  instrument  requires  focusing  for  each  sighting  distance, 
this  being  accomplished  by  an  ingenious  device  due  to  Fery.  Two 
semicircular  mirrors  (a),  Fig.  14,  inclined  to  one  another  at  an  angle  of 


FIG.  13. — FERY  RADIATION  PYROMETER. 

5°  to  10°,  are  mounted  in  the  thermocouple  box,  an  opening  of  about  1.5 
mm.  at  the  center  of  the  mirrors  forming  the  limiting  diaphragm  imme- 
diately in  front  of  the  couple.  The  observer  views,  by  means  of  the  tele- 
scope D,  the  image  of  the  furnace  formed  by  the  large  concave  mirror  MM' 


FIG.  14. — OPTICAL  SYSTEM  OP  FERY  RADIATION  PYROMETER. 

and  reflected  by  the  inclined  mirrors  xxr  and  yy'  through  a  hole  in  the  large 
mirror.  If  the  image  is  not  correctly  focused  at  0,  the  intersection  of 
the  two  small  mirrors,  the  image  appears  broken  in  half,  as  shown  by 


346 


OPTICAL   AND    RADIATION    PYROMETRY 


(6).  Correct  focus  is  obtained  when  the  two  halves  of  the  image  are  in 
alignment  (c).  This  breaking  of  a  line  is  illustrated  on  a  magnified 
and  distorted  scale  by  (d).  Suppose  that  the  pyrometer  were  incorrectly 
focused  upon  a  line  source,  say .  an  arrow,  the  image  falling  at  position 
A B  instead  of  at  0.  The  image  of  the  arrow  reflected  from  the  mirror 
yy'  lies  at  A"B"  and  that  reflected  from  the  mirror  xx'  at  A'B',  and 
to  the  observer  at  D  the  projections  of  these  images  appear  as  two  distinct 
arrows.  As  the  pyrometer  is  brought  into  the  correct  focus  by  turning 
the  pinion  screw  and  moving  the  large  concave  mirror  in  the  direction 


FIG.  15. — FERY  RADIATION  PYROMETER  IN  WEATHER  HOOD,  SIGHTED  INTO  A  FIRE- 
CLAY TUBE. 

OD,  the  points  P'  and  P"  of  the  reflected  images  move  along  the  lines 
P'O  and  P"0,  coinciding  at  0,  when  the  correct  focus  is  obtained. 

For  the  measurement  of  very  high  temperatures,  usually  above  1500° 
C.,  the  cover  to  the  front  of  the  telescope  is  provided  with  a  sectored 
opening  which  may  be  adjusted  to  reduce  the  radiation  falling  upon  the 
receiver  by  any  definite  amount,  and  in  this  manner  the  upper  tempera- 
ture limit  of  the  instrument  may  be  greatly  extended.  This  adjustment 
is  made  by  the  manufacturer  and  should  not  be  altered;  for  the  lower 
scale  range  the  cover  is  open,  as  shown  in  Fig.  15. 

The  readings  with  a  Fery  pyrometer,  when  properly  focused,  and 
neglecting  secondary  errors  discussed  later,  are  independent  of  the  sight- 
ing distance,  as  is  the  case  with  the  fixed-focus  radiation  pyrometer.3 

1  For  the  geometrical  demonstration  of  this  fact,  see  U.  S.  Bureau  of  Standards 
Sci.  Paper  250,  97. 


PAUL    D.    FOOTE    AND    C.    O.    FAIRCHILD 


347 


The  image  of  the  source,  as  viewed  through  the  small  telescope,  must 
cover  completely  the  limiting  diaphragm  of  the  thermocouple.  This 
diaphragm  appears  as  a  black  circular  area,  shown  at  the  center  of  the 
field  on  Fig.  14  (6)  and  (c).  An  excellent  rule  is  to  sight  at  such  a  distance 
that  the  area  of  the  image  overlaps  this  hole  and  extends  half-way  to  the 
edge  of  the  focusing  mirrors.  The  Fery  pyrometer  requires  a  smaller 
source  than  the  fixed-focus  instruments.  Table  7  indicates  the  size  of 
source  required  for  various  sighting  distances,  in  accordance  with  the 
above  rule. 

TABLE  7 


Sighting  Distance, 
Cm. 

Diameter  of  Source, 
.  -1    .     Cm. 

Sighting  Distance, 
Cm. 

Diameter  of  Source, 
Cm. 

70 

3.2 

200 

11.2 

80 

3.7 

300 

16.8 

100 

4.8 

500 

28.5 

150 

8.3 

Errors  to  which  Radiation  Pyrometers  are  Subject 

Dust  and  dirt  allowed  to  accumulate  upon  the  concave  reflecting  mir- 
ror may  so  decrease  its  reflection  coefficient  as  to  develop  errors  amount- 
ing to  100°  or  even  200°  C.  Frequently  the  dust  can  be  removed  from 
the  mirror  by  carefully  brushing  with  a  camel's  hair  brush.  The  mirror 
may  be  removed  from  the  instrument  and  washed,  but  this  must  be  done 
with  great  care  to  insure  that  the  delicate  thermocouple  or  its  mount- 
ing is  not  disturbed.  The  safest  practice  is  to  take  all  possible  precau- 
tions to  prevent  dust  from  entering  the  instrument.  Keep  the  case 
closed,  or  the  front  diaphragm  of  the  fixed-focus  instrument  plugged  with 
a  cork  when  not  in  use. 

As  shown  above,  radiation  pyrometer  readings  are  theoretically  inde- 
pendent of  the  sighting  distances  or  the  size  of  source,  provided  the  source 
is  larger  than  the  minimum  size  demanded  by  the  geometry  of  the  instru- 
ment. Actually  this  ideal  condition  is  not  always  realized.  Some  stray 
radiation  is  reflected  down  the  walls  of  the  telescope  case,  which  become 
heated  and  re-radiate  to  the  couple;  the  same  is  true  of  the  limiting  dia- 
phragms. For  these  reasons,  a  radiation  pyrometer  tends  to  read  low 
the  greater  the  sighting  distance  or  the  smaller  the  size  of  source.  It  is 
therefore  desirable  to  use  a  radiation  pyrometer  as  nearly  as  possible 
in  the  same  manner  from  day  to  day,  and  to  have  it  specially  calibrated 
for  such  conditions.  In  the  Fery  pyrometer,  both  the  proper  size  of 
source  and  the  correct  focusing  distances  are  secured  by  following  the 
rule  previously  suggested. 


348  OPTICAL   AND    RADIATION    PYROMETRY 

Advantages  and  Disadvantages  of  Radiation  Pyrometers 

For  temperatures  above  1400°  or  1500°  C.,  either  a  radiation  or  an 
optical  pyrometer  must  be  employed.  The  optical  pyrometer  is  capable 
of  higher  accuracy  and  is  less  susceptible  to  errors  than  the  radiation 
pyrometer.  Smoke  and  dust  affect  the  readings  of  both  instruments, 
but  the  radiation  pyrometer  is  seriously  affected  by  the  presence  of  cooler 
strata  of  carbon  dioxide  and  other  gaseous  combustion  products  in  the 
furnace.  Carbon  dioxide  and  water  vapor  absorb  the  heat  rays,  and  hence 
the  radiation  pyrometer  will  read  too  low  when  sighted  through  such 
gases.  The  main  advantage  of  the  radiation  pyrometer  is  the  fact  that 
it  can  be  made  automatically  recording.  The  recording  mechanism  is 
the  same  as  that  employed  for  ordinary  thermocouples.  The  radiation 
pyrometer  is  desirable  for  many  processes  requiring  lower  temperatures, 
where  thermocouples  can  not  be  conveniently  installed.  It  is  also  useful 
in  measuring  the  surface  temperature  of  large  ingots. 

Black-body  and  Non-black-body  Conditions 

Radiation  pyrometers  are  calibrated  to  read  correctly  when  sighted 
upon  a  black  body.  Most  furnaces  approximate  black-body  conditions 
sufficiently  well,  but  when  sighting  on  materials  in  the  open,  certain 
corrections  must  be  applied  to  the  observed  temperatures.  For  tem- 
perature control  or  reproducibility  the  apparent  temperatures  may  be 


•  FiG.  16. — METHOD  FOR  DETERMINING  PROPER  DIAMETER  OF  SIGHTING  TUBE. 

used  uncorrected  if  desired,  since,  although  known  to  be  low,  they  will 
be  low  by  the  same  amount  from  day  to  day.  In  case  the  tempera- 
ture of  one  portion  of  an  unequally  heated  furnace  is  required,  or  if 
the  furnace  contains  much  smoke  or  dust,  it  is  frequently  desirable  to 
sight  the  radiation  pyrometer  into  the  bottom  of  a  fire-clay  or  porce- 
lain tube,  as  illustrated  by  Fig.  15.  The  tube  should  be  uniformly 
heated  at  its  end  for  a  length  at  least  three  times  its  diameter.  The  tube 
must  also  be  of  such  diameter  that  the  cone  of  rays  entering  the  tele- 
scope is  not  intercepted  by  the  front  of  the  tube.  In  Fig.  16,  suppose  the 
distance  from  the  bottom  of  the  tube  to  the  mirror  of  the  Fery  pyrometer 
is  100  cm.  Referring  to  the  above  table,  the  required  diameter  of  the 
source  is  4.8  cm.  Lay  off  the  distance  6  =  4.8  cm.  and  draw  straight 
lines  from  the  bottom  of  b  to  the  bottom  of  the  mirror  c,  and  from  the 
top  of  6  to  the  top  of  the  mirror.  The  tube  must  have  such  a  diameter 
at  its  front  end  a  that  it  does  not  cut  in  on  the  cone  of  rays  represented  by 
these  two  straight  lines.  This  can  be  determined  only  by  actually  mak- 


PAUL  D.  FOOTE  AND  C.  O.  FAIRCHILD 


349 


ing  the  above  drawing  to  scale,  or  by  computing;  it  cannot  be  decided 
by  looking  through  the  telescope  of  the  pyrometer,  since  the  front  end  of 
the  tube  would  not  necessarily  appear  even  if  it  did  cut  in  on  the  cone  of 
rays.  In  the  case  of  the  fixed-focus  instruments,  the  diameter  of  the 
tube  must  be  such  that  the  cone  of  rays  represented  by  the  diameter 
A' A",  Fig.  12,  cuts  the  tube  in  the  region  which  is  uniformly  heated. 

Table  8  shows  the  true  temperatures  corresponding  to  the  apparent 
temperatures  observed  with  a  radiation  pyrometer  when  sighted  upon 
various  materials  in  the  open.  This  table  must  not  be  confused  with 
Table  2  relating  to  the  optical  pyrometer;  it  will  be  noticed  that  the  cor- 
rections are  entirely  different  for  the  two  types  of  pyrometer. 

TABLE  8. — True  Temperatures  and  Apparent  Temperatures  Measured  by 

Radiation  Pyrometers  when  Sighted  Upon  Various  Materials 

in  the  Open 

ObservedTem-  True  Temperature,  Degrees  C. 

perature,          

Degrees 

C.  Molten  Iron        Molten  Copper       Copper  Oxide 


Iron  Oxide 


Nickel  Oxide 


600                1130        720 

630        710 

650       1210        775 

755 

700 

1290        830 

735        800 

750 

890 

845 

800 

1200 

945 

840        895 

850 

1270 

1000 

940 

900 

1340 

1060 

945        985 

950 

1410 

1115 

1030 

1000 

1475 

1170 

1050 

1075 

1050 

1550 

....        .... 

1120 

1100 

1610 

1155 

1165 

1150 

1680 

1210 

1200       1750       

1260 

1255 

DISCUSSION 

E.  F.  Northrup,*  Trenton,  N.  J. — The  theory  of  the  optical  pyro- 
meter is  so  simple,  I  am  speaking  now  of  the  disappearing-filament  type, 
and  the  instrument  works  so  well  in  the  laboratory  and  is,  comparatively, 
so  inexpensive  that  one  wonders  there  should  be  any  use  of  any  kind  of 
direct  insertion  pyrometers  at  all,  because  all  industrial  temperatures  we 
wish  to  measure  very  accurately  are  high  enough  to  give  off  luminous 
rays.  In  the  field,  however,  there  are  a  few  features  with  which  the 
laboratory  does  not  have  to  contend. 

I  was  visiting  a  copper  works  at  Perth  Amboy  where  the  copper 

*  President,  Pyrolectric  Instrument  Co. 


350  OPTICAL   AND    RADIATION    PYROMETRY 

was  run  into  the  molds  as  they  were  carried  on  a  belt.  At  one  time  a 
dozen  of  these  molds  were  filled  at  once  when  I  stood  about  15  ft.  away. 
When  the  copper  ran  in,  its  surface  was  very  bright'  and  I  felt  a  certain 
amount  of  radiation  on  my  face.  In  a  moment,  quite  suddenly,  an  oxide 
film  spread  all  over  these  molds  at  the  same  time,  and  those  standing 
by  me  said  they  noticed  a  great  increase  in  the  amount  of  radiation 
striking  the  face,  but  the  surface  of  the  copper  was  very  much  duller. 
Now,  if  there  had  been  set  up  at  that  place,  directed  upon  this  molten 
surface  of  copper,  a  radiation  pyrometer  and  an  optical  pyrometer,  be- 
fore the  oxide  film  had  formed,  the  radiation  pyrometer  would  have  indi- 
cated a  lower  temperature  than  the  optical  pyrometer.  Immediately 
after  the  film  had  formed,  the  reverse  would  have  been  true.  In 
foundries  and  in  glass  works,  physical  conditions  limit  the  general  use- 
fulness of  the  optical  pyrometer  in  many  cases,  for  which  reason  the 
direct-insertion  pyrometers  are  used  in  large  numbers. 

Doctor  Foote  did  not  mention  the  fact  that  the  radiation  pyrometer, 
which  generates  an  e.m.f.  in  a  thermocouple,  can  be  made  with  suitable 
contrivances  for  recording  the  results.  An  optical  pyrometer,  where  the 
filament  of  a  lamp  is  matched  in  brightness  against  the  brightness  of  the 
background,  affects  only  the  human  eye  and  there  is  no  apparent  means 
of  making  the  optical  pyrometer  record. 

THE  CHAIRMAN  (G.  K.  BURGESS,  Washington,  D.  C.) — Both  the 
radiation  and  the  optical  pyrometer  will  have  higher  readings  when 
sighted  on  the  copper  oxide  at  the  same  temperature  as  the  liquid  copper. 

E.  F.  NORTHRUP. — You  mean  it  will  look  brighter  to  the  optical  pyro- 
meter after  the  film  has  formed? 

CHAIRMAN  BURGESS. — Yes. 

E.  F.  NORTHRUP. — It  does  not  look  brighter  to  the  eye. 

A.  G.  WORTHING,  Nela  Park,  Cleveland,  0. — Doctor  Foote  spoke 
of  the  optical  pyrometer  as  being  a  photometer.  In  the  laboratory, 
at  least,  there  are  dangers  that  may  occur  on  the  assumption  that  that  is 
true.  With  a  photometer,  a  person  naturally  moves  the  sight  box  along 
the  bars  until  he  has  two  illuminations  that  appear  to  be  equal ;  in  other 
words,  he  equates  the  illuminations.  In  the  case  of  the  disappearing- 
filament  pyrometer,  that  is  not  quite  true.  Apparently  the  pyrometer 
filament  is  as  bright  as  the  background  filament,  but  the  equality  is  not 
necessarily  real.  Doctor  Forsythe  and  I  found,  in  some  cases,  by  small 
variations  of  angular  aperture,  that  for  an  apparent  match  we  could  have 
the  pyrometer  filament  as  much  as  1.6  times  as  bright  as  the  background 
it  was  sighted  upon,  that  is,  1.6  times  as  bright  after  we  had  made  correc- 
tions for  the  glassware  in  between.  In  other  cases  it  might  be  only  nine- 
tenths  as  bright  as  the  background.  If  a  person  uses  the  pyrometer 


DISCUSSION 


351 


in  the  right  way,  there  is  no  trouble,  but  there  have  been  applications 
of  this  pyrometer  in  the  past  in  which  that  method  has  not  been  employed 
and  erroneous  results  have  been  obtained.  Such  troubles,  of  course, 
are  not  likely  to  appear  in  the  industrial  world;  they  are  more  likely 
to  appear  in  the  laboratory. 

P.  D.  FOOTE. — It  is  true  that  the  brightness  of  the  filament  of  the 
pyrometer  lamp  may  be  different  from  that  of  the  background  when 
the  condition  of  a  match  is  obtained.  It  is,  however,  correct  to  call 
the  optical  pyrometer  a  photometer  since,  in  use,  the  brightness  of  the 
source  sighted  upon  is  compared  to  the  brightness  of  the  black  body  used 
in  the  calibration  of  the  pyrometer,  the  lamp  serving  merely  as  a  trans- 
ferring device. 

F.  A.  HARVEY,  Syracuse,  N.  Y. — In  focusing  the  optical  pyrometer,  it 
is  necessary  to  focus  both  the  eyepiece  and  the  objective.  The  usual 
method  is  to  focus  the  eyes  on  infinity  and  adjust  the  eyepiece  until  the 
filament  is  sharply  defined  and  then  the  objective.  In  the  types  with 
which  I  am  familiar  the  pyrometer  must  be  removed  from  the  eye  while 
the  eyepiece  is  focused,  by  screwing  it  in  or  out.  This  makes  the  process 
of  focusing  very  tedious  and  difficult. 

Dr.  Foote  spoke  of  the  unreliability  of  a  platinum  couple  for  high 
temperatures.  When  a  controversy  arises  over  specifications  on  fire- 
brick, if  you  bought  firebricks  under  certain  temperature  specifications, 
the  first  thing  the  firebrick  manufacturer  questions  is  your  temperature. 
If  that  is  measured  by  an  optical  pyrometer,  it  is  necessary  to  know  the 
skill  and  honesty  of  the  one  who  made  the  test.  But  with  a  recording 
pyrometer,  it  is  possible  to  show  the  records,  which  may  also  show  a 
calibration  of  the  couple.  He  will  seldom  question  these.  We  find 
the  optical  pyrometer  exceedingly  useful  as  a  check  up  but  not  as  an 
operating  instrument. 


352  DISAPPEARING-FILAMENT    OPTICAL   PYROMETER 


Industrial  Applications  of  Disappearing-filament  Optical  Pyrometer 

BY    F.    E.   BASH,*   CH.    E.,    PHILADELPHIA,    PA. 
(Chicago  Meeting,  September,  1919) 

A  GREAT  many  industrial  operations  require  the  application  of  heat 
to  carry  on  or  complete  processes,  in  which  cases  the  temperatures  must 
often  be  controlled  within  very  narrow  limits.  For  the  lower  tempera- 
tures, this  control  is  not,  as  a  rule,  difficult  as  a  number  of  types  of  reliable 
pyrometers  are  available  for  the  work;  for  example,  the  mercury  and 
resistance  thermometers  and  the  thermocouple.  For  temperatures 
above  a  red  heat,  thermocouples,  platinum  resistance  thermometers,  and 
total  radiation  and  optical  pyrometers  may  be  used,  depending  on 
conditions. 

It  is  often  undesirable,  and  sometimes  impossible,  to  take  temperatures 
of  material  by  immersing  the  pyrometer  in  it  or  by  placing  the  tempera- 
ture-measuring device  in  a  position  in  which  it  will  attain  the  tempera- 
ture of  the  material.  The  pyrometer  may  contaminate  the  product;  the 
temperature  may  be  so  high  that  the  pyrometer  will  not  hold  its  calibra- 
tion, due  to  ineffectual  protection  and  consequent  contamination;  the 
object  may  be  inaccessible,  as  a  lamp  filament,  etc.  In  such  cases,  a  total 
radiation  or  optical  pyrometer  must  be  used. 

The  disappearing-filament,  or  Morse,  type  of  optical  pyrometer  works 
on  the  following  principle:  An  objective  lens,  Fig.  1,  focuses  the  image 
of  the  object  whose  temperature  is  desired  in  the  plane  of  the  lamp  filament 
at  F,  which  is  at  the  principal  focus  of  the  ocular  lens  shown  in  the  eye- 
piece. By  this  arrangement  the  eye  will  not  have  to  accommodate  itself 
first  to  the  lamp  filament  and  then  to  the  object  at  a  distance,  and  the 
lamp  filament  will  appear  superposed  on  the  object.  To  balance  the  in- 
strument, the  current  is  adjusted  by  means  of  a  rheostat  until  the  tip 
of  the  lamp  filament  just  disappears  against  the  object  as  a  background. 
Figs.  2,  3,  and  4  show  the  appearance  of  the  filament  when  too  cold, 
too  bright,  and  just  balanced. 

To  find  the  temperature  of  the  incandescent  body,  the  current  through 
the  lamp  is  read  from  the  ammeter  and  reference  made  to  a  calibration 
sheet  on  which  are  tabulated  values  of  temperature  corresponding  to 
different  currents. 

The  calibration  of  an  optical  pyrometer  is  for  "black-body"  tempera- 
tures. A  black  body  is  a  perfect  radiator,  or  a  body  that  absorbs  all 
radiation  which  falls  upon  it,  reflects  none,  and  transmits  none  away. 

*  Research  Engineer,  Leeds  &  Northrup  Co. 


F.    E.   BASH 


353 


Any  material  that  is  not  a  black-body  radiator  gives  off  less  radiation 
than  a  perfect  radiator  and,  if  it  is  incandescent,  appears  colder  and  less 
bright  than  a  black  body  at  the  same  temperature.  For  this  reason 
corrections  have  to  be  made  to  optical  pyrometer  readings  on  selectively 
radiating  bodies  that  depend  on  their  "emissivity,"  or  the  amount  of 
light  they  emit.  If  the  emissivity  is  known,  the  following  formula  de- 


VNAAAA^ 


FIG.  1. 


rived  from  Wien's  Law  may  be  used  to  find  the  relation  between  apparent 
temperature  and  true  temperature. 

C2  log  e 
Log  E  =  — ^e 

In  which  E    =    emissivity; 

X  =  wave  length  of  light  used; 
C2  =  14,500; 

e  =  base  of  Napierien  logarithms; 
TI  =  apparent  temperature  of  body; 
>T2  =  true  temperature  of  body. 

If  the  emissivity  is  not  known,  it  can  be  found  by  this  relation,  if 
the  true  and  apparent  temperatures  are  determined,  by  various  methods 
that  have  been  discussed  in  publications  of  the  Bureau  of  Standards1  and 
elsewhere. 

1  Burgess  and  Waltenberg :  U.  S.  Bureau  of  Standards  Sci.  Paper  242. 

23 


354 


DISAPPEARING-FILAMENT   OPTICAL   PYROMETER 


Black-body  conditions  are  closely  approximated  in  furnaces  and  uni- 
formly heated  enclosures  and  by  a  number  of  commonly  used  materials, 
so  that  small  errors,  if  any,  are  encountered  for  the  majority  of  indus- 
trial conditions.  However,  for  molten  metals  and  material  with  reflect- 
ing surfaces,  certain  corrections  must  be  made  or  black-body  conditions 
secured  by  some  means  or  other  if  the  true  temperature  is  desired.  Often 
all  that  is  required  is  to  be  able  to  repeat  a  certain  temperature  under 
the  same  conditions.  In  such  cases,  the  "apparent"  temperature  suffices 


FIG.  2. — FILAMENT  WHEN  TOO  COLD. 


FIG.  3. — FILAMENT  WHEN 
TOO  BRIGHT. 


FIG.  4. — FILAMENT  WHEN  JUST 
BALANCED. 


without  its  being  necessary  to  determine  the  true  temperature.  It  is 
generally  desirable,  however,  to  know  the  true  temperature  in  order 
to  be  able  to  compare  with  other  operations. 

One  precaution  that  must  be  observed  in  making  optical  pyrometer 
readings  is  that  the  instrument  must  not  be  sighted  through  any  con- 
siderable amount  of  smoke  or  flame,  particularly  smoke.  Flame  in- 
creases the  reading  but  is  not  generally  serious  for  light  flames;  smoke 
decreases  the  reading.  Smoke  absorbs  the  light  from  the  object  and 
has  a  large  effect  on  the  readings.  It  is  generally  possible,  however,  to 


F.    E.   BASH  355 

sight  under  the  smoke  and  flame  or  to  get  rid  of  them  while  the  reading 
is  being  made. 

To  secure  black-body  conditions,  a  number  of  methods  may  be  used. 
If  the  material  is  not  reflecting  and  is  in  a  uniformly  heated  furnace,  the 
pyrometer  may  be  sighted  directly  on  it  and  true  temperatures  will  be 
read.  If  the  material  has  cracks  or  holes  in  it,  true  temperatures  will 
be  obtained  by  sighting  into  them  as  the  radiation  from  them  closely 
approximates  that  of  a  perfect  radiator.  If  the  temperature  of  a  gas  is 
desired,  a  closed-end  tube  may  be  placed  so  that  the  gas  will  heat  the  end 
and  its  temperature  can  be  ascertained  by  sighting  down  the  tube  at  the 
end.  Another  method  is  to  hang  a  metal  target  in  the  gas  and  sight  on 
that.  The  temperature  of  molten  metal  can  also  be  taken  by  pushing 
a  closed-end  tube  into  it. 

APPLICATION  OF  OPTICAL  PYROMETER  TO  STEEL  INDUSTRY 

It  has  been  determined  that  oxide  of  iron  is  approximately  a  black 
body  and  that  the  difference  between  true  and  apparent  temperatures 
for  an  optical  pyrometer  using  red  light  is  only  a  few  degrees,  so  that  no 
correction  is  made  when  sighting  on  an  iron  billet  either  in  a  furnace  or 
outside.  This  is  a  fortunate  circumstance  as  no  precautions-  need  be 
observed  to  procure  black-body  conditions.  The  main  difficulty  is  that 
the  scale  on  an  iron  billet  is  generally  loose  and  does  not  adhere  closely 
to  the  piece.  This  means  that  the  scale  will  be  much  colder  than  the 
billet,  but  if  a  little  judgment  is  used  when  readings  are  made  and  the 
observations  are  made  on  the  bright  spots  or  the  loose  scale  is  knocked 
off  where  it  is  desired  to  read,  no  difficulty  will  be  encountered  from  this 
source.  If  a  piece  has  an  angle  or  hole  or  crack  in  it,  the  reading  can  be 
made  at  those  points. 

One  of  the  advantages  of  the  disappearing-filament  type  optical 
pyrometer  is  that  the  object  can  actually  be  seen  through  the  telescope 
and  it  is  an  easy  matter  to  select  any  portion  or  particular  spot,  no  matter 
how  small,  and  balance  on  that.  This  is  particularly  desirable  in  the  case 
of  hot  steel  that  is  being  worked.  It  has  a  mottled  appearance  due  to 
the  loose  scale  on  it  so  that  any  instrument  which  has  to  focus  on  a  com- 
paratively large  field  will  give  a  temperature  that  depends  on  the  average 
brightness  of  that  portion  of  the  billet. 

Molten  Steel  and  Slag. — In  the  case  of  molten  steel  or  slag,  we  have 
conditions  that  are  not  black-body  and  corrections  must  be  made  to 
optical  pyrometer  readings.  In  the  open-hearth  furnace,  according 
to  Burgess,  temperatures  can  be  taken  of  the  walls  and  roof  and  slag 
surface  without  making  any  correction  for  emissivity.2  It  is  question- 
able, however,  how  close  the  temperature  of  the  slag  surface  is  to  the 

2  Temperature  Measurements  in  Bessemer  and  Open-hearth  Practice.  U.  S.  Bureau 
of  Standards  Tech.  Paper  91. 


356 


DISAPPEARING-FILAMENT    OPTICAL  PYROMETER 


steel  temperature;  as  a  matter  of  fact,  quite  large  differences  are  often 
noted  when  tapping.  Sometimes  the  steel  is  hotter  and  sometimes  colder 
than  the  slag,  depending  on  furnace  conditions. 

Tapping  temperatures  can  be  taken  very  readily  by  sighting  on  the 
steel  or  slag  stream  and  applying  the  corrections  as  worked  out  by 
Burgess.  These  corrections  are  for  an  emissivity  of  0.40  for  steel  and 
an  average  emissivity  of  0.65  for  slag.  Curves  showing  the  relation 
between  true  and  apparent  temperatures  for  steel  and  slag  are  shown  in 


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APPARENT     TEMPERATURE     (FAHR) 

FIG.  5.  —  EMISSIVITY  CORRECTIONS  FOR  STEEL  AND  SLAQ. 
Curve  1—  Steel.    Curve  2—  Slag. 


Fig.  5.  When  taking  tapping  temperatures,  the  observer  should  stand 
on  the  windward  side  of  the  stream,  if  possible,  and  thus  avoid  smoke. 
Readings  can  be  made  on  the  slag  or  steel  stream  with  ease  and,  when 
corrected,  very  consistent  results  will  be  noted. 

Teeming  temperatures  can  also  be  taken  very  readily  by  sighting  on 
the  metal  stream  from  the  ladle  and  thus  obtaining  the  temperature  at 
which  each  ingot  is  teemed.  The  temperature  of  the  metal  rising  in 


F.    E.   BASH  357 

bottom-poured  molds  can  also  be  taken  if  there  is  no  smoke  to  interfere. 

Stream  temperatures  of  molten  cast  iron  and  pig  iron  have  also  been 
taken  successfully,  the  emissivity  of  0.40  being  used,  although  this  value 
has  not  been  satisfactorily  checked.  It  serves,  however,  to  get  com- 
parable results  and  is  probably  very  close  to  the  actual  emissivity. 

To  take  temperatures  in  the  blast  furnace  is  a  difficult,  although  not 
impossible,  matter  if  the  proper  precautions  are  taken.  If  the  tuyere 
glass  is  first  calibrated  for  its  absorption  effect  on  the  optical  reading, 
observations  may  be  made  and  very  interesting  and  instructive  data  can 
be  obtained. 

Forging. — Temperatures  in  forging  furnaces  can  be  taken  with  the 
disappearing-filament  type  optical  pyrometer  and  can  be  used  to 
good  advantage  in  determining  when  an  ingot  is  sufficiently  hot  to  forge. 
To  make  sure  that  the  temperature  taken  on  a  billet  of  steel  in  a  forging 
furnace  is  not  too  high,  due  to  loose  scale  becoming  hotter  than  the  piece, 
it  is  sometimes  well  to  run  a  bar  into  the  furnace  and  clean  the  surface 
at  the  point  where  the  reading  is  made. 

Forging  temperatures  are  a  more  or  less  uncertain  matter  due  to  the 
cooling  of  the  outside  surface  and  loose  scale.  Readings  can  be  made, 
however,  as  the  scale  falls  off  after  the  first  blows  of  the  hammer.  If 
the  temperature  gradient  through  the  piece  is  known  approximately, 
the  optical  reading  on  the  surface  will  serve  as  an  indication  of  the  tem- 
perature of  the  piece.  The  rule  to  follow  is  always  to  pick  the  brightest 
spot  and  a  place  free  from  loose  scale.  With  a  little  practice  and  judg- 
ment, much  useful  information  can  be  obtained. 

Rolling. — It  is  often  very  desirable  to  know  the  finishing  temperatures 
of  rolling  operations;  they  may  be  easily  obtained  with  the  disappearing- 
filament  optical  pyrometer.  If  small  stock  is  being  rolled,  temperatures 
may  be  determined  accurately  after  the  first  pass,  as  all  loose  scale  has 
dropped  off  and  the  iron  oxide  is  tightly  packed  and  is,  therefore,  at  the 
temperature  of  the  steel.  The  pyrometer  operator  can  judge  if  the  con- 
ditions are  favorable  for  temperature  measurements  by  noting  if  there 
are  black  spots  on  the  piece,  these  being  indications  of  loose  scale. 

In  the  case  of  material  of  greater  cross-section  than  2  in.  or  there- 
abouts, it  generally  takes  more  than  one  pass  to  free  it  of  scale.  This 
depends  a  great  deal  on  conditions,  however,  and  each  case  can  readily 
be  settled  by  bearing  in  mind  to  sight  on  the  bright  spots.  This  particu- 
larly applies  to  rolling  sheets  where  the  scale  lies  loosely  on  the  surface. 
In  this  case  the  temperature  reading  can  be  made  very  readily  just  after 
a  pass  while  it  is  momentarily  stationary  before  reversing. 

To  make  quick  readings,  it  has  been  found  advantageous  to  set  the 
current  through  the  lamp  at  a  value  corresponding  to  the  approximate 
temperature  of  the  piece  so  that  a  slight  adjustment  will  serve  to  make 
a  balance.  If  this  is  done,  temperatures  of  small  rolling  stock  can  be 


358  DISAPPEARING-PILAMENT   OPTICAL   PYROMETER 

taken  in  a  couple  of  seconds  and  it  will  not  be  necessary  to  halt  the 
operation  to  make  the  reading. 

Heat  Treating. — Temperatures  of  steel  in  heat-treating  or  annealing 
furnaces  can  be  taken  accurately  by  the  use  of  the  optical  pyrometer, 
which  has  the  added  advantage  of  being  able  to  take  temperatures  of 
any  piece  in  the  furnace  or  any  section  of  a  particular  piece,  whereas  a 
thermocouple  may  give  the  gas  temperature,  which  may  not  bear  any 
relation  to  the  piece  being  treated. 

USE  OF  OPTICAL  PYROMETERS  IN  THE  GLASS  INDUSTRY 

The  optical  pyrometer  may  be  used  in  determining  the  temperature 
of  molten  glass  under  various  conditions  but  proper  precautions  must 
be  taken  in  each  case  to  avoid  incorrect  results.  The  errors  arise  from 
a  lack  of  black-body  conditions  and  may  be  avoided  in  all  cases  by  using 
a  closed-end  sight  tube.  This,  however,  is  not  always  necessary  and  a 
number  of  experiments  have  been  made  to  determine  the  practicability  of 
a  sight  tube  and  other  methods  by  which  glass  temperatures  could  be  most 
readily  taken  under  practical  working  conditions.  To  this  end,  observa- 
tions of  glass  temperatures  were  made  in  various  factories  on  different 
types  of  furnaces.  Until  experience  indicated  no  further  need,  results 
were  checked  by  using  a  thermocouple  as  well  as  the  optical  pyrometer. 
The  thermocouple  was  finally  discarded,  as  it  was  possible  to  secure  the 
same  results  with  greater  ease  by  producing  black-body  conditions 
The  employment  of  thermocouples  in  practical  work  is  somewhat  diffi- 
cult for  a  number  of  reasons.  If  a  silica  or  porcelain  protecting  tube  is 
used,  it  will  withstand  the  effects  of  the  glass  long  enough  to  get  a  few 
checks  but  the  couple  is  rendered  useless  for  practical  routine  tests  be- 
cause the  tubes  soon  crack  due  to  the  contraction  of  the  cooling  glass, 
which  adheres  to  them.  If  a  noble-metal  couple  is  immersed  bare  in 
the  glass,  it  becomes  contaminated  and  brittle  so  that  incorrect  results 
are  obtained  as  a  consequence. 

Iron  or  nichrome  protecting  tubes  were  not  used  for  fear  that  the 
batch  of  glass  might  become  colored  by  material  being  dissolved  from 
them.  They  can  be  used  occasionally  in  window  glass,  as  iron  tools  are 
used  continually  for  stirring  and  skimming.  This  would  probably  be 
objectionable  with  plate  or  optical  glass,  in  which  cases,  however,  clay 
tubes  could  be  and  have  been  used  successfully. 

After  a  number  of  checks  between  a  thermocouple  immersed  in  the 
glass  and  an  optical  pyrometer  reading  on  the  bottom  of  a  closed-end 
tube  beside  the  thermocouple,  in  which  the  two  checked  each  other  with- 
in 10°  F.,  it  was  decided  that  there  was  no  further  necessity  of  using  a 
thermocouple  for  checking  as  the  tube  was  much  simpler  and  the  extra 
apparatus  could  be  dispensed  with. 

In  most  cases,  errors  due  to  reflection  will  arise  when  sighting    on 


F.    E.   BASH  359 

the  surface  of  molten  glass.  In  one  tank  furnace,  a  difference  of  117°  F. 
in  apparent  temperature  was  found  by  sighting  on  the  same  spot  from 
different  angles,  due  to  reflection  from  flames  and  walls.  These  reflec- 
tions are  entirely  done  away  with  when  a  black-body  tube  is  used. 

The  best  way  to  determine  the  proper  method  of  taking  temperatures 
in  a  glass  furnace  is  to  immerse  a  tube  in  the  glass  and,  when  it  has  come 
to  temperature,  select  a  point  free  from  reflections,  if  there  is  any,  and 
compare  the  readings.  A  sight  in  the  shadow  of  a  clay  floater  that  lay 
close  to  the  wall  in  a  large  tank  furnace  was  found  to  agree  with  the  opti- 
cal reading  in  a  tube  to  18  °F.  A  similar  test  in  a  pot  furnace,  where  the 
optical  reading  was  made  on  the  angle  of  intersection  of  the  black  wall 
of  the  pot  with  the  glass  surface,  gave  a  check  of  5°  F. 

It  appears  probable,  although  it  has  not  been  conclusively  proved, 
that  the  radiation  from  an  uncovered  pot  or  kiln  of  glass  that  has  no 
hotter  bodies  surrounding  it  is  that  of  black  body  for  a  wave 
length  of  approximately  0.65  microns.  The  evidence  to  support 
this  is  that  readings  made  with  the  optical  pyrometer  of  the  disappear- 
ing-filament  type  on  the  center  of  a  window-glass  kiln  checked  a  thermo- 
couple immersed  in  the  glass  to  3°  F.  The  optical-pyrometer  readings 
were  made  almost  from  a  vertical  position  from  an  operating  tower  and 
also  from  an  angle  of  about  60°  with  the  glass  surface  and  both  seemed 
to  agree  equally  well  with  the  thermocouple.  It  would  be  well  to  check 
up  each  individual  case  with  a  black-body  tube  or  thermocouple,  how- 
ever, as  conditions  vary  from  one  plant  to  another. 

In  each  case  it  is  usually  possible  to  find  a  point,  the  optical  reading 
on  which  will  give  the  true  temperature.  This  can  be  determined  by 
experiment  in  each  case.  In  a  large  tank  furnace  of  the  reverberatory, 
regenerative  type,  for  example,  the  surface  of  the  "metal"  will  give  very 
discordant  results  unless  care  is  taken  to  avoid  reflection  from  the  gas 
flame.  In  this  case,  however,  by  sighting  in  the  shadow  on  the  vertical 
face  of  a  floater,  the  true  temperature  will  be  secured,  providing  there 
are  no  surfaces  of  large  extent  illuminating  that  face.  In  any  case,  it 
is  possible  to  check  the  accuracy  of  such  measurements  and  their  con- 
stancy by  temporarily  employing  a  black-body  tube  of  iron  or  clay.  In 
a  pot  furnace  where  the  pots  are  open  and  in  tank  furnaces,  care  must 
be  taken  in  selecting  a  sighting  point  because  of  the  strong  reflections. 
In  a  furnace  where  there  are  flames  playing  or  the  walls  are  hotter 
than  the  glass  sighted  on,  the  optical  reading  will  always  be  high  due  to 
reflections  unless  a  place  can  be  found  that  is  protected  from  such 
reflections. 

A  method  that  has  proved  satisfactory  for  taking  tank  temperatures  is 
to  drill  diagonal  holes  through  the  walls  just  above  the  glass  surface  and  to 
insert  through  them  closed-end  clay  tubes  that  project  into  the  glass 
and  remain  there  permanently.  In  this  manner,  it  is  possible  to  keep  a 


360  DTSAPPEARING-FILAMENT   OPTICAL    PYROMETER 

check  on  the  glass  temperatures  around  the  tank  and  also  control  them 
to  the  desired  value.  The  use  of  the  optical  pyrometer  has  the  added 
advantage,  in  this  case,  that  when  the  clay  tubes  break  no  damage  results, 
as  would  be  the  case  if  platinum  couples  were  used. 

When  determining  temperatures  in  a  pot  furnace  where  the  individual 
pots  have  closed  tops,  it  will  doubtless  prove  correct  to  take  the  tem- 
perature as  that  optically  observed  on  the  pot  wall  near  the  glass,  owing 
to  the  fact  that  in  the  angle  practically  black-body  conditions  prevail. 
In  any  case,  it  is  well  to  remember  that  the  material  of  the  pot  and  its 
.top,  while  they  may  be  such  as  to  give  black-body  radiation,  will  not 
necessarily  do  so  when  covered  with  a  surface  of  molten  glass. 

In  plate-glass  pot  furnaces,  it  was  found  that  readings  made  on  the 
intersection  of  the  glass  surface  and  the  pot  wall  gave  the  true  glass  tem- 
perature; but  readings  on  the  glass  surface  in  the  center  of  the  pot  were 
too  high  due  to  flames  playing  over  them. 

When  the  purpose  of  the  measurement  is  to  insure  repetition  of 
working  conditions,  the  actual  temperature  may  not  be  necessary  and, 
therefore,  a  single  point  may  be  chosen  for  observation  from  day  to  day 
without  regard  to  the  real  temperature.  It  is  recommended,  however, 
that  true  temperatures  be  found  in  every  case  and  recorded  as  the 
actual  operating  condition.  The  value  of  so  doing  is  evident,  for  it  makes 
data  of  value  in  comparison  with  other  observations  and  other  condi- 
tions. Furthermore,  any  new  furnaces  that  are  to  be  installed  may 
differ  in  their  natures  from  older  ones.  In  such  a  case  true  temperatures 
would  be  necessary  for  comparison. 

A  black-body  tube  of  clay  may  be  used  if  care  is  taken  to  heat  it 
gradually  before  plunging  it  into  the  molten  glass.  An  ordinary  gas 
pipe  also  makes  a  very  satisfactory  black  body,  but  should  be  well  burned 
out  to  remove  oil  and  grease  from  the  interior  before  being  used.  The 
presence  of  any  smoke  within  the  pipe  will,  by  reason  of  its  absorption, 
give  false  readings.  A  2-in.  (5-cm.)  gas  or  water  pipe  capped  at  the  far 
end  has  been  found  very  satisfactory  as  a  black  body.  Such  a  pipe  has 
been  used  successfully  up  to  a  length  of  10  or  12  ft.  (3  or  3. 6m. )  in  furnace 
gases. 

The  accuracy  of  measurement  with  the  disappearing-filament  optical 
pyrometer  depends  somewhat,  but  not  greatly,  on  the  skill  of  the  observer. 
Even  those  using  the  instrument  for  the  first  time  will  find  that  they  can 
check  each  other  to  within  5  to  10°  F.,  depending  on  the  care  they  may 
exercise  in  balancing. 

OPTICAL  PYROMETERS  IN  THE  NON-FERROUS  ^FOUNDRY 

Many  alloys  of  widely  varying  composition  are  made  in  the  non- 
ferrous  foundry  at  the  present  time  and  for  many  of  them  the  tempera- 


F.    E.   BASH  361 

ture  at  which  the  metal  is  poured  is  a  vital  factor,  as  the  mechanical 
properties  of  an  alloy  often  depend  largely  on  the  casting  temperature. 
The  writer  ventures  to  state  that  a  great  many  rejections  would  never 
occur  if  the  foundry  pouring  temperatures  were  carefully  controlled. 
In  many  foundries,  as  soon  as  a  pot  of  metal  is  pulled  from  the  furnace 
and  skimmed,  the  molder  is  anxious  to  pour  immediately  for  fear  of  its 
getting  too  cold,  even  though  it  may  be  a  great  deal  hotter  than  neces- 
sary. Temperatures  of  a  brass  alloy  from  a  number  of  different  pots, 
all  for  the  same  kind  of  castings,  varied  in  temperature  from  1937°  to 
2337°  F.  (1058°  to  1281°  C.). 

The  taking  of  temperatures  of  non-ferrous  alloys  may  be  done  in  a 
number  of  ways.  A  protected  base-metal  thermocouple  may  be  used 
for  the  lower  melting  alloys  and  platinum  thermocouples  for  the  higher 
melting  alloys.  There  are  a  number  of  disadvantages,  however,  in  their 
use.  If  a  metal  protecting  tube  is  used,  it  will  be  attacked  by  the  molten 
metal  and  finally  spoil  the  couple.  Another  disadvantage  is  that  the 
metal  of  the  protecting  tube  goes  into  solution  in  the  alloy,  which  in 
many  cases  is  very  undesirable.  Protecting  tubes  of  different  ceramic 
materials  may  be  used  but,  as  they  are  somewhat  fragile,  the  couple  is 
soon  contaminated,  which  in  the  case  .of  platinum  thermocouples  is 
somewhat  expensive. 

The  difficulty  in  using  an  optical  pyrometer  on  non-ferrous  alloys 
is  that  practically  all  of  them  contain  copper  and  oxidize  readily.  As  a 
result,  when  a  molten  metal  surface  is  exposed,  it  immediately  oxidizes; 
and  the  longer  it  is  exposed,  the  thicker  the  coat  of  oxide  becomes,  so 
that  the  reading  of  the  optical  varies  greatly,  depending  on  the  instant 
at  which  the  balance  was  made.  To  remedy  this  difficulty,  a  number  of 
experiments  were  tried  with  closed-end,  quartz,  sight  tubes  immersed  in 
the  metal  and  the  readings  were  checked  against  a  protected  platinum- 
rhodium  thermocouple.  The  ends  of  the  quartz  tubes  were  immersed  to 
the  same  depth  as  the  thermocouple,  which  was  enclosed  in  a  silica  pro- 
tecting tube.  The  following  readings  were  obtained  on  nickel-silver 
from  different  pots. 


THERMOCOUPLE     OPTICAL  PYROMETER 
No.  TEMPERATURE,       TEMPERATURE, 

DEGREES  F.         DEGREES  F. 


ni»» 

i;lrl   iN 
' 


1  2165  2161  4 

2  2170  2170  0 

3  2207  2200  7 

4  2193  2193  0 

5  2112  2115  3 

6  2142  2147  5 

7  2211  2210  1 

8  2211  2213  2 

9  2226  2227  1 


362  DISAPPEARING-FILAMENT   OPTICAL   PYROMETER 

The  thermocouple  was  read  with  a  potentiometer  indicator  with 
automatic  cold-junction  compensation. 

Similar  tests  were  made  on  manganese-bronze,  yellow  brass,  and  other 
alloys  of  which  the  following  readings  are  representative. 

THERMOCOUPLE  OPTICAL  PYROMETER  ^ 

No.  TEMPERATURE,  TEMPERATURE,  Yi  i? 

DEGREES  F.  DEGREES  F. 

1  1698  1699  1 

2  1836  1830  6 

3  1800  1805  5 

4  2067  2070  3 

From  the  above  results,  it  was  concluded  that  no  errors  would  result 
from  using  a  sight  tube  in  taking  these  molten  metal  temperatures  so  the 
device  shown  in  Fig.  6  was  designed.  The  optical  pyrometer  A  is  clamped 
on  one  end  of  a  light  steel  tube  B  and  the  sight  tube  C  on  the  other  in 
such  a  manner  that  the  pyrometer  is  always  sighted  on  the  bottom  of  the 
sight  tube.  To  make  a  temperature  observation,  the  quartz  sight  tube  is 

A  c 

X— n 


B 


TJ 


B' 
FIG.    6. — OPTICAL  PYROMETER  WITH  SIGHT  TUBE  ATTACHED. 


plunged  directly  into  the  metal  and  after  a  period  of  approximately  45 
sec.,  while  the  tube  is  coming  to  temperature,  a  balance  is  made  and  the 
tube  withdrawn.  In  this  manner  the  temperature  of  metal  in  a  pot 
may  be  taken  before  it  is  withdrawn  from  the  furnace,  the  tube  being 
pushed  down  through  the  charcoal  on  the  surface,  and  the  final  tempera- 
ture may  thus  be  controlled  at  will. 

It  was  attempted  to  determine  the  approximate  emissivity  of  some 
alloys  by  taking  a  reading  in  the  tube  just  after  a  pot  was  skimmed  and 
immediately  sighting  on  the  oxidized  surface  of  the  metal  and  determining 
the  emissivity  from  the  two  readings,  using  the  formula  previously 
given.  The  emissivity  values  obtained  would  only  be  approximate  as 
the  reading  in  the  tube  would  give  the  temperature  of  the  metal  about  6 
in.  below  the  surface  and  not  the  true  surface  temperature.  However, 
they  would  give  the  desired  relation  between  surface  and  true  tempera- 
ture of  metal.  In  Table  1  are  given  a  few  readings  taken  in  this  manner. 
The  first  column  gives  the  true  temperature  T,  the  second  the  apparent 
temperature  S,  the  third  the  calculated  emissivity  E,  the  fourth  gives 
the  temperature  T\  calculated  from  the  apparent  temperature  and  the 
mean  emissivity  found.  This  would  be  the  calculated  temperature  if  no 
tube  were  used  and  a  correction  for  a  mean  emissivity  of  0.69  is  made  to 
readings  on  the  surface  of  the  oxidized  metal.  The  fifth  column  gives 


F.    E.   BASH 


363 


the  errors  that  would  result  in  using  this  value  instead  of  actually  measur- 
ing the  true  temperature  with  the  aid  of  a  sight  tube. 

TABLE  1. — Pyrometer  Readings  Taken  to  Determine  Emissivity  of 
Non-ferrous  Alloy 


Differ- 

Appar- 

Calcu- 

ence in 

Tube 

ent 

lated 

Tem- 

Metal 

Tem- 
perature, 

Tem- 
perature, 

Calcu- 
lated 

Tem- 
perature, 

pera- 
ture,0 

Remarks 

Degrees 

Degrees 

Emis- 

Degrees 

Degrees 

F. 

F. 

sivity 

F. 

F. 

T 

S 

E 

Ti 

T  -  Ti 

1.  No.  1  composition. 

2147 

2110 

0.80 

2170 

-23 

Mean  emissivity 

2.  No.  1  composition. 

1978 

1928 

0.71 

1980 

-2 

for  Nos.  1  and  2 

3.  No.  1  composition. 

1934 

1934 

composition  is 

4.  No.  1  composition. 

1945 

1928 

(?) 

0.69.  Both  metals 

5.  No.  2  composition  . 

2258 

2184 

0.66 

2247 

+  11 

have     practically 

6.  No.  2  composition. 

2337 

2206 

same  composition. 

7.  No.  2  composition. 

2170 

2117 

0.73 

2178 

0 

—  o 

No.  2  is  more  im- 

8. No.  2  composition. 

2275 

2217 

0.73 

2283 

-8 

pure  and  all  virgin 

9.  No.  2  composition  . 

2170 

2110 

0.71 

2173 

-3 

metal  is  in  No.  1. 

10.  No.  2  composition. 

2183 

2102 

0.63 

2160 

+23 

11.  Monel  

2364 

2263 

0.59 

12.  Gun  metal  

2247 

2165 

0.63 

2226 

+21 

13.  Gun  metal  

2254 

2173 

0.64 

2234 

+20 

14.  Gun  metal  

2185 

2147 

0.80 

2209 

-26 

"This  column  shows  the  error  in  calculated  temperature  due  to  using  mean 
emissivity. 

It  will  be  seen  that  an  error  of  +  or  —  25°  F.  may  result  from  using 
a  mean  emissivity  value  and  taking  the  temperature  by  sighting  on  the 
surface  of  the  skimmed  metal.  It  is  questionable,  however,  if  this  would 
ever  prove  to  be  a  satisfactory  method  of  taking  temperatures  as  there 
are  bright  and  dark  spots  on  the  surface  of  the  metal  in  the  pots  and 
various  readings  may  be  obtained.  In  this  tabulation  the  brightest 
spot  was  always  chosen  to  sight  upon.  In  the  case  of  brasses,  the  zinc 
fumes  make  it  very  questionable  what  the  reading  would  be  and  it  was 
not  attempted  to  make  any  observations  for  them  other  than  in  the  sight 
tube. 

After  making  temperature  measurements  on  a  number  of  different 
alloys  in  three  different  foundries  under  variable  conditions,  it  was  con- 
cluded that  the  use  of  the  optical  pyrometer  in  a  foundry  for  determin- 
ing the  temperatures  of  brasses,  bronzes,  nickel-silver,  monel,  and  special 
alloys  of  various  kinds  is  entirely  feasible  and  satisfactory  if  a  sight  tube 
is  used;  or  if  certain  precautions  are  observed  and  no  great  accuracy  re- 
quired, the  optical  pyrometer  may  be  used  without  a  sight  tube  by  sight- 
ing on  the  surface  of  the  skimmed  metal  and  applying  a  predetermined 


364  DISAPPEARING-FILAMENT    OPTICAL    PYROMETER 

correction.  The  optical  pyrometer  with  sight-tube  attachment  is  par- 
ticularly useful  in  determining  metal  temperatures  in  the  furnace  previous 
to  pulling  the  pot  and  pouring  the  metal'.  In  this  manner  great  uniform- 
ity of  pouring  temperatures  may  be  obtained.  It  was  shown  conclusively 
that  a  silica  tube  immersed  in  a  metal  has  black-body  characteristics 
as  borne  out  by  the  excellent  checks  against  a  carefully  calibrated 
thermocouple. 

The  time  required  to  make  a  temperature  observation  is  short  and 
the  use  of  a  quartz  sight  tube  allows  it  to  be  plunged  cold  into  the  metal 
without  breaking,  which  is  a  big  advantage.  If  the  tube  breaks  it  is 
easily  and  quickly  replaced  and  the  instrument  is  not  injured  in  any  way, 
as  a  thermocouple  would  be. 

COPPER,  NICKEL,  SILVER,  AND  GOLD  TEMPERATURES 

Temperature  observations  were  made  on  streams  of  copper  from  re- 
fining furnaces  with  the  optical  pyrometer  and  the  emissivity  determined 
by  finding  the  true  temperature  both  with  a  thermocouple  and  a  sight- 
tube  attachment  to  the  optical.  The  values  obtained  agreed  very  well 
with  that  determined  by  Burgess,  which  was  0.17.  Readings  made  on 
the  oxidized  surface  of  a  freezing  ingot  of  pure  copper  and  corrected  for 
the  emissivity  of  copper  oxide,  as  determined  by  Burgess,3  gave  the  fol- 
lowing results: 

APPARENT  TEMPERATURE,  TRUE  TEMPERATURE, 

DEGREES  F.  DEGREES  F. 

1915  1983 

1920  1990 


Mean 1917.5  1986.5 

When  considering  that  the  melting  point  of  copper  is  given  as  1985°F., 
this  is  a  very  good  agreement  and  shows  Burgess'  value  for  emissivity 
to  be  correct  for  these  conditions. 

No  readings  were  made  on  nickel  but  a  number  were  taken  on  streams 
of  approximately  55-45  cupro-nickel  with  very  good  results.  The  emis- 
sivity was  not  determined  but,  from  observations  in  the  furnace  and  on  the 
stream,  the  value  of  0.285  was  chosen.  This  may  be  considerably  in 
error,  but  served  for  comparative  values. 

A  number  of  observations  were  made  on  both  fine  and  sterling-silver 
streams,  but  there  is  great  difficulty  in  determining  the  proper  emissivity 
value  to  use  as  silver  is  so  a  good  a  reflector.  The  reflection  of  windows 
and  lights  can  easily  be  seen  in  a  stream  of  silver,  so  that  the  optical 
reading  may  easily  be  in  error  if  great  care  is  not  taken.  Sterling  silver 
has  a  greater  tendency  to  oxidize,  due  to  the  presence  of  copper,  so  that 

3  U.  S.  Bureau  of  Standards  Bull.  6. 


F.    E.   BASH  365 

care  must  be  taken  in  choosing  the  place  where  the  sight  is  made.  For 
these  reasons  it  was  decided  not  to  be  practicable,  as  an  every-day  opera- 
tion, to  take  silver  temperatures  by  sighting  on  the  stream.  The  method 
of  the  optical  sight  tube,  however,  was  found  to  be  entirely  satisfactory. 
Silver  is  usually  melted  in  tilting  furnaces  or  small  open  hearths  so  that 
it  is  not  difficult  to  insert  the  tube  into  the  molten  metal  and  take  a 
reading  while  refining  or  just  before  casting  and  the  temperature 
controlled  as  desired. 

In  the  determination  of  gold  temperatures,  approximately  the  same 
problems  are  met  with  as  with  silver  except  that  the  gold  temperatures 
are  higher.  The  sight-tube  method  can  be  used  as  with  silver  with  success. 

CEMENT  TEMPERATURES 

Clinker  temperatures  may  be  taken  very  readily  as  they  fall  in  a 
stream  from  a  kiln;  or  if  it  is  desired  to  take  them  part  way  up  a  revolv- 
ing kiln  into  the  end  of  which  a  flame  is  projected,  the  flame  can  be  shut 
off  momentarily  while  the  reading  is  taken. 

FUEL  BEDS  AND  GAS  TEMPERATURES 

The  temperature  of  the  surface  of  beds  of  incandescent  coal  or  coke 
may  be  taken  with  the  disappearing-filament  optical  pyrometer,  pro- 
viding there  is  no  smoke  or  much  flame  intervening.  It  is  questionable, 
however,  what  relation  there  is  between  the  surface  temperature  and 
that  of  the  center  of  a  large  mass.  It  is  also  possible  to  pick  out  hot  spots 
in  a  fuel  bed  and  take  their  temperature  with  the  optical  pyrometer. 
This  should  give  the  clinkering  temperature  under  the  actual  firing  con- 
ditions and  not  under  artificial  ones. 

If  it  is  desired  to  take  gas  temperatures  in  flues  or  firing  chambers, 
a  closed-end  ceramic  tube  can  be  set  in  the  wall  and  readings  made  in  it. 
Porcelain,  alundum,  clay,  and  carborundum  have  been  successfully  used 
under  these  conditions  in  taking  gas  temperatures  of  forging  furnaces, 
marine  boilers,  etc.,  etc. 

DETERMINING  TEMPERATURES  IN  CERAMIC  INDUSTRIES 

In  the  ceramic  industries,  temperatures  are  often  very  high  and  a  mat- 
ter of  great  importance,  but  often  can  be  taken  only  with  a  radiation  or 
optical  pyrometer.  The  disappearing-filament  type  has  the  advantage 
that  the  temperature  of  any  particular  object  can  be  determined  regard- 
less of  the  temperature  of  surrounding  walls  or  objects,  providing,  of  course, 
that  the  object  is  a  black  body,  which  would  generally  be  near  enough  to 
the  case  in  a  furnace  or  kiln  unless  the  material  was  glazed.  The  uni- 
formity of  a  kiln  may  also  very  readily  be  obtained  by  making  observa- 
tions on  different  points.  This  is  often  very  important  and  may  have 
a  great  effect  on  the  quality  of  the  product. 


366  DISAPPEARING-FILAMENT  OPTICAL  PYROMETER 

MISCELLANEOUS  USES  OF  OPTICAL  PYROMETERS 

For  electric-furnace  temperatures,  particularly  laboratory  and  ex- 
perimental furnaces,  the  optical  pyrometer  can  be  used  to  great  advan- 
tage in  taking  either  the  arc  temperature  or  that  of  the  material  it  heats 
or  the  walls  of  the  furnace.  Another  use  for  this  type  of  optical  py- 
rometer is  the  checking  of  base-metal  thermocouples.  This  may  seem 
strange  to  many  who  consider  that  an  optical  pyrometer  does  not  ap- 
proach the  accuracy  of  a  thermocouple.  As  a  matter  of  fact,  the  optical 
pyrometer,  in  the  useful  range  of  a  base-metal  thermocouple,  has  an 
absolute  accuracy  of  +  or  —  15°  F.  or  better  and  can  repeat  readings 
to  5°  F.,  while  the  thermocouple  may  be  much  further  off  calibration  than 
that.  The  advantage  of  this  method  of  checking  is  that  a  couple  may 
be  very  quickly  checked  as  it  is  used  by  sighting  on  the  fire  end  and  com- 
paring the  reading  with  that  of  the  thermocouple  indicator.  As  this 
takes  only  a  few  minutes  a  large  number  of  checks  can  be  made  in  the 
•course  of  half  a  day;  while  if  it  were  attempted  to  check  the  couples  with 
another  couple,  at  least  %  hr.  would  be  consumed  in  each  check.  This 
time  would  be  required  for  the  two  couples  to  come  to  equilibrium,  if 
they  were  protected  as  most  couples  are;  and  even  then,  unless  the  fur- 
nace conditions  were  good,  it  would  be  difficult  to  tell  if  both  couples 
were  at  the  same  temperature. 

In  concluding,  the  writer  can  say  that  the  Morse  type  of  disappear- 
ing-filament  optical  pyrometer  has  been  found  to  be  very  useful  in  taking 
temperatures  in  many  industries  where  thermocouples  cannot  be.  used  or 
where  it  is  not  necessary  to  have  a  continuous  record  of  temperatures. 
To  obtain  true  temperatures,  black-body  conditions  must  prevail  or  a 
correction  for  emissivity  must  be  made.  Temperature  observations 
taken  in  furnaces  are  generally  true  temperatures  if  the  material  in  them 
is  not  reflecting.  All  molten  metals  must  have  emissivity  corrections. 
Materials  like  iron  oxide,  graphite,  and  most  rough  materials  give  black- 
body  radiation  in  the  open. 

To  obtain  black-body  conditions,  it  is  necessary  for  a  black  body 
to  take  the  temperature  of  the  material  and  sight  on  that  or  at  an  angle 
or  hole  in  the  piece  that  will  radiate  as  a  black  body. 

The  use  of  the  Morse  type  optical  pyrometer  does  not  require  a  highly 
skilled  operator;  a  man  can  generally  be  trained  for  the  works  use  of 
the  instrument  in  a  few  days.  The  balancing  of  the  instrument  is  very 
simple  and  a  very  short  time  is  required  to  acquire  speed. 


EMISSIVE    POWERS    AND    TEMPERATURES    OF    NON-BLACK   BODIES         367 


Emissive  Powers  and  Temperatures  of  Non-black  Bodies 

BY   A.    Q.    WORTHING,*   PH.   D.,    CLEVELAND,    OHIO 
(Chicago  Meeting,  September*  1919) 

Black  Bodies. — In  the  ordinary  conception,  a  black  object  is  an  opaque 
object  that  reflects  but  little  of  the  light  that  is  incident  on  it.  This 
means  naturally  that  such  an  object  is  a  good  absorber  of  luminous  radia- 
tion. The  black  body,  which  is  so  closely  connected  with  high-tempera- 
ture measurements,  represents  the  limit  in  blackness  of  all  black  objects. 
It  is  a  body  that  not  only  absorbs  all  visible  radiations  but  also  all  other 
radiations  in  the  infra-red  and  the  ultra-violet  regions  of  the  spectrum 
which  are  incident  on  it.  No  substance  is  known  that  is  completely 
black;  platinum  black,  one  of  the  nearest  if  not  the  nearest  representative 
of  the  ideal,  reflects  1  per  cent,  or  so  of  ordinary  visible  radiation.  The 
closest  approach  to  the  ideal  is  a  small  opening  in  the  wall  of  a  relatively 
large  opaque  cavity.  Any  radiation  incident  on  this  black  body  will  be 
almost  completely  absorbed  if  the  relative  dimensions  are  properly  chosen 
no  matter  what  the  material  may  be.  The  great  difference  between  the 
blackness  of  such  a  cavity  and  of  lampblack,  for  instance,  will  be  appar- 
ent to  one  who  makes  the 'simple  test. 

Non-black  Bodies. — The  field  of  non-black  bodies  covers  all  real 
bodies.  Their  characteristics  are  extremely  varied.  So  far  as  absorp- 
tion of  radiation  is  concerned,  these  bodies  range  practically  from  the 
almost  completely  black,  platinum-black,  and  lampblack,  which  absorb 
all  but  a  fractional  part  of  1  per  cent,  or  so  of  the  incident  radiation,  by 
an  infinite  variety  of  steps  to  freshly  polished  silver,  which  ordinarily 
absorbs  only  a  few  per  cent.  The  exact  percentage  absorbed  depends 
not  only  on  the  nature  of  the  incident  radiation,  but  also  on  the  tempera- 
ture of  the  body,  the  condition  of  its  surface,  the  angle  of  incidence,  the 
nature  of  the  surrounding  medium,  and  on  certain  other  less  important 
conditions.  As  consequences  of  these  varied  characteristics,  we  have 
not  only  the  differences  of  color  of  ordinary  objects,  but  also  the  changes 
of  color  that  any  one  object  undergoes  on  heating  or  when  lighted  from 
various  directions.  This  complexity  of  the  phenomena,  in  a  way,  is 
responsible  for  the  meagerness  of  knowledge  regarding  materials  at 
elevated  temperatures. 

Measurement  of  High  Temperatures. — Excepting  in  cases  where  the 
thermocouple  or  the  gas  thermometer  is  applicable,  in  which  case  the 

*  Physicist,  Nela  Research  Laboratory. 


368        EMISSIVE    POWERS   AND   TEMPERATURES   OF   NON-BLACK  BODIES 

temperatures  obtained  are  true  temperatures  regardless  of  the  material 
studied,  high  temperatures  are  almost  always,  if  not  always,  determined 
from  measurements  of  radiation.  In  fact,  determinations  of  tempera- 
ture from  this  latter  point  of  view  are  often  so  much  more  convenient 
than  the  thermocouple  and  especially  the  gas-thermometer  determina- 
tions that,  in  the  region  where  all  of  these  types  of  measurements  may  be 
made,  the  radiation  measurements  are  often  preferred.  This  is  true 
even  though  the  temperatures  usually  thus  directly  determined  are  not 
the  actual  temperatures,  a  consequence  of  the  sources  of  the  radiation 
being  non-black  bodies;  in  other  words,  a  consequence  of  being  concerned 
with  iron,  or  glass,  or  tungsten  instead  of  small  openings  leading  to  rela- 
tively large  cavities  uniformly  heated. 

Coordination  of  Experimental  Results. — In  order  that  data  on  iron, 
glass,  tungsten,  or  other  non-black  bodies  obtained  in  one  laboratory, 
may  be  correlated  with  similar  data  obtained  elsewhere,  temperatures 
must  be  expressed  on  a  common  scale.  The  true  temperature  scale  is  the 
natural  scale  for  this  purpose.  It  is  ordinarily  the  scale  to  which  all 
other  temperatures  can  be  most  directly  reduced.  When  the  measure- 
ments depend  on  the  radiation,  a  knowledge  of  emissive  powers  repre- 
sents a  means  whereby  such  measured  temperatures  may  be  converted 
simply  into  the  actual  temperatures.  Because  of  this,  we  herewith  give 
consideration  to  the  significance  of  emissive  powers,  how  they  are 
obtained,  and  how  they  are  applied  in  the  determination  of  the  real  true 
temperatures  of  non-black  bodies. 

FUNDAMENTAL  CONCEPTS  REGARDING  EMISSIVE  POWERS 

Two  Classes  of  Emissive  Powers. — There  are  two  classes  of  emissive 
powers  that  are  commonly  applied  to  temperature  measurements.  One 
of  these,  called  the  total  emissive  power,  is  based  on  the  total  heating 
effects  due  to  the  radiation  from  a  non-black  body  source;  the  other, 
called  spectral  emissive  power,  is  based  on  the  monochromatic  brightness 
of  the  source.  Total  emissive  powers  are  used  generally  in  connection 
with  radiation  pyrometry  to  determine  .true  temperatures;  spectral 
emissive  powers  are  similarly  used  in  connection  with  optical  pyrometry. 
We  shall  consider  these  later  in  considerably  more  detail. 

Kirchhoff's  Law. — A  fundamental  law  underlying  the  temperature 
radiations  from  non-black  bodies  was  originally  put  forth  by  Kirchhoff 
and  is  universally  known  as  Kirchhoff's  law.  The  fundamental  aspect  of 
this  law  may  be  readily  demonstrated  in  the  home.  Let  one  take,  for 
instance,  a  piece  of  broken  crockery  with  a  decorative  pattern,  and  heat 
it  over  a  gas  flame  in  a  dark  room.  When  the  crockery  becomes  incan- 
descent, the  darker  portions  of  the  pattern  will  be  noticeably  the  brighter. 
Those  portions  that  were  good  absorbers  of  light  show  themselves,  when 


A.    G.    WORTHING  369 

heated,  to  be  good  emitters.  This  effect  is  not  due  to  a  change  in  the 
characteristics  of  the  pottery  or  the  painted  design,  for  if  the  pottery  when 
hot  is  illuminated  relatively  strongly  by  means  of  some  other  light,  the 
different  parts  of  the  pattern  will  appear  in  their  natural  colors. 

Kirchhoff  's  law,  which  is  directly  applicable  here,  states  that  the  rate 
of  emission  of  radiant  energy  (the  radiant  flux)  by  a  non-black  body 
divided  by  its  absorption  factor  (the  ratio  of  the  radiation  absorbed  by 
the  body  to  the  incident  radiation  —  strictly  speaking,  this  radiation 
should  be  like  that  from  a  black  body  at  the  same  temperature)  is  equal 
to  the  rate  of  emission  of  radiant  energy  (the  radiant  flux)  by  a  black 
body  of  the  same  size  and  temperature.  Thus  using  concrete  values, 
it  would  appear,  from  the  work  of  Thwing1  on  cast  iron  and  of  others  on 
black-body  radiation,  that  cast  iron  at  1600°  K.  (1327°  C.)  radiates 
energy  at  a  rate  of  10.8  watts  per  sq  cm.,  that  it  absorbs  at  this  tem- 
perature about  29  per  cent,  of  the  radiation  from  a  black  body  at  the 
same  temperature  that  is  incident  on  it,  and  that  in  agreement  with 

this  a  black  body  at  1600°  K.  radiates  energy  at  a  rate  of  pr^ 

u.^y  cm* 

watts 


0_  _ 
or  37.3 

cm. 

The  reasoning  underlying  this  law  is  simple.  Suppose  a  non-black 
body  and  a  black  body  are  placed  in  an  opaque  enclosure  possessing  a 
uniform  temperature.  According  to  experience  these  two  bodies  would, 
in  time,  come  to  the  same  temperature.  Moreover,  according  to  the 
common  conception,  these  two  bodies  will  function  as  ordinarily;  in  other 
words,  they  will  radiate  energy,  absorb  radiant  energy,  and  reflect  radiant 
energy.  In  order  that  these  two  bodies  shall  finally  maintain  a  steady 
common  temperature,  they  must  radiate  energy  at  the  same  rate  that 
they  absorb  it.  The  black  body  will  absorb  all  of  the  radiation  falling 
on  it  and  must  radiate  at  that  same  rate.  The  non-black  body  will 
absorb  only  a  certain  part  of  the  radiation  falling  on  it  (29  per  cent,  if 
cast  iron  at  a  temperature  of  1600°  K.),  and  must  therefore  radiate  energy 
at  a  lower  rate  given  by  that  same  percentage  of  the  black-body  rate  — 
a  give-and-take  policy  of  the  fairest  kind.  Kirchhoff  's  law,  the  generali- 
zation resulting  from  this  reasoning,  as  already  given,  states  that  the  rate 
of  radiation  of  energy  by  a  non-black  body  divided  by  its  absorption 
factor  is  equal  to  the  rate  of  radiation  of  energy  by  a  black  body  of  the 
same  surface  area  at  the  same  temperature,  a  statement  that  might 
pithily  be  replaced  by  "As  a  body  absorbs,  so  does  it  radiate."  One 
of  the  most  striking  facts  expressed  by  this  law  is  that  all  non-black 
bodies  radiate  energy  at  lower  rates  per  unit  of  area  than  do  black  bodies 
at  the  same  temperature.  The  same  reasoning  may  be  applied  to  radia- 

1  Phys.  Rev.  (1908)  26,  190. 

24 


370        EMISSIVE   POWERS   AND   TEMPERATURES   OF  NON-BLACK  BODIES 

tion  of  any  particular  wave-length  of  radiation,  with  exactly  similar 
results. 

The  relation  just  developed  may  be  put  in  mathematical  form,  thus 


at 


(1) 


where  En  and  E  represent  respectively  the  rates  of  emission  of  energy  per 
unit  area  for  a  non-black  body  and  for  a  black  body  and  at  is  the  total 
absorption  factor  for  the  non-black  body. 

Anticipating  later  developments,  let  us  carry  this  discussion  slightly 
forward.  Evidently,  the  opaque  non-black  body  must  reflect  all  inci- 
dent radiation  that  it  does  not  absorb.  If  molten  iron  has  an  absorption 
factor  of  0.29,  it  must  also  have  a  reflection  factor  of  0.71,  that  is, 

r,  =  1  -  af  (2) 

where  rt  is  the  total  reflection  factor.2  Similar  reasoning  may  be  applied 
to  radiation  of  any  wave-length  with  corresponding  similar  results. 


J 


(.6          2.0          2A 
Wave  Length  ir 

FIG.  1. — SPECTRAL  DISTRIBUTION  CURVES  FOR  THE  RADIANT  FLUXES  PROM:  a,  TUNGSTEN 
AT  2450°  K.;  b,  A  BLACK  BODY  AT  2450°  K.;  c,  A  BLACK  BODY  AT  2500°  K. 

It  is  well  to  emphasize  the  fundamental  basic  character  of  this  law 
for  the  measurement  of  high  temperatures.  Exactly  how  this  is  the 
case  will  appear  to  some  extent  as  we  proceed.  That  it  also  underlies 
the  laws  of  Stefan  and  Boltzmann  and  of  Planck,  to  be  considered  later, 
will  not  be  made  evident  there,  though  true. 

Illustrative  Data. — Let  us  consider,  further,  specimen  data  illustrating 
the  relation  between  emissive  powers  and  radiation.  In  Fig.  1,  curve 
a  represents  the  spectral  distribution  of  the  radiant  flux  from  tungsten 
at  2450°  K.,  the  normal  operating  temperature  of  the  60-watt  vacuum 
tungsten  lamp.  It  shows  the  relative  heating  effects  of  radiant  flux  associ- 

2  For  radiation  having  the  same  spectral  distribution  as  that  from  a  black  body 
having  the  same  temperature. 


source  per  unit  area,  a  quantity  to  be  measured  ordinarily  in 


A.    G.    WORTHING  371 

ateH  with  the  various  wave-lengths  of  the  source.  Thus,  for  instance,  the 
maximum  effect  is  in  the  infra-red  spectrum  at  1.05/x,  being  about  a  third 
greater  than  at  the  red  edge  of  the  visible  spectrum  (0.78/u)  and  about 
three  and  one-half  times  as  great  as  at  the  blue  edge  of  the  visible  spec- 
trum (0.38/*)-  The  area  enclosed  by  this  curve  and  the  X-axis  in  accord 
with  this  represents  the  total  heating  effect  or  radiant  flux  from  the 

watts 
cm.5 

Similarly,  curve  6  shows  the  relative  heating  effects  or  radiant  flux 
associated  with  the  various  wave-lengths  of  the  radiation  from  a  black 
body  at  2450°  K.  The  area  enclosed  by  this  curve  and  the  X-axis  like- 
wise represents  the  total  heating  effect  or  radiant  flux  from  a  black 

body  per  unit  of  area,  a  quantity  likewise  expressed  ordinarily  in  - 

Curve  c  refers  in  an  exactly  similar  manner  to  a  black  body  at  2500°  K. 
At  this  temperature  a  black  body  possesses  the  same  color  as  does  the 
•  tungsten  at  2450°  K. 

The  total  emissive  power  of  a  substance  is  the  ratio  of  the  radiant  flux 
per  unit  of  area  from  that  substance  to  the  radiant  flux  per  unit  of  area  from 
a  black  body  at  the  same  temperature.  Thus,  the  ratio  of  the  area  under 
curve  a  to  the  area  under  curve  b  gives  at  once  the  total  emissive  power  for 
tungsten  at  2450°  K.  Taking  due  account  of  the  units  in  which  the  ordi- 
nates  and  abscissas  of  Fig.  1  are  expressed,  we  find  from  the  area  under 
curve  b  that  the  rate  of  emission  of  energy  or  the  radiant  flux  from  a  black 

TO7Q  "f"f"G 

body  at  2450°  K.  is  about  205 ^  and  that  the  corresponding  radiant 

TXTQ  "|"{"O 

flux  from  tungsten  at  the  same  temperature  is  about  50         2-    This3 

leads  to  0.27  for  the  total  emissive  power  of  tungsten  at  2450°  K.  This 
ratio  for  tungsten  is  not  constant  with  temperature. 

From  the  same  set  of  curves,  in  Fig.  1,  the  significance  of  spectral  emis- 
sive powers  may  be  obtained;  but,  since  they  are  more  commonly  obtained 
for  high-temperature  work  from  visual  observations,  we  shall  consider 
them  from  this  point  of  view.  A  similar  platting  of  the  visual  effects 
against  the  wave-length  for  the  radiation  from  a  tungsten  filament  at 
2450°  K.  and  a  black  body  at  2450°  K.  and  2500°  K.  lead  to  the  spectral 
luminosity  or  more  truly  the  spectral  brightness  distribution  curves  of 
Fig.  2.  Curve  a  thus  shows  the  relative  visual  effects  associated  with 
the  various  wave-lengths  of  the  luminous  flux  from  tungsten  at  2450°  K. 
It  shows  that,  for  a  certain  small  wave-length  interval,  the  visual  effect 


3  In  computing  curve  a,  it  was  assumed  that  Lambert's  cosine  law  of  emission 
was  fulfilled.  There  are  marked  deviations  from  this,  however,  which  undoubtedly 
explain  largely  the  discrepancy  between  the  value  for  the  radiant  flux  per  unit  of 
area  here  given  and  the  value  obtained  from  direct  measurement. 


372        EMISSIVE   POWERS   AND   TEMPERATURES    OF   NON-BLACK  BODIES 

is  greater  at  about  0.575/*  than  elsewhere,  that  at  0.5/i  the  visual  effect 
is  roughly  one-sixth  of  the  maximum  value,  and  at  0.6/z  about  five-sixths 
of  the  maximum  value.  Curves  6  and  c  represent  similar  spectral  bright- 
ness distribution  curves  for  a  black  body  at  2450°  and  2500°  K.  As  in  the 
case  of  the  spectral  radiant-flux  curves,  the  areas  included  under  the 
curves  represented  the  rates  of  emission  of  radiant  energy  per  unit  of 


area,  quantities  measurable  in 


watts 
cm.5 


so  here  the  areas  included  under  the 


curves  may  be  taken  to  represent  the  brightness  of  the  tungsten  filament 


.52  .56          .60 

Wave  Length  in 

FIG.  2. — SPECTRAL  DISTRIBUTION  CURVES  FOR  THE  LUMINOUS  FLUXES  FROM:  a,  TUNG- 
STEN AT  2450°  K.;  b,  A  BLACK  BODY  AT  2450°  K.;  c,  A  BLACK  BODY  AT  2500°  K. 


or  of  the  black  body;  and,  if  due  account  of  the  units  in  which  the  ordinates 

and  abscissas  are  expressed  is  taken,  one  will  obtain  about  195  - 

cm.2 

c&n  flips 
arid  440  ^-  for  the  tungsten  filament  and  for  the  black  body  at 

2450°  K.,  respectively. 

The  spectral  emissive  power  at  a  given  wave-length  and  temperature  for  a 
certain  substance  is  defined  as  the  ratio  of  the  spectral  brightness  for  that 
substance  for  the  given  temperature  and  wave-length  to  the  corresponding 
spectral  brightness  of  a  black  body  at  the  same  temperature.  Thus,  the 
ratio  of  the  ordinate  at  0.6/i  for  curve  a  to  the  ordinate  at  the  same  wave- 
length for  curve  b  gives  the  spectral  emissive  power  at  0.6/i  for  tungsten 
at  2450°  K.  Actual  carrying  out  of  the  operation  in  the  present  case 
yields  quite  closely  44  per  cent.  Similarly,  the  ratio  of  the  correspond- 


A.    G.    WORTHING  373 

ing  ordinates  at  0.7ju  gives  the  spectral  emissive  power  connected  with 
that  wave-length.  The  actual  value  is  about  42  per  cent.  These 
values  might  be  obtained  also,  as  stated,  from  the  spectral  radiant-flux 
curves;  and  in  conformity  with  this  the  ratios  of  their  ordinates  at  0.6/z 
and  0.7/t  should  likewise  be  44  and  42  per  cent. 

TOTAL  EMISSIVE  POWERS 

Theoretical  Basis.  —  The  theoretical  basis  underlying  total  emissive 
power  determinations  and  applications  is  the  relation  applicable  to  black- 
body  radiation,  known  as  the  Stefan-Boltzmann  law, 

E  =  aT*  (3) 

It  states  that  E,  the  radiant-flux  density  (that  is  the  rate  of  radiation  of 
energy  per  unit  of  area)  by  a  black  body  is  a  constant  times  T  its  tempera- 
ture on  the  absolute  scale  raised  to  the  fourth  power.  When  E  is  ex- 

"IX7Q  "j"f~C 

pressed  in          ,   and  T  in  °K.,  a  is  as  a  result  of  a  great  deal  of  experi- 
cm.2 

mental  work  commonly  taken4  as  about  5.70  X  10~12  -  —.•      We 

cm.  deg. 

find  thus  that  for  a  black  body  at  1000°  K.,  the  radiant-flux  density  is 


5.70  ;  at  2000°  K,  16  X  5.70  or  91.2  ;  and  at  2450°  K., 

cm.2  cm.2  cm.2 

the  normal  operating  temperature  of  the  vacuum  tungsten  lamp,  as 

already  mentioned,  206        '-„• 

cm.2 

For  a  non-black  body  the  radiant-flux  density  En  may  be  represented 
by  a  similar  equation: 

En  =  e^r*  (4) 


where  e<  represents  the  total  emissive  power.  This  equation  serves,  in 
fact,  as  the  defining  equation  for  this  quantity.  As  has  been  indicated 
elsewhere,  e«  may,  and  usually  will,  vary  with  the  temperature.  In 
Fig.  1,  curves  a  and  6,  En  and  E  are  graphically  represented  for  a  tungsten 
filament  and  a  black  body  at  2450°  K.,  by  the  areas  included  between  the 
X  axis  and  the  corresponding  curves. 
Evidently  we  may  also  write, 

En  =  erZV,  (5) 

an  equation  exactly  similar  to  (3)  in  which  TR,  a  temperature  less  than 
the  true  temperature  T,  is  the  temperature  of  a  black  body  that  radiates 
energy  at  the  same  rate  per  unit  of  area  as  does  the  non-black  body  at  T. 
So  far  as  the  writer  is  aware,  no  definite  name  has  been  attached  to  this 

4  Coblentz:  II.  S.  Bureau  of  Standards  Bull.  13  (1916)  459. 


374        EMISSIVE    POWERS   AND   TEMPERATURES   OF   NON-BLACK  BODIES 

temperature  heretofore.  For  the  purposes  of  this  paper5  let  us  call  it 
"radiation  temperature"  in  analogy  with  brightness  temperature  and 
color  temperature.  We  may  thus  expect  to  say,  for  instance,  that  molten 
iron  at  a  true  temperature  of  1600°  K.  has  a  radiation  temperature  of 
1174°  K.,  or  that  silver  at  a  true  temperature  of  1200°  K.  has  a  radiation 
temperature  of  720°  K.,  or  that  iron  oxide  at  a  true  temperature  of  1200°  K. 
has  a  radiation  temperature  of  1160°  K. 

From  equations  (4)  and  (5)  there  follows: 

T*  =  J/etT.  (6) 

Evidently  we  may  determine  et,  the  total  emissive  power,  when  T  and 
TR  are  known,  or  we  may  determine  T  when  once  et  and  TR  are  known. 

Radiation  Pyrometry. — At  present  there  are  several  types  of  radiation 
pyrometers  that  may  be  used  in  determining  total  emissive  powers  or, 
once  these  total  emissive  powers  are  known,  to  determine  true  tempera- 
tures. Radiation  pyrometers,  their  underlying  principles,  methods  of  use, 
and  sources  of  error,  have  been  discussed  by  Burgess  and  Foote.6  In 
this  discussion  of  total  emissive  powers,  this  important  contribution  is 
freely  referred  to.  While  different  types  of  radiation  pyrometers  differ 
greatly  in  construction,  .they  all  depend  necessarily  on  effects  due  to  the 
absorption  of  radiant  energy  that  is  focused,  by  one  means  or  another, 
'upon  a  receiving  instrument  sensitive  to  heating  effects,  a  thermocouple, 
a  bimetallic  coil,  or  some  other  radiation-sensitive  device. 

Let  us  fix  our  attention,  for  illustrative  purposes,  upon  the  Fery 
mirror  thermocouple  radiation  pyrometer  in  which  the  radiation  from  the 
object  whose  temperature  is  being  measured  is  reflected  by  a  gold-plated 
mirror  on  to  a  blackened  thermocouple  receiver.  For  a  description  of 
the  apparatus  and  the  method  of  operating,  reference  should  be  made  to 
the  paper  by  Burgess  and  Foote  or  to  another  paper  of  this  symposium 
dealing  with  radiation  pyrometers.  It  would  seem  perfectly  simple  to 
make  use  of  such  a  pyrometer  in  the  determination  of  emissive  powers, 
by  sighting  a  radiation  pyrometer  with  a  known  black-body  calibration 
(see  Fig.  3,  in  which  is  shown  such  a  calibration  curve  as  determined  by 
Burgess  and  Foote)  at  a  non-black  body  at  a  known  temperature  and 
computing  the  result  by  means  of  equation  (6).  In  practice,  difficulty 
is  often  experienced  in  determining  the  true  temperature.  Further, 
account  must  be  taken  of  reflected  radiations. 

True  temperatures  are  usually  determined  by  means  of  a  thermo- 
couple, or  by  shaping  the  material  so  that  from  certain  portions  black- 
body  radiation  is  obtained,  or  by  placing  in  contact  with  the  non-black 


5  See  also  paper  by  E.  P.  Hyde:   High-temperature    Scale  and   its  Application 
in  the  Measurement  of  True,  Brightness  and  Color  Temperatures.     This  volume. 

6  U.  S.  Bureau  of  Standards  Bull.  12  (1915)  91 


A.   G.    WORTHING 


375 


body  some  other  body  whose  temperature  is  known  or  measurable.  As 
an  illustration,  making  use  of  the  results  obtained  by  Thwing7  for  molten 
iron,  and  of  the  calibration  curve  of  Fig.  3,  we  should  find  for  a  particular 
case  an  e.m.f.  of  1.32  millivolts  when  the  pyrometer  is  sighted  on  a  certain 
mass  of  iron  and  4.21  millivolts  when  sighted  on  some  body  in  contact 
shaped  so  as  to  give  black-body  radiation.  From  the  calibration  curve 
it  follows  at  once  that  the  temperature  of  the  iron  is  1600°  K.  (1327°  C.) 
and  that  the  molten  iron  has  a  radiation  temperature  (subject  to  a  slight 
correction,  shown  in  the  next  paragraph)  of  1174°  K.,  that  is,  molten  iron 


900        1000 


1100 


1200         1300        1*00         1500 
Temperature  in  Degrees  K 


1600 


1700 


1800 


FIG.  3. — CALIBRATION  CURVE  OF  A  FERY  MIRROR  PYROMETER,  ACCORDING  TO  BURGESS 

'AND  FOOTE. 

at  1600°  K.  radiates  energy  at  the  same  rate  as  does  a  black  body  at  1174° 

.    .              watts 
K.,  the  common  rate  in  this  case  being  10.8 ^-,  as  will  appear  from  the 

C/IJLl« 

Stefan-Boltzmann  equation.     Using  equation  (6)  we  have  at  once  0.29 
as  the  total  emissive  power. 

That  account  must  be  taken  of  radiations  reflected  by  the  non-black 
body  the  radiation  pyrometer  is  sighted  on  is  readily  seen.  In  this  case, 
the  radiation  leaving  the  non-black  body  is  made  up  in  part  of  the  natural 
radiation  from  the  non-black  body  and  in  part  of  radiation  reflected 
by  it.  Naturally  the  effect  of  the  combined  radiations  on  the  receiver 
of  the  pyrometer  is  greater  than  that  due  to  the  non-black  body  alone. 
Exactly  how  much  the  reflected  portion  amounts  to  in  per  cent,  depends 


Loc.  cit. 


376        EMISSIVE    POWERS   AND   TEMPERATURES   OF   NON-BLACK  BODIES 

on  the  temperature  of  the  non-black  body,  the  temperature  of  the  sur- 
roundings, and  the  reflectivity  of  the  non-black  body.  Under  ordinary 
conditions  the  portion  reflected  may  often  be  neglected,  but  it  is  a  factor 
that  must  be  considered.  Let  us  consider  expressions  for  the  net  rates 
of  transfer  of  radiant  energy  to  a  black  receiver  at  temperature  TI  from 
a  non-black  source  at  temperature  T  in  surroundings  assumed  opaque 
and  at  a  temperature  T0,  and  from  a  black  body  of  the  same  size  at  the 
same  distance  from  the  receiver  and  in  the  same  surroundings  but  at  a 
temperature  TV,  an  uncorrected  radiation  temperature  for  the  non-black 
body  such  that  the  effects  on  the  black  receiver  will  be  the  same  in  the 
two  cases.  Suppose  the  dimensions  of  the  sources  and  the  receiver  to  be 
small  in  comparison  with  the  distance  I  from  the  source  to  the  receiver 
and  the  areas  of  their  projections  on  a  plane  normal  to  the  line  join- 
ing them  to  be,  respectively,  A\  and  A2.  For  the  common  net  rate  of 

transfer  of  radiant  energy  from  the  source  to  the  receiver  -,-•  we  have 

for  the  black-body  source  and  the  non-black-body  source,  respectively, 
from  purely  geometrical  considerations, 


and 

d 


where  r  t  is  the  total  reflectivity  of  the  non-black  body  for  radiation  from 
a  black  body  at  T0.     From  equations  (7)  and  (8)  we  have,  at  once, 

_  TV  -rfgy 
€<  ~        ~~fi~ 

Replacing  rt  by  1  —  et,  we  have  another  form  which  has  been  considerably 
used:8 


, 
dt    '         fir     (    *  l 


_    m  4 


8  Dr.  Foote  has  called  the  writer's  attention  to  the  fact  that  (10)  is  not  exact, 
due  to  rt  being  equal  to  1  —  «j  only  when  the  incident  radiation  has  the  same 
spectral  distribution  as  that  from  a  black  body  at  the  same  temperature.  Follow- 
ing much  the  plan  adopted  by  Aschkinass  lAschkinass:  Ann.  d.  Phys.  (1905)  17, 
960;  Foote:  U.  S.  Bureau  of  Standards  Bull.  11  (1915)  607]  in  arriving  at  an  ex- 
pression for  the  total  emissive  power  of  a  metal  as  a  function  of  temperature,  Foote 
has  arrived  at  the  following  equation, 


T*  -  TVVfo  (101) 

T 

which  likewise  is  applicable  to  metals  only.  Equation  (101)  eliminates  the  very 
difficult  and  tedious  determination  of  rt  and  may  therefore  usually  be  used.  For 
precision  work,  however,  equation  (9)  should  be  used. 


A.    G.    WORTHING 


377 


Evidently  equations  (6)  and  (9)  give  for  the  radiation  temperature  of 
the  non -black  body  in  terms  of  the  uncorrected  observed  temperature, 

TV  =  TV4  -  n!ZV  (11) 

The  discrepancy  between  TR  and  TR>  is  often  negligible  but,  on  the  other 
hand,  may  become  of  considerable  importance.     A  few  random  illustra- 
te* 

tions  showing  this  discrepancy  and  — the  error  in  the  emissive  power  due 

ft 

to  neglecting  the  reflected  radiation  term  of  (9)  are  incorporated  in  Table 
1.  The  discrepancies  are  small  for  high  temperatures  and  high  emissive 
powers  of  the  source  and  for  low  temperatures  of  the  surroundings. 

TABLE  1. — Errors  Resulting  from  Neglecting  Reflected  Radiation   Term 

of  Equation  (9) 


Material 

T, 
Degrees  K. 

To, 
Degrees  K. 

ft 

r< 

TR, 
Degrees  K. 

TR'-TR, 
Degrees  K. 

At,, 
Per  Cent. 

Platinum  

1000 

300 

0  100 

0.945 

562 

10  4 

8   0 

Platinum  

1200 

300 

0  118 

0.941 

704 

5  4 

3   1 

Platinum  

1500 

300 

0.142 

0.936 

921 

*  2  4 

1  0 

Platinum  .... 

2000 

300 

0  177 

0  931 

1297 

0  8 

0  3 

Platinum  

1000 

400 

0  100 

0  937 

562 

31 

24 

Silver  

1000 

300 

0  045 

0  975 

461 

19 

17 

Iron  oxide  

1000 

300 

0.86 

0.14 

963 

0  3 

0  13 

Black  body  

1000 

300 

1.00 

0.00 

1000 

0.0 

0.0 

A  second  method  of  obtaining  total  emissive  power  consists  in  apply- 
ing Kirchhoff's  law.     As  already  stated  this  law  is  represented  by 

E  =^"- 
at 

From  our  definition  of  emissive  power,  it  follows  that  the  emissive  power  of 
any  substance  is  equal  to  the  absorption  factor  for  black-body  radiation 
corresponding  to  the  same  temperature  as  that  possessed  by  the  non- 
black  body.  This  equality  has  been  used  in  the  discussions  of  the 
absorption  factor  and  total  emissive  power  for  molten  iron  at  1600°  K., 
for  which  the  common  value  is  0.29.  As  at  =  1  —  rt  where  rt  is  the 
reflection  factor, 


=  1  -rt 


(12) 


The  method  embodied  in  this  equation  consists  essentially  in  measuring 
for  a  non-black  body  the  fractional  part  of  the  radiation  from  some 
other  source  reflected  by  the  non-black  body.  Only  in  relatively  few 
cases  will  the  method  be  satisfactory  and  not  entail  many  corrections. 
These  simple  cases  demand  non-black  bodies  that  have  either  well  polished 


378        EMISSIVE   POWERS  AND  TEMPERATURES   OF  NON-BLACK  BODIES 

or  perfectly  matt  surfaces  and  black-body  or  very  similar  sources  at  about 
the  same  temperature  as  the  non-black  body. 

Given  the  total  emissive  power  of  a  substance,  it  is  possible  to  com- 
pute its  true  temperature  when  once  the  radiation  temperature  is  known. 
The  expression  employed  here  is  the  same  as  the  one  used  in  determining 
the  emissive  power,  or  equation  (6) .  The  radiation  temperature  TR  will 
be  determined  ordinarily  from  a  direct  reading  of  the  e.m.f .  of  a  radiation 
pyrometer  receiver  and  an  interpolation  from  a  calibration  curve  such  as 
is  given  in  Fig.  3.  Corrections  as  shown  above,  if  appreciable,  must  be 
made  in  order  to  obtain  the  corrected  radiation  temperatures.  Suppose 
Tg  for  a  certain  specimen  of  molten  iron  to  be  1200°  K.;  then  using  0.29 
as  €t,  1635°  K.  is  its  true  temperature. 


True  Temperature  Degrees  C. 

S  §  c!  § 
,.°  o  o  o 

/ 

0  ( 

7 

^6 
°9r 

/°° 

00 

o&& 

y 

o 

•/ 

/ 

/ 

A 

/ 

0 

00                               600                                 700                               800 

Temperature  by  Radiation  Pyrometer  Degrees  C. 

FIG.  4. — RADIATION  TEMPERATURE  AND  TRUE  TEMPERATURE  RELATION  FOR  IRON 
OXIDE  ACCORDING  TO  BURGESS  AND  FoOTE. 

Ordinarily,  when  one  has  occasion  to  work  repeatedly  on  samples  of 
the  same  material  at  different  temperatures,  he  will  have  computed  a 
curve  showing  the  relation  between  radiation  temperature  and  true  tem- 
perature. In  case  one  has  measured  the  total  emissive  powers  by  the  first 
method  outlined,  the  data  obtained  may  be  platted  directly  to  give  this 
relation  without  going  through  emissive  power  computations.  In  Fig.  4 
such  a  plat  of  results  by  Burgess  and  Foote9  on  iron  oxide  is  shown.  An 
average  deviation  of  about  5°  from  the  curve  is  shown  by  the  individual 

9  U.  S.  Bureau  of  Standards  Bull.  12  (1915),  83. 


A.    G.    WORTHING  379 

readings.  From  such  a  plat,  the  corrections  to  be  applied  to  radiation 
temperatures  in  order  that  true  temperatures  may  be  obtained  may  be 
readily  determined.  Thus,  according  to  this  plat,  at  a  radiation  tempera- 
ture of  500°  C.  (773°  K.),  we  have  to  add  about  33°  to  get  the  true  tempera- 
ture; for  600°  C.,  35°;  for  750°  C.,  39°.  These  correspond  to  an  emissive 
power  varying  from  somewhat  less  than  0.85  for  the  lowest  temperature 
to  0.86  for  the  highest  temperature.  For  the  lowest  of  these  temperatures, 
the  authors  have  assigned  a  total  emissive  power  of  0.85  and,  for  reasons 
we  need  not  consider  here,  for  the  highest  temperature  0.87.  Consider- 
ing difficulties,  especially  the  relatively  large  temperature  drop  between 
metal  iron  and  the  outer  surface  when  oxidized  as  shown  by  Burgess  and 
Foote,  this  is  not  a  bad  agreement  with  the  careful  work  by  Randolph  and 
Overholser10  who,  apparently  not  aware  of  this  difference,  obtained  0.78 
and  0.79  respectively  for  oxidized  cast  iron  and  oxidized  steel  at  600°  .C. 

Data  of  this  type  are  rather  meager.  There  should  be  mentioned, 
however,  among  recent  works,  including  those  already  alluded  to,  that 
by  Thwing11  on  molten  copper  and  molten  iron,  that  by  Burgess12  on 
molten  copper  and  cuprous  oxide,  that  by  Randolph  and  Overholser13  on 
cast  iron  and  the  oxides  of  zinc,  aluminum,  copper,  nickel,  lead,  calorized 
copper,  monel,  calorized  steel,  brass,  cast  iron,  and  steel,  that  by  Burgess 
and  Foote  u  on  iron  oxide  and  nickel  oxide,  and  that  by  Foote15  on  platinum. 

The  question  might  well  be  asked,  why  we  pay  any  attention  to  such  a 
quantity  as  total  emissive  power  since  usually  we  obtain  it  from  measure- 
ments of  radiation  temperature  and  true  temperature.  The  answer  is 
that  emissive  powers  give  to  the  individual  who  tries  to  compare  various 
radiating  substances  and  to  rate  them  accordingly,  a  more  convenient 
and  significant  basis  for  these  purposes  than  does  a  statement  of  the 
radiation-temperature  true-temperature  difference. 

Solar  Radiation  Temperature. — As  another  application  of  radiation 
pyrometers,  which  for  this  type  of  work  are  usually  known  as  pyrheli- 
ometers,  there  are  the  investigations  of  Pouillet,  Langley  and  others,  and 
finally  of  Abbot,  Fowle,  and  Aldrich,16  on  the  intensity  of  the  solar 
radiation  and  the  radiation  temperature  of  the  sun.  The  work  in  this 
case  has  been  greatly  complicated  by  the  fact  that  all  observations  were 
necessarily  made  with  a  great  thickness  of  the  earth's  atmosphere  be- 
tween the  observer  and  the  source.  In  order  to  take  account  of  the 
absorption  of  the  sun's  radiation  by  the  earth's  atmosphere,  Abbot, 
Fowle,  and  Aldrich  have  used  stations  at  Mt.  Whitney  and  Mt.  Wilson, 
in  California,  Bassour,  Algeria,  and  Washington,  D.  C.,  which  are 


10  Phys.  Rev.  [2]  (1913)  2,  144.  "  Loc.  tit. 

12  U.  S.  Bureau  of  Standards  Bull.  6  (1910)  111.  13  Loc.  cit. 

14  U.  S.  Bureau  of  Standards  Bull.  11  (1914)  41;  Bull.  12  (1915)  83. 

15  U.  S.  Bureau  of  Standards  Bull.  11,  607. 

16  Annals,  Astrophys.  Obs.,  Smithsonian  Inst.,  2  and  3;  Abbot:  "The  Sun." 


380        EMISSIVE   POWERS   AND   TEMPERATURES   OF   NON-BLACK  BODIES 

approximately  3  mi.,  1  mi.,  %  mi.,  and  0  mi.,  respectively,  above  sea 
level.  The  precise  method  of  obtaining  the  transmission  of  the  earth's 
atmosphere  will  be  found  in  the  paper  already  referred  to  and  probably 
in  another  paper  to  be  presented  at  this  symposium.  With  such  integral 
transmissions,  it  is  theoretically  simple  to  obtain  the  radiation  tempera- 
ture of  the  sun,  as  viewed  outside  the  earth's  atmosphere  TR  from  the 
radiation  temperature  at  the  earth's  surface  T'R  observed  with  a  cali- 
brated pyrheliometer.  Evidently 


T'R  =  -VtTR  (13) 

where  t  is  the  total  transmission  of  the  earth's  atmosphere. 

The  method  employed,  however,  has  been  somewhat  different.  In 
calibrating  pyrheliometers,  instead  of  sighting  the  receiver  on  a  calibrated 
black  body,  heat  was  usually  developed  electrically  at  known  rates 
within  it,  and  the  effect  on  the  recording  instrument  noted.  It  is  this 
effect,  suitably  chosen  in  the  calibration  work,  that  was  compared  with 
the  effect  noted  when  the  pyrheliometer  was  sighted  at  the  sun.  Thus 
there  were  obtained  measurements  in  absolute  units  of  the  sun's  radia- 
tion at  the  various  stations.  For  instance,  during  a  certain  interval 
on  Sept.  5,  1912,  at  Mt.  Wilson,  the  pyrheliometer  indicated  that  radia- 

tion was  being  received  at  a  rate  given  by  1.264  ;  the  integral 

cm.  mm. 

transmission  of  the  atmosphere  was  found  to  be  63.8  per  cent,  at  this 


time.     There  results  by  simple  division  1.985  -  2  —  ;  —    For  the  average 

cm.   mm. 

option  GS 

for  the  day  in  question,   2.014  ---—  ,  —  —  was  obtained.     From  results 

cm.2  mm. 

extending  over  many  years  Abbot,  Fowle,  and  Aldrich  have  arrived  at 


1  .932  —      —  r—  as  giving  the  most  probable  value  according  to  their  obser- 
cm.2  mm. 

vations.     Combining  this  with  149,560,000  km.  as  the  mean  radius  of  the 
earth's  orbit,  and  696,000  km.  as  the  mean  solar  radius,  and  using  equa- 


tion (3)  with  5.70  X  10~12  —  TJ^II  or  what  is  the  same  thing  81.8  X 

cm.2  deg.4' 

pa  1  OT*lf*^ 

as  the  up-to-date  value  of  a,  one  obtains  5750°  K. 


^          . 
cm.2  mm.  deg. 

as  the  radiation  temperature  of  the  sun. 

The  determination  of  the  true  temperature  of  the  sun  is  a  matter  of 
great  scientific  interest.  It  appears  not  to  be  possible  to  make  use  of 
any  total  emissive  power  in  connection  with  the  radiation  temperature 
obtained,  for  from  our  present  understanding  of  the  situation,  the  radia- 
tions in  different  parts  of  the  sun's  spectrum  effectively  have  their  sources 
at  different  depths  within  the  sun  and,  therefore,  at  different  tempera- 
tures. Attempts  have  been  made  to  determine  true  temperatures  from 
other  points  of  view  than  that  of  the  total  radiation.  Generally,  as  was 


A.    G.    WORTHING  381 

expected,  they  have  given  temperatures  noticeably  in  excess  of  the  radia- 
tion temperature  values,  in  the  neighborhood  of  6200°  K.  to  7000°  K. 

SPECTRAL'  EMISSIVE  POWERS 

The  theoretical  basis  underlying  spectral  emissive  power  measure- 
ments is  given  by  Planck's  equation,  a  law  mathematically  formed  that 
gives  very  closely  the  spectral  distribution  of  radiant  flux  found  for 
black-body  radiation.  It  is 

J  =  Cl\-»  -- 


in  which  J  represents  the  spectral  intensity  of  the  radiant  flux  at  wave- 
length X  and  temperature  T,  e  the  base  of  natural  logarithms,  and  ci  and 
cz  constants.  For  a  black  body  at  2450°  K.,  this  equation  is  graphically 
represented  by  curve  b  of  Fig.  1.  The  various  ordinates  J  of  this  curve 
represent  the  relative  heating  effects  associated  with  the  corresponding 
wave-lengths.  Curve  c  represents  the  spectral  distribution  of  radiant 
flux  from  a  black  body  at  2500°  K.  Equation  (14)  shows  not  only  how 
J  varies  with  wave-length  X  at  any  one  temperature  T  but  also  how  it 
varies  with  the  temperature  T  at  any  one  wave-length.  Thus  curves  6 
and  c  show  that  at  0.467/t,  for  instance,  the  values  of  J  at  2450°  and 
2500°  K.  are  in  the  ratio  2.51  and  at  0.665/i  in  the  ratio  1.76.  This  latter 
characteristic  of  Planck's  law,  representing  the  variation  with  tempera- 
ture, is  the  one  of  vital  importance  in  the  theoretical  discussion  of  spectral 
emissive  powers. 

The  corresponding  spectral  radiant-flux  curve  of  a  non-black  body 
may  be  represented  by  a  similar  equation 

Jn  =  ex/ 


e\T  —   1 

in  which  an  added  term  ex,  the  emissive  power  of  the  non-black  body  at 
wave-length  X  is  included.  Ordinarily  ex  is  a  variable  changing  both 
with  temperature  and  wave-length.  Curve  a  of  Fig.  1  represents  such  a 
distribution  for  tungsten  at  2450°  K.  The  ratio  of  an  ordinate  of  this 
curve  for  any  given  wave-length  X  co  the  corresponding  ordinate  of  curve 
b  gives  ex  for  tungsten  at  2450°  K.  at  that  wave-length.  Thus  for 
0.467/i  and  0.665/i  we  have,  respectively,  0.464  and  0.428  as  the  corre- 
sponding emissive  powers  of  tungsten  at  2450°  K. 

In  any  special  case  where  the  product  ^,  the  exponent  of  e  in  the 

equations  (14)  and  (15),  is  more  than,  say  6,  the  term  —1  may  be 
neglected  without  appreciable  error.  For  this  limit,  the  error  introduced 
in  J  or  Jn  is  about  0.25  per  cent.  For  this  condition,  which  includes  all 
optical  measurements  where  X  does  not  exceed  0.7ju  up  to  temperatures  of 


382        EMISSIVE    POWERS   AND   TEMPERATURES    OF   NON-BLACK  BODIES 

about  3500°  K.,   the  simplified  approximation  known  as  Wien's  equa- 
tion is  employed.     We  then  have 

J  =  ci\-5e~\T  (16) 

and 


e-xr  (17) 

Since  for  most  purposes  these  equations  are  accurate  well  within  the 
limits  set  by  the  uncertainties  of  experiment,  we  shall  assume  these  latter 
equations  for  the  further  discussion,  with  the  understanding  that  the 
reasoning  employed  may  be  carried  without  loss  of  generality  to  the  cases 
where  Planck's  equation  must  be  used. 

For  the  further  consideration  of  spectral  emissive  powers,  let  us 
consider  the  disappearing-filament  type  of  pyrometer,17  in  which  one 
views  through  a  more  or  less  monochromatic  screen  a  pyrometer  filament 
projected  against  the  object  whose  temperature  is  being  measured. 
Several  precautions18  should  be  observed  in  using  such  an  instrument, 
primary  among  which  are  the  keeping  of  the  entrance  cone  angle  constant 
and  the  obtaining  of  an  axial  alignment.  The  setting  of  the  instrument 
consists  in  changing  the  current  through  the  pyrometer  filament  until  it 
disappears  against  the  background.  In  the  operation,  we  are  not  directly 
concerned  with  distributions  given  by  the  curves  of  Fig.  1.  Instead  we 
are  directly  concerned  with  spectral  distributions  of  luminous  flux,  or 
more  truly  the  spectral  brightness  distributions,  such  as  are  shown  in 
Fig.  2,  the  equations  for  which  are  usually  derived  from  the  relation 

V  =  VJ  (18) 

where  6',  a  quantity  like  J  varying  with  wave-length  and  temperature, 
represents  the  luminosity  or  luminous  effect  of  the  radiation  associated 
with  the  heating  effect  /.  Fx  represents  the  visibility,  a  factor  varying 
with  the  wave-length,  which  permits  of  a  mathematical  representation  for 
the  spectral  brightness  distribution  when  once,  the  spectral  distribution  of 
the  radiant-flux  emission  density  is  known.  Thus  the  curves  of  Fig.  2, 
may  be,  and  actually  were,  computed  from  the  data  used  in  platting  the 
curves  of  Fig.  1,  and  the  values  of  relative  visibility  accepted  by  the 
nomenclature  and  standards  committee  of  the  Illuminating  Engineering 
Society. 19  Equations  for  the  spectral  brightness  distributions  of  a  black 
body  and  a  non-black  body  follow  at  once  from  the  last  three.  Since  Fx 
changes  with  wave-length  only,  it  is  evident  that  the  ratio  of  an  ordinate 
of  curve  a  of  Fig.  2,  to  the  ordinate  of  curve  6  for  any  wave-length  is  the 
same  as  the  corresponding  ratio  for  curves  a  and  6  of  Fig.  1.  Thus  at 


17  Forsythe:  Bull  153  (September, 

18  Worthing  and  Forsythe:  Phys.  Rev.  [2]  (1914)  4,  163. 
Hyde,  Cady  and  Forsythe:  Astrophys.  Jnl.  (1915)  42,  303. 

19  Trans  111.  Engng  Soc.  (1918)  13,  512. 


A.    G.    WORTHING  383 

0.467/u  and  0.665^,  the  ratio  of  the  ordinates  determined  visually  are  also 
0.464  and  0.428. 

In  case  the  disappearing-filament  pyrometer  is  provided  with  a 
spectrometer  so  that  the  luminous  flux  employed  in  the  determination  of 
emissive  powers  or  of  temperatures  is  quite  homogeneous,  it  is  merely 
necessary  to  determine  the  ratio  of  the  luminosity  of  the  non-black  body 
at  the  desired  wave-length  to  the  corresponding  luminosity  of  a  black 
body  at  the  same  temperature.  The  working  equation  becomes  merely: 

ex  =   5',"  (19) 

There  is,  of  course,  no  question  then  as  to  what  wave-length  to  ascribe 
the  emissive  power  determinations. 

In  case  the  disappearing-filament  pyrometer  is  not  so  provided  with  a 
spectrometer  and  the  comparisons  of  black-body  and  non-black-body 
radiations  are  made  with  a  considerably  extended  spectral  range  of  wave- 
lengths, the  procedure  is  not  so  simple.  So-called  monochromatic 
screens,  usually  transmitting  a  considerable  range  of  wave-lengths  at  the 
red  end  of  the  spectrum,  are  commonly  used  in  order  to  eliminate  color 
differences  between  the  source  and  the  pyrometer  filament.  What  is 
really  compared,  in  this  case,  is  a  certain  brightness  of  the  non-black 
body  with  that  of  a  black  body  at  the  same  temperature;  the  ratio  repre- 
sents an  average  emissive  power,  and  as  such  must  naturally  be  ascribed 
to  some  wave-length.  For  a  particular  so-called  monochromatic  screen 
one  might  expect  this  wave-length  to  change  with  a  change  in  the  tempera- 
ture of  the  sources.  This  is  the  case.  How  it  changes  with  the  tempera- 
ture will  be  discussed  later.  Up  to  within  a  few  years  it  was  assumed, 
questioningly  in  many  cases,  that  the  wave-length  to  be  used  is  that 
which  visually  appears  to  be  at  the  center  of  the  transmitted  band  on 
the  passage  of  the  flux  through  a  spectrometer.  On  this  assumption, 
one  naturally  proceeds  using  equation  (19)  just  as  in  the  case  where  the 
transmission  band  is  limited  by  means  of  a  spectrometer. 

Just  as  in  measuring  non-black  bodies  with  the  total  radiation 
pyrometer  we  make  use  of  radiation  temperatures,  so  here  we  use  a 
similar  quantity  called  brightness  temperature.20  It  is  defined  by  the 
equation  : 

b'n  =  FxexdX-Wr  =  TVA"5™  (20) 

or  more  simply  by 

'         '  :: 


where  S  is  the  brightness  temperature  at  wave-length  X  corresponding  to 
the  true  temperature  T.     This  is  analogous  to  equation  (6),  relating 

20  Hyde,  Cady  and  Forsythe:  Phys.  Rev.  [2]  (1917)  10,  395. 
Hyde:  Bull,  153  (September,  1919). 


384        EMISSIVE    POWERS   AND   TEMPERATURES   OF   NON-BLACK  BODIES 


to  total  radiation  pyrometry.  As  in  the  application  of  the  former  equa- 
tion, we  may  determine  S  in  terms  of  T  without  consideration  of  ex 
or  we  may  determine  ex  in  terms  of  T  without  consideration  of  S.  The 
determination  of  either  cx  or  S  as  a  function  of  T  is  effectually  also  a 
determination  of  the  other  one  of  the  two  quantities  as  a  function  of  T, 
since  the  one  unknown  may  at  once  be  obtained  with  the  aid  of  equation 
(21).  As  is  true  of  total  emissive  powers,  it  is  likewise  true  of  spectral 
emissive  powers  that  their  values  give  a  more  convenient  and  significant 
basis  for  comparing  radiating  substances  than  do  statements  of  differ- 
ences between  brightness  temperatures  and  true  temperatures. 


180 


160 


g   140 


120 


100 


80 


2   60 


40 


jL 


900        1000 


1100 


1200        1300        1400         1500        1600         1700         1800       1900 
Temperature  in  Degrees  K. 

FIG.  5. — BRIGHTNESS  TEMPERATURE,  TRUE  TEMPERATURE  RELATIONS  FOR  PLAT- 
INUM ACCORDING  TO  MEASUREMENTS  MADE  WITH  DISAPPEARING-FILAMENT  PYRO- 
METERS WITH  RED-GLASS  ABSORBING  SCREENS.  °  ,  HoLBORN  AND  KuRLBAUMJ 

X  ,  WAIDNER  AND  BURGESS;  —  ,  MENDENHALL;  +  ,  SPENCE. 

Experimental  Methods  and  Results. — In  the  experimental  work,  the 
greatest  difficulty  is  experienced  in  determining  the  true  temperature 
of  the  material  investigated.  In  connection  with  their  original  work  on 
the  disappearing-filament  pyrometer,  Holborn  and  Kurlbaum21  investi- 
gated the  radiation  properties  of  platinum  and  palladium,  obtaining  S  as 
a  function  of  T.  This  was  accomplished  by  forming  an  enclosure  of 
platinum  sheet  that  was  heated  electrically.  True  temperatures  were 
measured  by  means  of  a  thermocouple,  one  junction  of  which  was  located 
within  the  enclosure.  Brightness  temperatures  were  measured  by  sight- 
ing a  pyrometer,  previously  calibrated  against  a  black  body,  on  the  outer 
surface  of  the  enclosure.  Their  results  are  incorporated  in  Fig.  5,  along 


21  Ann.  Phys.  [4]  (1903)  10,  225. 


A.    G.    WORTHING  385 

with  the  results  of  Waidner  and  Burgess,22  who  somewhat  later  used 
essentially  the  same  method,  and  of  Mendenhall23  and  Spence,24  both  of 
whom  made  use  of  a  wedge  opening  to  be  described  presently.  Due  to 
lack  of  specification  of  the  characteristics  of  the  so-called  monochromatic 
screens  used,  it  is  impossible  to  be  sure  to  what  wave-lengths  the  measured 
brightness  temperature  should  be  ascribed.  On  the  assumption  that  the 
monochromatic  screens  were  much  the  same  and  that  the  wave-length  to 
which  these  should  be  ascribed  is  roughly  0.66|u,  the  value  assumed  by 
Waidner  and  Burgess,  these  results  lead  to  approximately  0.36,  0.32,  and 
0.29  as  the  emissive  powers  at  1000°  K.,  1400°  K.,  and  1800°  K.,  respectively. 
Emissive  power  determination  using  a  spectrophotometer  have  been 
carried  out  by  Stubbs  and  Prideaux25  on  liquid  and.  solid  gold  and  by 
Stubbs26  on  liquid  and  solid  copper  and  silver.  Bidwell27  determined,  with 
a  disappearing-filament  pyrometer,  the  spectral  emissive  powers  of  silver, 
gold,  copper,  steel,  and  nickel.  In  the  three  last  mentioned  investi- 
gations, temperatures  were  obtained  from  thermocouples  embedded  in 
the  heated  material. 

Mendenhall28  seems  to  have  been  the  first  to  suggest  a  fundamentally 
sound  way  to  eliminate  uncertainties  as  to  the  true  temperature  of  the 
radiating  material  being  investigated.  '  This  he  accomplished  by  means 
of  a  narrow  wedge  opening  formed  by  folding  on  itself  a  sheet  of  the 
material  being  studied.  Regarding  this  he  says: 


The  device  to  be  described  promises  to  be  of  some  value  because  it  enables  one 
with  a  calibrated  optical  pyrometer  to  determine  the  true  temperature  of  a  radiating 
surface.  It  is,  of  course,  nothing  but  a  special  scheme  for  obtaining  the  Kirchhoff 
black-body  conditions — a  black  body  being  denned,  as  usual,  by  the  conditions,  a  = 
absorptive  power  =  1;  it  will  have,  of  course,  the  maximum  possible  emissive  power 
at  any  temperature.  The  special  scheme  referred  to  is  shown  in  Fig.  6  (this  paper) 
where  F  is  a  flat  conducting  ribbon,  heated  by  a  longitudinal  electric  current,  as 
shown,  and  folded  on  a  line  parallel  to  the  length  so  that  the  resulting  cross-section 
perpendicular  to  the  current-flow  is  a  very  narrow  V — say  with  about  10°  angular 
opening.  If  the  ribbon  is  of  uniform  thickness  and  width,  it  will  be  raised  to  a  uni- 
form temperature  by  a  given  current,  except  near  the  ends.  The  inside  of  the  V  might 
be  then  expected  to  be  a  close  approximation  to  a  black  body,  or  total  radiator, 
since  it  has  but  a  small  opening  and  uniformly  heated  walls,  and  if  this  were  so, 
observations  on  it  with  an  optical  pyrometer  would  give  the  true  and  not  the  "black- 
body"  (brightness)  temperature  of  its  insicle  walls.  The  outside  of  the  V  will  give 
radiation  characteristics  of  the  material  of  the  ribbon,  and  could  be  used  to  study 
this  radiation;  but  before  we  can  draw  conclusions  as  to  the  temperature  of  the  out- 
side surface  we  must  evidently  consider  two  questions: 

"  U.  S.  Bureau  of  Standards  Bull.  3  (1907)  163.     "  Astrophys.  Jnl.  (1911)  33,  191. 
l<  Astrophys.  Jnl.  (1913)  37,  194.  25  Proc.  Roy.  Soc.  Lond.  (1912)  87A,  451. 

"  Proc.  Roy.  Soc.  Lond.  (1913)  88A,  195.  27  Phys.  Rev.  [2]  (1914)  3,  439. 

28  Loc.  cit. 

25 


386        EMISSIVE   POWERS   AND   TEMPERATURES    OF   NON-BLACK  BODIES 

1.  How  closely  does  the  radiation  from  the  inside  of  the  V  approximate  that  of  a 
black  body  at  the  temperature  of  the  inside  walls? 

2.  How  much  real  temperature  difference  is  there  between  the  inside  and  outside 
surface  of  the  wall  of  the  V? 


-WEDGE  OPENING  USED  BY  MENDENHALL  FOR  TRUE-TEMPERATURE  MEASURE- 
MENTS. 


The  first  of  these  two  questions  was  answered  by  considering  the 
building  up  of  radiation  within  the  V-opening  to  black  radiation.  In 
Fig.  7,  a  V-opening  is  formed  by  bending  a  specular  reflecting  sheet. 
Points  A,  B,  C,  D,  E,  and  F  are  points  of  reflections  for  a  ray  which  may  be 
imagined  as  entering  at  P.  If  the  material  of  the  V  is  radiating,  in  con- 
sequence of  its  temperature,  for  any  range  of  wave-lengths,  the  bright- 


FIG.  7. — DIAGRAM  SHOWING  HOW  RADIATION  WITHIN  A  NARROW  V  BUILDS  UP  TOWARD 

BLACK-BODY  RADIATION. 

ness  of  the  point  F,  as  viewed  from  Q  may  be  considered  as  made  up  of 
various  components:  first,  that  due  to  the  natural  radiation  from  F, 
second,  that  due  to  the  natural  radiation  from  E  reflected  at  F,  third 
that  due  to  the  natural  radiation  from  D  which  is  twice  reflected  at  E 
.and  at  F,  etc.  Limiting  ourselves  to  a  small  wave-length  interval,  remem- 
bering according  to  Kirchhoff  s  law  that  the  reflection  factor  rx  is  equal  to 
1  —  cx,  and  representing  by  b'  the  spectral  brightness  of  a  black  body  at 


A.    G.    WORTHING  387 

the  temperature  of  the  material  of  the  V,  and  by  6"  the  corresponding 
spectral  brightness  of  the  point  F  as  viewed  from  Q  we  have, 

b"  =  <J>'  +  rX6X6'  +  rVx&'  +  .    .    .    .  r\exb'  =  b'(l  -  r-x) 
With  a  V-opening  of  10°,  as  suggested  by  Mendenhall,  n  will  be  equal  to 
18.     Thus  with  rx  equal  to  0.7  (about  that  for  the  material  used  originally 

b" 
by  Mendenhall)  , ,   is  found  to  be  99.8  per  cent.,  that  is  the  radiation 

from  the  V-cavity  may  be  said  to  be  99.8  per  cent,  black,  a  satisfactory 
approach  to  black-body  radiation. 

The  second  question  relating  to  the  temperature  difference  between 
the  inside  and  the  outside  of  the  V-opening  was  settled  by  computing  the 
difference  in  temperature  from  the  known  dimensions,  the  electrical  input, 
and  the  thermal  conductivity  of  the  material.  For  the  platinum  wedges 
used,  Mendenhall  found  a  difference  of  the  order  of  a  few  tenths  of  a 
degree.  His  results  on  platinum  agreed  quite  well  with  the  previ- 
ously mentioned  results  by  Holborn  and  Kurlbaum  and  by  Waidner 


FIG.  8. — PYROMETER  FILAMENT  PROJECTED  AGAINST  HOLE  AND  SURFACE  OF  A 
PERFORATED  TUBULAR  TUNGSTEN  FILAMENT  AS  BACKGROUND,  AS  USED  IN  EMISSIVE 
POWER  DETERMINATIONS  OF  TUNGSTEN. 

and  Burgess  (see  Fig.  5).  Later  Mendenhall  and  Forsythe29  applied 
this  method  with  considerable  success  to  tungsten,  tantalum,  molybdenum, 
and  carbon. 

While  the  V-method  of  obtaining  the  true  temperature  of  the  material 
being  investigated  was  theoretically  a  considerable  advance,  it  left 
some  uncertainties.  The  method  demanded  a  uniform  temperature  over 
a  relatively  large  plane  surface.  Moreover,  in  certain  cases,  particularly 
in  connection  with  tungsten,  trouble  was  experienced  due  to  the  two 
separate  sheets,  found  necessary  at  that  time  in  making  up  the  V,  separat- 
ing so  as  to  leave  a  gap  between  the  two  parts. 

The  question  of  the  emissive  power  as  a  function  of  the  temperature, 
and  therewith  also  the  relation  between  brightness  and  true  tempera- 
ture, for  tungsten  has  also  been  attempted  by  the  writer.30  It  is  believed  . 

29  Astrophys.  JnL.  (1913)  37,  380.  30  Phys.  Rev.  [2]  (1917)  10,  377. 


388        EMISSIVE   POWERS  AND   TEMPERATURES   OF  NON-BLACK  BODIES 

that  in  so  doing  the  uncertainties  occurring  in  the  work  using  the  V- 
mettiod  have  been  largely  eliminated.  In  this  work  long  tubular  fila- 
ments with  small  holes,  Fig.  8,  penetrating  the  side  walls  at  various  places 
have  been  used.  Here  again  the  desirability  of  the  method  has  depended 
on  the  fact  that  we  have  accurate  means  of  measuring  the  temperature 
of  the  material  being  studied.  In  general  terms,  the  procedure  consisted 
of  determining  with  an  optical  pyrometer  the  ratio  of  the  brightness  of 
the  filament  surface  adjacent  to  a  hole,  to  the  brightness  of  the  hole,  in  a 
region  suitably  chosen  from  the  standpoint  of  constancy  of  temperature, 
when  the  filament  was  heated  to  incandescence  in  a  vacuum  or  in  an 
atmosphere  chemically  inert.  On  the  assumption  that  the  radiation 
from  the  hole  is  black  and  that  there  is  a  negligibly  small  difference  of 
temperature  between  the  interior  and  the  surface,  such  a  ratio  represents 
an  emissive  power  for  a  wave-length  depending  on  the  light  transmitted 
by  the  pyrometer  glass  screen  and  for  a  temperature  corresponding  to 
that  of  the  radiation  from  the  hole.  This  latter  temperature  was  ob- 
tained in  the  standard  manner  with  the  aid  of  Wien's  law  by  comparing 
the  black  radiation  with  that  from  a  calibrated  black  body  of  the  ordinary 
type  at  the  palladium  point.  A  brightness-temperature  true-temperature 
.relation  follows  simply. 

Several  sources  of  error  were  of  course  necessarily  considered  and 
corrections  were  made  on  account  of  two  of  these;  one  on  the  difference 
in  temperature  between  the  interior  and  the  exterior  surfaces  of  the  tubu- 
lar filament,  the  other  for  the  lack  of  monochromatism  in  the  light  used. 
These  corrections  were  both  small;  the  first  correction  was  by  far  the 
more  important.  A  formula  proposed  by  Mendenhall  and  used  by 
Angell,31  together  with  data  previously  obtained  by  the  writer32  were 
used.  How  important  this  correction  was  is  shown  in  a  later  figure 
giving  emissive  power  results.  The  latter  correction,  one  essentially 
of  determining  to  what  wave-lengths  measured  results  shall  be  applied, 
will  be  now  considered. 

In  Fig.  9,  exaggerated,  curves  a,  /3, 7  and  5  represent  for  a  given  filament, 
at  a  temperature  T,  certain  spectral  brightness  V  distributions  related  to 
the  luminous  flux  transmitted  through  the  pyrometer  system  including 
the  colored-glass  screen  at  the  eyepiece.  Let  a  refer  to  the  black-body 
radiation  at  the  temperature  T  coming  from  a  hole  in  the  filament  wall; 
/3  the  natural  tungsten  radiation  arising  from  the  adjacent  external  sur- 
face; 7  the  radiation  from  a  black  body  having  the  temperature  S,  the 
measured  brightness  temperature  of  the  natural  tungsten  radiation; 
and  8  that  black-body  radiation  whose  relative  brightness  distribution 
is  the  same  as  that  given  by  /3.  These  diagrammatic  distributions  assume 


81  Phys.  Rev.  (1911)  33,  421.  32  Phys.  Rev.  [2]  (1914)  4,  535. 


A.    G.    WORTHING 


389 


the  possibility  of  color  matching  the  tungsten  radiation  with  black-body 
radiation.  Thus  curve  5  is,  according  to  Hyde,  Cady  and  Forsythe,33 
the  brightness  distribution  of  a  black  body  at  a  temperature  given  by 
the  color  temperature  of  the  natural  radiation.  Evidently,  from  the 
definition  of  brightness  temperature,  the  areas  included  under  curves  j3 
and  7  are  equal.  It  is  also  evident  that  only  at  the  wave-length  X'  is 
the  brightness  temperature  of  the  natural  radiation  equal  to  S,  being 
progressively  less  than  S  as  the  wave-length  is  increased  beyond  X'  and 
progressively  greater  than  S  as  the  wave-length  is  decreased  below  X'. 


FIG.  9. — VARIOUS    SPECTRAL   BRIGHTNESS,   b',   DISTRIBUTIONS  CONNECTED  WITH 

TUNGSTEN  FILAMENTS,  WHICH  ARE  HELPFUL  IN  DETERMINING  THE  WAVE-LENGTH 
TO  WHICH  TO  ASCRIBE  BRIGHTNESS  TEMPERATURE  MEASUREMENTS. 

Representing  by  &'„,  b'ff,  etc.,  values  of  V  corresponding  to  curves 

/»OJ 

a,  ft,  etc.  and  by  ba,  etc.,  the  total  brightnesses  I  b'ad\,  etc.  we  then  have 

6,  =  by  (23) 

£-  =  !!-  =  £- =  (fe)  (24) 

o's        os       bs        \b  s  /  x, 

where  in  the  first  member  X,  of  course,  refers  to  any  wave-length  within 
the  range  concerned.  The  last  of  these  equations,  according  to  Hyde, 
Cady  and  Forsythe,34  is  also  the  defining  equation  of  the  effective  wave- 
length of  the  pyrometer  screen  for  black-body  radiation  for  the  tempera- 


33P%s.  Rev.  [2]  (1917)  10,  395. 


34  Astrophys.  JnL.  (1915)  42,294. 


390        EMISSIVE    POWERS    AND   TEMPERATURES   OF   NON-BLACK  BODIES 


ture  interval  given  by  curves  7  and  8.  It  follows,  therefore,  that  the 
wave-length  X'  to  which  the  brightness  temperature  S  is  to  be  ascribed 
is  the  effective  wave-length  of  the  screen  for  black  radiation  in  going 
from  the  brightness  temperature  of  the  tungsten  to  its  color  temperature. 
In  the  writer's  work,  X'  for  tungsten  has  varied  from  0.6662/1  at  1600°  K. 
true  temperature  to  0.6628/x  for  3200°  K. 


.50 

.48 

I 

(£.46 

UJ 

-.44 

I 

in 

42 

.40 
38 

X 

^ 

"x 

^  z  a, 

Pseudo  Emissive  Power  (Dash  line) 

^^v 

\ 

*X 

*x 

'•^ 

"*X 

+ 

•v.^ 

^x 

>v 

^x 

x 

N, 

L-, 

^^ 

X 

^  o** 

^sc, 

^S 

« 

-^^ 

V, 

% 

5>s 

c 

^3 

X 

^•N 

&  ° 

^ 

^ 

^ 

+ 

°^ 

>* 

X:" 

Ss 

o 

JL-0.4 

67/i. 

0^ 

x^k, 
•g^s 

-o-ofc 

^ 

4 

& 

o 

^& 

D'V^^ 

^00 

\c' 

x    o 

0 

*cft    x 

+ 

X 

<® 

^•*' 

N*' 

3 
O 
0 

°N 
c 

o 

°^v 

| 
1 

1 

* 

oX^ 

w 
X 

JL-0. 

»^a 

^ 

X 

^ 

1 

^ 

X 

s 

00             600              1000              1400              1800              2200             2600            3000             3400 

3800 

Temperature  in  Degrees  K. 

FIG.  10. — EMISSIVE  POWER  RESULTS  AND  A  COMPUTED  PSEUDO-EMISSIVE  POWER 
CURVE  FOR  TUNGSTEN  AS  A  FUNCTION  OF  THE  TEMPERATURE  AT  0.665/i  AND  0.467/i. 
X  VALUES  OBTAINED  ON  UNPOLISHED  FILAMENT  IN  MUCH  STRIATED  BULBSJ  +  VALUES 
OBTAINED  ON  POLISHED  FILAMENTS  IN  MUCH  STRIATED  BULBSJ  °  VALUES  OBTAINED  ON 
POLISHED  FILAMENT  IN  FAIRLY  CLEAR  BULBS J  ©  VALUES  OBTAINED  AT  ROOM  TEM- 
PERATURE BY  REFLECTION  METHOD;  a,  a',  WEIGHTED  CURVES  FOR  DATA  OBTAINED;  6', 
CURVE  a'  CORRECTED  FOR  LACK  OF  MONOCHROMATISM  FOR  THE  UVIOL  GLASSJ  C,  c' , 
FINAL  CURVES  CONTAINING  CORRECTIONS  FOR  DIFFERENCES  IN  TEMPERATURE  BETWEEN 
INTERIOR  AND  EXTERIOR  SURFACES  OF  THE  FILAMENTS. 

Having  once  determined  \r,  the  method  of  determining  S0,  the  bright- 
ness temperature  that  will  correspond  to  some  common  wave-length  X0 
arbitrarily  chosen,  is  simple.  It  consists  in  finding  the  temperature  of 
a  black  body  corresponding  to  70  (Fig.  9).  Choosing  X0  as  0.665/i 
means,  in  the  writer's  work,  that  the  values  of  S0  —  S  for  tungsten  for 
red  light  are  respectively  +0.2°  and  —  1.4°  at  true  temperatures  1600° 
and  3200°  K.  The  corrections  for  the  blue  uviol  screen  are  somewhat 
greater. 

In  a  similar  way,  the  wave-length  to  which  to  ascribe  the  emissive 


A.    G.    WORTHING  391 

power  measurement  may  be  determined.  Imagine  another  spectral 
brightness  distribution  curve  added  to  the  somewhat  complicated  figure, 
which  will  enclose  underneath  it  an  area  equal  to  that  enclosed  by  /3, 
and  which  will  bear  the  same  relation  to  a  that  /3  does  to  6.  Call  this 
curve  $'.  The  ratio  of  its  ordinates  to  that  of  a  will  everywhere  be  equal 
to  the  measured  emissive  power.  It  will  cross  the  curve  /3  at  some  wave- 
length X".  Evidently  at  this  wave-length  only  is  the  ratio  of  the  ordi- 
nate  of  $  to  that  of  a  equal  to  the  measured  emissive  power.  Hence 
strictly  the  emissive  power  measured  should  be  ascribed  to  X".  As 
in  the  case  of  X'  just  described,  X"  may  be  shown  to  be  the  effective  wave- 
length for  the  optical  system  in  passing  from  distribution  a  to  distribu- 
tion 8,  X"  is  slightly  shorter  than  X'.  On  considering  later  the  change 
in  emissive  power  in  going  from  0.665ju  to  0.467/i,  together  with  color- 
matching  possibilities,  it  will  be  seen  that  the  changes  in  the  emissive 
power  in  going  from  X"  to  Xo  are  very  small.  In  this  work  such  correc- 
tions at  0.665/x  were  inappreciable,  those  at  0.467X  were  just  appreciable, 
as  will  appear  later. 

The  emissive  power  results  are  given  in  Fig.  10.  In  accord  with 
what  has  previously  been  stated,  at  0.665ju  and  0.467/i,  the  emissive  powers 
for  2450°  K,  the  normal  operating  temperature  of  the  vacuum  tungsten 
lamp,  are  respectively  0.428  and  0.464.  These  values  have  been  used  in 
computing  the  curves  a  of  Figs.  1  and  2.  It  is  evidently  now  possible, 
with  the  aid  of  equation  (21),  to  compute  for  tungsten  the  relation 
between  the  brightness  temperature  and  the  true  temperature,  the 
method  of  determining  to  what  wave-length  to  assign  the  brightness  tem- 
perature having  been  shown.  Previous  to  the  publication  of  these  results 
(see  Table  2),  there  had  been  various  temperature  scales35  (relation 
between  the  brightness  temperature  at  some  wave-length  and  the  true 
temperature)  proposed  for  tungsten.  Each  of  these  appeared  to  be 
faulty  in  one  respect  or  another  in  comparison  with  that  reported  in 
Table  2.  As  a  consequence  thereof,  these  results  have  been  accepted 
by  the  laboratories  of  the  General  Electric  Co.  as  their  temperature 
scale  for  tungsten. 

For  other  methods  for  determining  emissive  powers,  reference  should 
be  made  to  papers  by  Langmuir,36  Shackelford,37  Hulburt,38  and  Weniger 
and  Pfund.39  In  Langmuir's  work  the  emissive  power  of  molten  tungsten 
was  determined  from  the  brightness  of  the  images  of  opposing  electrodes 


38  Pirani:  Phys.  Zeit.  (1912)  13,  753. 

Mendenhall  and  Forsythe:  Astrophys.  Jnl.  (1913)  37,  380. 

Pirani  and  Meyer:  Elektrotech.  u.  Masch.  (1915)  33,  397,  414. 

JLangmuir,  Phys.  Rev.  [2]  (1915)  6,  138  and  (1916)  7,  302. 
3*Phys.  Rev.  [2]  (1915)  6,  183.  37  Phys.  Rev.  [2]  (1916)  8,  470. 

38  Astroptiys  Jnl.  (1917)  46,  149.         3»  Jnl.  Frank.  Inst.  (1917)  183,  354 


392        EMISSIVE   POWERS   AND   TEMPERATURES   OF   NON-BLACK  BODIES 

in  a  tungsten-  arc.  The  main  difficulty  with  this  method  lay  in  the  tem- 
perature variations  across  the  arc  terminal  and  in  the  extraneous  bright- 
ness, due  to  the  arc  proper.  Shackelford,  using  helical  tungsten  coils 
of  varying  pitch,  determined  with  the  aid  of  a  disappearing-filament 
pyrometer  the  ratio  of  the  external  brightness  to  the  maximum  internal 
helix  brightness  and  extrapolated  to  the  case  of  a  closed  cylinder  to  ob- 
tain emissive  powers.  In  theory,  the  method  is  very  nice,  though  one 
is  not  inclined  to  give  such  results,  when  unsupported,  the  full  confidence 
they  deserve.  The  actual  values  obtained  agreed  well  with  those  re- 
ported by  the  writer.  Hulburt  worked  in  the  ultra-violet  region  and 
used  the  photo-electric  effect  of  such  radiations.  Comparisons  were  made 
directly  with  a  black  body  operated  at  the  palladium  point.  Tempera- 
tures of  the  tungsten  filament  were  obtained  from  other  work.  The 
actual  values  of  emissive  power  depend  considerably  on  the  accuracy  of 
this  temperature.  Weniger  and  Pfund  have  worked  in  the  infra-red 
region  of  the  spectrum.  They  measured  the  reflectivities  of  tungsten 
for  various  wave-lengths  directly.  From  Kirchhoff's  law,  it  follows  at 
once  that  unity  less  the  measured  reflectivities  give  the  corresponding 
emissive  powers.  Their  results  slightly  extrapolated  as  to  temperature 
have  been  used  in  computing  the  infra-red  parts  of  curve  a,  Fig.  1,  in 
which  is  given  the  spectral  radiant  flux  distribution  for  a  vacuum  tung- 
sten lamp  at  its  normal  operating  temperature. 


TABLE  2. — Emissive  Power  and  Allied  Data  for  Tungsten 


T 
Degrees  K. 

o 
«                                           0.665M 

«-MMI          Degrees  K. 

'o.467M 

T 

c(a.  467/1  0.666M) 

Degrees  K. 

f   (0.467/1.0.665M) 

T.t 

1200 

0.458 

1150 

0.493 

1211 

0.391 

1600 

0.448 

1510 

0.484 

1620 

0.380 

1611 

2000 

0.438 

1858 

0.474 

2032 

0  370 

2031 

2400 

0.429 

2193 

0.465 

2448 

0.360 

2442 

2800 

0.419 

2516 

0.455 

2868 

0.350 

3200 

0.409 

2826 

0.446 

3292 

0.340 

3600 

0.399 

3121 

0.436 

3720 

0.329 

3675* 

0.398 

3176 

0.435 

3800 

0  328 

*  Melting  point. 

t  Measurements  using  integral  color  match  method,  according  to  Hyde,  Cady  and 
Forsythe. 

The  method  of  employing  emissive  power,  or  the  brightness-tempera- 
ture true-temperature  relation,  in  determining  true  temperature  when 
once  the  brightness  temperature  is  obtained  is  obvious.  For  example, 
suppose  it  is  desired  to  find  the  true  temperature  of  tungsten  at  its  melt- 
ing point.  Suppose,  as  in  Table  2,  that  3176°  K.  is  the  observed  bright- 
ness temperature  of  just  molten  tungsten,  the  use  of  a  table  or  a  plat 


A.    G.    WORTHING  393 

giving  T  —  S  or  ex  as  a  function  of  S  .leads  either  directly,  or  through 
equation  (21),  to  3674°  K. 

Variations  in  Emissive  Powers  of  Substances. — There  are  many  possible 
types  of  variation  of  which  only  three  will  be  considered  here;  viz., 
variation  with  wave-length,  temperature,  and  angle  of  emission.  Data 
referring  to  certain  of  these,  particularly  that  due  to  temperature,  have 
already  been  given  at  least  in  part. 

One  of  the  most  fruitful  works  relating  to  emissive  power  variations 
was  the  derivation  based  on  Maxwell's  theory  which  was  obtained  by 
Drude.  It  is 


ex  =  const..  A  (25) 

\  X 

where  p  is  the  resistivity  of  the  material  and  X  the  wave-length.  When 
p  is  measured  in  ohm  cm.  and  X  in  n,  the  constant  is  numerically  equal 
to  0.365.  This  equation  has  been  subjected  to  a  very  great  number  of 
tests,  most  of  which  have  referred  to  measurements  at  moderate  tempera- 
tures. However,  measurements  at  high  temperatures  have  been  made; 
particularly,  the  works  of  Hagen  and  Rubens,40  McCauley,41  Weniger  and 
Pfund42  may  be  referred  to.  Generally  speaking,  this  law  which  applies 
to  a  temperature  variation  as  well  as  a  wave-length  variation  has  been 
found  satisfactorily  fulfilled  at  long  wave-lengths,  usually  those  far  in  the 
infra-red,  beyond  14ju  for  silver,  beyond  about  2/*  for  tungsten.  Devia- 
tions from  this  law,  as  one  proceeds  from  this  region  to  the  near  infra- 
red, have  usually,  if  not  always,  become  greater  and  greater.  In  the 
near  infra-red  and  the  visible,  the  formula  generally  fails  completely, 
the  variation  with  temperature  being  often  in  the  opposite  direction 
from  what  might  be  expected,  as  is  shown  by  the  results  on  platinum 
and  tungsten  already  quoted.  Theories  of  atomic  structure  have  been 
productive  in  explaining  these  variations,  but  no  completely  satisfactory 
working  theory  for  this  region  of  failure  of  equation  (25)  has  yet  been 
developed. 

The  question  of  selectivity  of  radiation,  a  question  of  very  great 
practical  interest  to  incandescent-lamp  manufacturers,  is  in  a  broad 
way  likewise  connected  with  spectral  emissive  powers.  By  saying  that 
a  body  radiates  selectively,  we  mean  that  its  spectral  radiant-flux  dis- 
tribution for  a  given  temperature  is  different  from  that  for  a  black  body 
at  the  same  temperature.  A  direct  method  of  testing  a  substance  for 
selectivity  would  be  to  compare  its  spectral  radiant-flux  curve  with  that 
for  a  black  body  at  the  same  temperature.  Thus  we  might  compare 
curve  a,  Fig.  1 ,  with  curve  b.  If  in  so  doing  we  should  arbitrarily  plot 
the  data  represented  by  curve  a  in  such  a  fashion  that  it  coincides  with 
curve  b  at  one  point,  let  us  say  0.7/t,  we  should  find  that  the  two  curves 

"Ann.  Phys.  (1903)  11,  888.        «  Astrophys.  Jnl.  (1913)  37,  164.        <2  Loc.  cit. 


394        EMISSIVE   POWERS   AND   TEMPERATURES   OF   NON-BLACK   BODIES 


would  not  continue  to  coincide  throughout  the  whole  range  of  wave-length. 
We  should  find,  in  fact,  that  for  wave-lengths  shorter  than  0.7/*  the 
curve  for  tungsten  would  lie  everywhere  above  the  curve  for  the  black 
body,  and  that  for  wave-lengths  greater  than  0.7 /j,  the  curve  for  the  tung- 
sten radiation  would  lie  everywhere  below  that  for  a  black  body.  In 
general,  the  discrepancy,  expressed  in  per  cent.,  would  become  greater  and 


90 


J2 


070 


< 

.£60 


£ 

-£50 


Si  40 


530 


u-  20 


10 


\ 


\ 


2.6 


3.0 


0.6>        1.0  1.4  1.8  2.2 

Wave  Length 

FIG.  11. — SPECTRAL  RADIANT  FLUX  CURVES  FOR  TUNGSTEN,  X,  AND  CARBON,  o,  AT  A 

COLOR  TEMPERATURE  OF  APPROXIMATELY  2200°  K.  ACCORDING  TO  COBLENTZ. 

greater  the  further  we  get  from  the  arbitrarily  chosen  wave-length  0.7/x. 
While  such  a  method  of  comparing  substances  is  the  most  direct,  it  gener- 
ally entails  a  great  amount  of  work  and  has  seemed  practically  impossible 
in  some  cases. 

There  is  another  method,  much  more  convenient,  dependent  on  the 
possibility  of  color-matching  various  light  sources,  a  method  that  seems 
first  to  have  been  employed  by  Morris,  Stroud,  and  Ellis.43  The  method 
has  been  developed  independently  by  Hyde  and  others44  in  a  much 


"Elec.  (1907)  59,  584  and  624. 

44  Hyde:  Phys.  Rev.  (1908)  27,  521;  Aslrophys.  Jnl.  (1912) .36,  89. 

Hyde,  Cady  and  Middlekauff:  Trans.  111.  Engng.  Soc.  (1909)  4,  334. 

Hyde:  Jnl.  Frank.  Inst.  (1910)  169,  439;  (1910)  170,  26. 

Hyde,  Cady  and  Forsythe:  Phys.  Rev.  [2]  (1917)  10,  395. 


A.    G.    WORTHING 


395 


more  far-reaching  way.  Data  obtained  by  them  for  various  light  sources 
are  shown  in  Table  3,  in  which  k  represents  the  ratio  of  the  relative 
change  in  candlepower  7  to  the  accompanying  relative  change  in  wattage 
W  for- the  source  studied.  It  is  to  be  noted  that  proceeding  down  the 
table  there  is  a  like  progressive  change  in  this  quantity  K  as  well  as  in 
the  efficiency.  Both  of  these  quantities  may  be  used  in  determining 
selectivities.  In  particular,  values  of  efficiency  are  of  interest  in  that 
they  show  in  a  very  direct  and  convenient  manner  how  the  different 
sources  differ  in  their  methods  of  radiating  energy.  In  order  that  two 
sources  shall  have  the  same  distribution  throughout  the  visible  spectrum 
and  have  different  efficiencies,  there  necessarily  must  be  a  difference  in 
the  infra-red  distribution.  Exactly  what  this  is  like  for  the  case  of 
carbon  and  tungsten  is  well  shown  in  Fig.  11,  taken  from  a  paper  by 
Coblentz.  In  his  work,  the  color  temperature  seems  to  have  been  some- 
where in  the  neighborhood  of  2200°  K.  Taking  account  of  the  results 
given  in  Table  3,  we  should  expect  that  the  ratio  of  the  areas  included 

3  5 
under  these  curves  should  be  about  ^ .  or  0.80.     This  is  fairly  closely 

0.76,  the  value  actually  obtained.  Just  how  data  of  this  type  are  valuable 
to  the  lamp  industries  is  perfectly  evident.  The  table  shows  a  good 


TABLE  3. — Radiating  Selectivities  of  Various  Sources  at  Two  Color  Tem- 
peratures, according  to  Hyde 

Tc  =  1700°  K.,  approximately   T*  =  2160°  K.,  approximately 


Source 

K      dl  /dw 
K  =  J/'W 

Efficiency  in 
lumens 

dl  ,dW 
K  =  I/~W 

Efficiency  in 
lumens 

watts 

watts 

Black  body  observed  

3  50 

Black  body  computed  

3  60 

3  05 

Untreated  carbon  

3.45 

0  39 

2  75 

3.5 

Flashed  carbon  

3  35 

0  41 

2  65 

3  7 

Graphitized  carbon  

3  40 

0  41 

2  75 

3.7 

Platinum  

3  10 

Tantalum  

3  00 

0  50 

2  35 

3  9 

Tungsten  

2  85 

0  59 

2  40 

4  4 

Osmium  

2  85 

0  72 

2  40 

4.9 

reason  (not  the  only  one)  for  the  transition  that  has  actually  occurred  in 
incandescent  lamps  from  the  use  of  untreated  carbon  to  flashed  carbon, 
to  graphitized  carbon,  to  tantalum,  and  finally  to  tungsten.  The  fact 
that  there  has  not  been  a  further  change  to  osmium  is  a  consequence  of 


396        EMISSIVE    POWERS   AND   TEMPERATURES   OF   NON-BLACK  BODIES 

other  properties  (particularly  the  rate  of  vaporization),  which  osmium 
possesses  to  a  less  favorable  extent  than  does  tungsten. 

From  the  standpoint  of  emissive  powers,  a  favorable  selectivity 
means  that  the  emissive  powers  of  the  substance  in  the  visible  spectrum 
are,  on  the  average,  greater  than  the  average  emissive  powers  in  the  infra- 
red spectrum;  and,  on  the  whole,  the  greater  the  favorable  selectivity 
from  the  light  production  point  of  view  the  greater  is  this  difference 
between  the  average  emissive  powers. 

Another  variation  of  emissive  powers  that  has  been  but  little  studied, 
but  which  may  be  of  considerable  importance,  is  that  which  occurs  with 
a  change  in  the  angle  of  emission  of  radiation.  The  cosine  law  of  emission, 
the  standard  by  which  these  variations  are  measured,  was  first  enunciated 


120 


HO 


100 


-£90 


80 


70 


60 


50 


20 


\ 


30          40  50          60 

Angle  of  Emission  in  Degrees 


FIG.  12. — VARIATION  IN  BRIGHTNESS  OF  TUNGSTEN  (UPPER  CURVE)  AND  OP  CARBON 
(LOWER  CURVE)  WITH  ANGLE  OF  EMISSION. 


by  Lambert.  It  states  that  the  luminous  intensity,  or  candlepower,  of 
an  element  of  source  varies  as  the  cosine  of  the  angle  of  emission;  a  con- 
sequence of  the  fact  that  the  element  of  source  when  viewed  at  greater 
and  greater  angles  of  emission  is  smaller  in  proportion  to  the  cosine  of 
the  angle.  Wherever  the  cosine  law  is  fulfilled,  a  source  appears  equally 
bright  from  all  directions.  Quantitative  measurements  relating  to  the 
fulfillment  of  Lambert's  law  seem  first  to  have  been  made  by  Moller.46 
He  investigated  the  light  emitted  by  a  glowing  strip  of  platinum  and  con- 


46  Ann.  Phys.  (1885)  24,266. 


A.   G.    WORTHING  397 

eluded  from  his  measurements  that  the  cosine  law  of  emission  was  ful- 
filled. Later  Uljanin,46  as  a  result  of  computation  and  measurement, 
concluded  that  the  law  was  not  fulfilled  for  platinum,  but  that  for  certain 
wave-lengths  in  the  infra-red  there  was  an  increase  from  the  normal 
brightness  for  a  zero  angle  of  emission  to  1.17  times  the  normal  brightness 
for  an  angle  of  70°  decreasing  at  somewhat  larger  angles.  The  writer47 
has  also  made  measurements  of  this  kind  on  tungsten  and  carbon  at 
incandescent  temperatures.  Inasmuch  as  these  results  represent,  so 
far  as  the  writer  knows,  the  extremes  of  variation  found  as  well  as  repre- 
senting a  use  of  the  optical  pyrometer,  the  results  obtained  are  given  in 
Fig.  12.  It  is  to  be  noticed  in  this  connection  that  the  brightness  of 
tungsten,  viewed  at  an  angle  of  75°,  for  the  temperature  considered  is 
about  17  per  cent,  greater  than  the  brightness  viewed  normally.  With 
increase  of  the  angle  beyond  this,  there  occurred  a  more  or  less  gradual 
diminution  in  brightness  to  a  zero  value  at  90°.  For  carbon,  on  the  other 
hand,  a  gradual  falling  off  in  brightness  with  increasing  angles  of  emission 
was  observed,  even  at  small  angles  of  emission.  Using  the  data  given  in 
the  figure,  it  is  easy  to  show,  by  computation,  that  the  average  brightness 
of  tungsten,  taking  into  account  the  light  emitted  in  all  directions,  is  a 
trifle  more  than  5  per  cent,  greater  than  the  brightness  viewed  normally. 
What  has  been  said  very  definitely  shows  the  importance  in  all  optical 
pyrometric  measurements  of  noting  whether  observations  are  made 
normally  on  the  material  studied,  or  if  not,  at  what  angles  of  emission 
they  are  made. 

PSEUDO-EMISSIVE  POWER  DEPENDING  ON  COLOR  OF 
AN  INCANDESCENT  BODY 

The  possibility  of  color-matching  various  light  sources  already  men- 
tioned leads  to  the  consideration  of  a  pseudo-emissive  power.  Consider 
in  this  connection,  curves  a  and  c,  of  Fig.  2,  which  represent  the  spectral 
brightness  distribution  of  tungsten  at  2450°  K.  and  a  black  body  at  2500° 
K.  at  which  temperatures  such  a  color  match  exists,  a  condition  sum- 
marized by  saying  that  the  color  temperature  of  tungsten  at  2450°  K.  is 
2500°  K.  Naturally,  it  follows  that  everywhere  through  the  visible  spec- 
trum the  ratio  of  the  spectral  brightnesses,  whatever  the  wave-length,  is 
constant  and  therefore  the  same  as  for  the  total  luminous  flux.  This  ratio, 
as  can  be  readily  verified,  is  quite  closely  0.358.  It  represents  a  pseudo- 
emissive  power.  Representing  by  T,  as  before,  the  true  temperature  of 
the  tungsten  filament,  by  Tc(XlXl)  its  color  temperature,  2500°  K.  in  the 
case  noted  above,  by  ex  and  by  e'  the  spectral  emissive  power  and  the 
pseudoemissive  power,  and  by  Xi  and  X2  two  arbitrarily  chosen  wave- 
lengths1, we  have,  following  the  applications  of  Wien's  equation, 

46  Ann.  Phys.  (1897)  62,  528.  47  Astrophys.  Jnl.  (1912)  36,  345. 


398        EMISSIVE    POWERS   AND   TEMPERATURES    OF   NON-BLACK   BODIES 

€Xle  ~x.r  =  e'(XiX2)  e'xIKo^j  (25) 

and 

Cl  _     C2 

cX2  e    xz  T  =  c'(XiX2)  e    **rc(MXt)  (26) 

whence  by  elimination  of  e'(XlXj)  we  have 

** 

1 _    J 

77  ~    71 

1  cxix          2 


From  this  equation,  we  may  compute  the  color  temperature  as  a  func- 
tion of  the  true  temperature  from  such  data  as  are  given  in  Fig.  10.  Pro- 
vided the  true  temperature  scale  of  tungsten  is  known,  this  relation 
may  also  be  determined  by  a  direct  comparison  of  tungsten  with  a  black 
body  in  which  the  integral  luminous  flux  is  used.  This  has  been  done 
by  Hyde,  Cady  and  Forsythe.48  How  well  the  two  methods  agree  is 
shown  by  a  comparison  of  the  fifth  and  seventh  columns  of  Table  2.  It 
should  be  noted  that  these  results  refer  to  tungsten  and  not  to  tungsten 
lamps.  The  principal  difference  is  due  to  the  cooling  effects  in  the  neigh- 
borhood of  junctions  and  supports  and  the  selective  absorption  of  the 
glass  bulb  that  occur  in  lamps.  The  variation  of  e'XlXt  with  temperature 
for  tungsten  as  computed  by  means  of  equations  (25),  (26),  and  (27)  is 
also  shown  in  Fig.  10  in  connection  with  the  true  emissive  powers.  For 
the  relation  between  brightness,  color,  and  true  temperature  for  tungsten 
lamps,  reference  must  be  made  to  the  paper  by  Hyde,  Cady,  and  Forsythe. 

A  method  of  determining  variations  in  pseudo-emissive  powers  that 
holds  rigidly  in  case  absolute  color  matches  exist  between  the  radiating 
source  and  a  black  body  at  appropriate  temperatures  makes  application 
of  what  Hyde49  has  called  criterion  1  in  his  synthetic  development  of 
radiation  laws.  In  case  a  rigid  color  match  cannot  be  obtained,  the 
variations  obtained  in  the  pseudo-emissive  powers  may  be  only  approxi- 
mate. The  fulfillment  of  the  criterion  requires  that,  when  a  radiating 
source  and  a  black  body  are  raised  from  one  condition  of  color  match 
to  another  color  match,  the  relative  increases  in  luminous  intensity,  or 
candlepower,  shall  be  the  same.  To  within  experimental  errors,  he 
found  that  this  criterion  was  fulfilled  in  the  radiation  from  carbon  and 
from  tantalum.  In  the  case  of  tungsten,  however,  he  found  that  the  cri- 
terion was  not  fulfilled.  In  going  from  a  color  temperature  of  about  1740° 
K.  to  another  of  about  2130°  K.  there  was  found  a  lack  of  fulfillment 
amounting  to  3.7  per  cent.  This  means  that,  when  tungsten  is  raised 
from  color  temperature  1740°  K.  to  color  temperature  2130°  K.,  the  rela- 
jtive  change  in  candlepower  is  only  96.3  per  cent  of  the  corresponding 
change  in  the  candlepower  of  a  black  body  raised  through  the  same  tem- 

*• bop.  pit,  «  Astrophys.  Jnl.  (1912)  36,  89. 


A.    G.   WORTHING  399 

perature  range.  A  comparison  of  this  result  with  what  is  given  in  Table 
2  indicates  a  moderately  good  agreement.  From  the  data  given  in  the 
sixth  column,  one  would  expect  instead  of  96.3  per  cent,  something  like 


. 

„'  _„  or  97.6  per  cent.     How  much  of  this  difference  may  be  ascribed  to 

(J.uiO 

experimental  error  and  how  much  to  an  inability  to  obtain  a  perfect 
color  match  is  of  course  uncertain.  The  agreement,  everything  consid- 
ered, is  quite  satisfactory  and  the  method  may  be  looked  upon  as  one 
of  considerable  value  in  studying  other  substances. 

CONCLUSION 

In  conclusion,  it  may  be  said  that  a  knowledge  of  the  temperature 
relation  for  non-black  bodies  is  necessary  if  industrial  processes  and  prod- 
ucts are  to  be  satisfactorily  described  and  standardized;  that  the  true 
temperature  in  a  broad  way  is  the  reasonable  basis  on  which  to  coordinate 
such  data;  that  a  study  of  emissive  powers  total,  spectral  and  pseudo, 
or  of  the  radiation-temperature  true-temperature,  brightness-tempera- 
ture true-temperature  or  color-temperature  true-temperature  relations 
represents  in  many  cases  our  only  means  of  measuring  high  true  tempera- 
tures; and  that  therefore  a  study  of  these  relations  is  of  the  highest 
commercial  as  well  as  scientific  significance. 


400  RECORDING  THERMOCOUPLE  PYROMETERS 


Recording  Thermocouple  Pyrometers 

BY   LEO  BEHR,*   M.   E.,    PHILADELPHIA,    PA. 
(Chicago  Meeting,  September,  1919) 

RECENT  years  have  seen  important  practical  advances  in  the  con- 
struction of  recording  instruments  for  use  with  thermocouples.  The 
difficulties  of  the  problem  will  be  appreciated  when  it  is  remembered  that 
a  10°  F.  change  in  temperature  of  a  base-metal  couple  at  1500°  means  a 
change  in  the  electromotive  force  of  about  0.00035  volt.  In  a  circuit 
having  a  resistance  of  350  ohms,  this  means  a  change  in  the  current  of 
0.000001  amp.;  and  despite  this  requirement  of  great  sensitivity  the  in- 
struments must  be  sufficiently  robust  to  withstand  the  rough  us'age  which 
is  so  often  their  lot. 

Two  methods  are  in  common  use  for  the  measurement  of  the  electro- 
motive force  produced  by  a  thermocouple : 

1.  The  milliammeter  method,  in  which  the  current  produced  by  the 
thermocouple  in  a  circuit  of  known  resistance  is  measured  and  from  this 
the  electromotive  force  is  deduced  by  Ohm's  law. 

2.  The  potentiometer  method  in  which  a  known  potential  difference 
is  opposed  to  the  thermocouple  electromotive  force. 

The  use  of  thermocouple  and  milliammeter  for  measuring  tempera- 
ture introduces  some  possible  sources  of  error,  for  the  reading  depends 
on  a  number  of  factors,  including  the  following:  (1)  Electromotive  force 
set  up^at  the  hot  junction,  (2)  electromotive  force  set  up  at  the  cold 
junction,  (3)  resistance  of  the  circuit  including  thermocouple,  lead  wires, 
and  meter,  (4)  field  strength  of  magnets,  (5)  strength  of  controlling 
springs  in  meter,  (6)  friction  at  pivots,  of  pointer  on  paper,  etc. 

It  is  obviously  necessary  to  so  design  and  maintain  the  installation 
that  the  reading  is  independent  of  all  the  factors  except  the  first.  The 
electromotive  force  set  up  at  the  cold  junction  can  easily  be  taken  care 
of  provided  it  is  constant.  This  has  been  approximately  accomplished 
by  burying  the  cold  junction  in  the  ground  or  actually  secured  by  locat- 
ing it  in  a  container,  the  temperature  of  which  is  kept  constant  by  means 
of  a  thermostat. 

A  more  flexible  arrangement  and  one  requiring  less  attention  from  the 
user  is  to  incorporate  some  automatic  cold-junction  temperature  com- 
pensator in  the  recording  instrument.  The  installation  of  the  pyrometer 

*  Research  Engineer,  Leeds  &  Northrup  Co. 


LEO  BEHR 


401 


is  then  reduced  to  merely  connecting  the  thermocouple  terminals  to  the 
recorder.  Numerous  schemes  for  automatic  cold-junction  compensa- 
tion have  been  proposed.  These  consist  essentially  of  resistances  shunted 
around  the  meter  or  in  series  with  it.  These  resistances  are  supposed  to 
be  so  affected  by  a  change  in  temperature  at  the  cold  end  as  to  properly 
compensate  for  such  changes  by  varying  the  amount  of  the  total  current 
passing  through  the  meter.  A  consideration  of 
Ohm's  law,  however,  shows  that  any  arrange- 
ment that  consists  solely  of  resistances  can  satis- 
factorily compensate  for  variations  in  the  cold- 
junction  temperature  for  only  a  limited  range 
of  temperatures  at  the  hot  end,  for  any  such 
arrangement  can  merely  fix  a  ratio  between  the 
currents  for  different  cold-end  temperatures  in- 
stead of  changing  the  current  by  an  additive 
constant. 

A  device  that  permits  of  accurate  compensa- 
tion is  shown  in  Fig.  1.  It  consists  of  a  Wheat- 
stone  bridge  network  in  series  with  the  thermo- 
couple, one  arm  of  the  bridge  is  of  nickel  and  is 
located  at  the  cold  end  of  the  thermocouple.  The 
remaining  arms  Ri,  #2,  and  Ra  are  of  manganin 
and  are  located  in  the  meter.  The  variable 
resistance  R  is  so  adjusted  that  the  pyrometer 
reads  the  temperature  of  the  cold  end  with  the 
thermocouple  out  of  circuit  and  the  leads  con- 
nected together.  Changes  in  temperature  of  the 
nickel  coil  vary  its  resistance  and  determine  the 

extent  of  unbalance  of  the  Wheatstone  bridge  and,  therefore,  the  amount 
of  current  added  (to  the  thermocouple  circuit.  If  the  thermocouple 
leads  are  extended  to  the  meter,  it  is  desirable  to  make  R2  also  of  nickel 
because  the  current  drawn  from  the  storage  battery  B  may  then  be 
reduced. 

Probably  the  most  insidious  source  of  error  is  the  change  in  resistance 
of  the  circuit  containing  the  thermocouple,  lead  wires,  and  meter.  All 
joints  that  are  not  welded  or  soldered  are  possible  sources  of  trouble 
because,  in  time,  they  will  corrode  or  work  loose.  If  a  central-station 
installation  is  in  use,  the  resistance  of  all  the  thermocouple  circuits  should 
be  equal  and  the  switches  must  be  kept  scrupulously  clean.  The  change 
in  the  resistance  of  the  thermocouple  through  oxidation  is  a  source  of 
error  that  does  not  make  its  presence  evident  but  gradually  increases  the 
amount  of  error.  The  percentage  error  in  the  temperature  reading  is 
directly  proportional  to  the  change  in  resistance  and  inversely 
proportional  to  the  total  resistance.  The  designer  can  reduce  this 

26 


FIG.  1. — DEVICE  PER- 
MITTING ACCURATE  COM- 
PENSATION FOR  TEMPERA- 
TURE CHANGES. 


402 


RECORDING  THERMOCOUPLE  PYROMETERS 


error  by  increasing  the  total  resistance  in  the  circuit.  Such  an  increase 
in  resistance  decreases  the  current  for  a  given  temperature  and  a  more 
sensitive  and,  therefore,  essentially  less  robust  meter  element  results. 
The  meters  are  generally  designed  to  operate  on  circuits  of  300  to  400 
ohms. 

The  temperature  coefficient  of  the  meters  is  a  summation  effect  of  the 
temperature  coefficients  of  resistance  of  the  coils  and  the  strength  of  the 
magnets  and  springs.  The  springs  are  weaker  at  higher  temperatures 
while  the  magnets  may  be  either  weaker  or  stronger.  The  copper  coil 


Std.  Cell 


FIG.  2. — SIMPLE  POTENTIOMETER  CIRCUIT. 

of  the  moving  system  is  in  series  with  a  coil  of  small  temperature  coeffi- 
cient and  their  combined  temperature  coefficient  is  of  the  order  of  0.05 
per  cent,  per  degree  Fahrenheit  or  more.  Among  the  methods  used  to 
compensate  for  the  change  in  sensitivity  of  the  meter  may  be  mentioned 
the  use  of  a  mercury  column  to  short-circuit  a  resistance  and  a  compound 
metallic  strip  to  change  the  field  strength. 

On  account  of  the  small  forces  used  to  turn  the  pointer,  friction  of  the 
moving  element  must  be  reduced  to  a  minimum  by  careful  design  and 
construction  of  the  pivots  and  pen.  The  meter  case  must  be  dust-proof, 
because  even  the  most  minute  particles  of  dirt  on  the  pivots  is  sufficient  to 
give  trouble.  There  are  several  devices  for  reducing  pen  friction.  The 
one  in  most  general  use  is  that  of  having  the  pen  touch  the  paper  inter- 
mittently so  that  the  moving  system  is  entirely  free  from  the  paper 
part  of  the  time  and  able  to  assume  its  equilibrium  position  Without 
being  affected  by  friction  on  the  chart. 

In  the  potentiometer  recorder,  the  electromotive  force  of  the  couple 
is  opposed  to  the  potential  drop  across  a  known  variable  resistance 
through  which  a  definite  current  is  flowing. 


LEO  BEHK 


403 


The  simple  potentiometer  circuit  is  shown  in  Fig.  2.  The  current 
in  the  circuit  is  adjusted  to  a  predetermined  value  by  means  of  the  bat- 
tery regulating  rheostat  R  until  the  voltage  drop  across  the  resistance  S  is 
equal  to  the  electromotive  force  of  the  standard  cell.  The  electro- 
motive force  of  the  couple  C  is  measured  by  opposing  it  to  the  potential 
drop  along  a  length  of  the  slide  wire.  A  balance  is  reached  when  the 
galvanometer  stands  at  zero,  at  which  time  no  current  is  drawn  from  the 
thermocouple  circuit  and  therefore  a  change  in  resistance  produces  no 
error.  Before  balance,  the  extent  and  direction  of  unbalance  is  indicated 
by  the  galvanometer  deflection. 


FIG.  3. — SPLIT-CIRCUIT  POTENTIOMETER. 

The  change  in  temperature  at  the  cold  junction  can  be  compensated 
for  by  the  use  of  a  "split  circuit"  potentiometer.  In  Fig.  3,  T,  U,  and 
M  are  fixed  resistances  of  manganin,  S  is  a  slide  wire,  and  N  is  a  nickel 
coil  the  resistance  of  which  is  a  function  of  the  temperature.  As  before, 
a  standard  current  is  obtained  by  adjusting  the  regulating  rheostat  until 
the  drop  across  M  is  equal  to  the  electromotive  force  of  the  standard 
cell.  The  potential  difference  opposed  to  the  couple  C  is  that  from  P  to 
the  brush  B  on  slide  wire  S.  For  a  fixed  temperature  at  the  hot  end, 
any  changes  in  the  cold-end  temperature  will  be  accompanied  by  a  change 
in  the  resistance  of  N.  This  in  effect  moves  P  electrically  an  amount  just 
sufficient  to  compensate  for  the  change  in  the  electromotive  force  so  that 
the  position  of  B,  and  therefore  the  temperature  reading,  is  independent 


404 


RECORDING  THERMOCOUPLE  PYROMETERS 


of  the  cold-junction  temperature.     This  compensation  for  cold-junction 
temperature  is  correct  for  all  values  of  hot-junction  temperature. 

The  problem  in  constructing  a  recording  potentiometer  lay  in  devising 
a  mechanism  capable  of  rotating  a  slide  wire  in  a  definite  manner  accord- 
ing to  the  indications  of  a  galvanometer.  Such  instruments  have  been 
constructed  in  which  the  galvanometer  pointer,  when  it  deflected,  closed 
a  circuit,  which  started  a  brush  moving  over  the  slide  wire.  This  con- 
struction has  always  given  trouble  due  to  overshooting  as  the  speed 
of  the  brush  is  not  dependent  on  the  amount  of  unbalance  and,  moreover, 
the  small  force  available  for  producing  a  contact  with  the  galvanometer 


FIG.  4. — MECHANISM  UNBALANCED.  FIG.  5. — MECHANISM  BALANCED. 

BALANCING  MECHANISM  OP  POTENTIOMETER  RECORDER. 

pointer  does  not  always  insure  the  necessary  electrical  circuit.  The 
mechanism  finally  devised  to  solve  this  problem  is  rather  interesting 
and  its  general  features  will  be  described. 

As  shown  in  Figs.  4  and  5,  the  position  of  the  galvanometer  indicates 
in  which  direction  and  to  what  extent  the  potentiometer  is  out  of  balance. 
When  the  measuring  circuit  is  balanced,  the  pointer  is,  as  in  Fig.  5, 
directly  under  the  space  between  the  two  right-angle  levers  4L  and  J^R 
which  are  pivoted  at  24E.  When  the  potentiometer  circuit  is  out  of 
balance,  the  galvanometer  pointer  will  move  under  the  horizontal 
arm  of  4L  or  4R-  The  rocker-arm  5  is  pivoted  at  24B  and  is  periodic- 
ally raised  by  the  cam  6B  on  the  motor-driven  shaft  6.  As  the  rocker- 
arm  moves  upward,  it  lifts  the  galvanometer  pointer.  When  the  measuring 


DISCUSSION  405 

circuit  is  balanced  the  pointer  will  merely  be  raised  into  the  space  between 
4L  and  4R-  If,  however,  the  measuring  circuit  is  unbalanced  and  the  gal- 
vanometer is  under  4L,  for  instance,  the  resultant  position  of  the  parts 
is  as  shown  in  Fig.  4.  As  the  galvanometer  pointer  is  now  raised  by 
the  rocker-arm  it  rotates  J^L  about  the  pivot  24-E.  This  motion  is  trans- 
mitted to  the  horizontal  lever  2,  which  is  rotated  in  a  counter-clockwise 
direction  by  the  action  of  the  lower  extremity  of  4L  on  the  pin  2C.  At 
the  completion  of  this  motion,  the  lever  2  is  forced  into  contact  with  disk 
1  through  the  action  of  the  cam  6C.  The  cam  6E  is  attached  to  the 
shaft  6  and  rotates  with  it.  The  shape  of  the  cams  is  such  that  they 
return  the  lever  2  to  its  horizontal  position.  In  this  particular  case 
the  lever  2  and  the  disk  1  with  which  the  former  is  now  engaged  must 
both  rotate  in  a  clockwise  direction.  The  power  required  for  this  rota- 
tion is  obtained  from  a  motor,  therefore  sufficient  can  be  supplied  to 
balance  the  measuring  circuit,  move  the  recording  pen,  and  operate 
signals  or  relays. 

The  amount  of  rotation  per  cycle  given  to  arm  2  and  disk  1  depends 
on  the  galvanometer  deflection,  for  as  the  pointer  moves  out  from  its 
zero  position  it  approaches  the  fulcrum  of  4L.  Consequently  the  rebalanc- 
ing is  large  or  small  according  as  the  unbalance  is  large  or  small. 

As  there  is  no  current  flowing  in  the  thermocouple  circuit,  variations 
in  resistance  there  have  no  effect  on  the  accuracy  of  the  meter.  As  the 
galvanometer  is  used  as  a  null  instrument,  change  in  the  strength  of  the 
springs  or  magnet  have  no  effect  on  its  accuracy.  The  galvanometer 
is  a  suspended  instrument;  this  construction  is  permissible  because 
the  only  requirement  imposed  on  the  galvanometer  is  that  it  has  a 
reasonably  stable  zero.  There  is  consequently  no  friction  trouble, 
such  as  is  found  in  pivot  and  jewel  instruments,  and  the  galvanometer 
is  much  more  sensitive. 

DISCUSSION 

R.  W.  NEWCOMB,  New  York,  N.  Y.  (written  discussion*). — In  this 
paper,  the  author  has  enumerated  the  various  sources  of  error  that  may, 
under  certain  conditions,  develop  in  instruments  of  the  direct-deflection 
type,  but  through  a  failure  to  mention  possible  sources  of  error  in  instru- 
ments of  the  potentiometer  type,  he  leaves  the  reader  with  a  wrong  im- 
pression. If  he  would  carry  his  criticisms  further  and  give  a  list  of 
the  possible  sources  of  error  on  potentiometer  instruments,  the  paper 
would  be  more  complete. 

*  Received  Oct.  15,  1919. 


406  RECORDING    PYROMETRY 


Recording  Pyrometry 

BY   C.    O.    FAIRCHILD,*  B.    S.,    AND   PAUL   D.    FOOTE,f    PH.    D.,    WASHINGTON,    D.    C. 
(Chicago  Meeting,  September,  1919) 

ONE  of  the  fundamental  principles  of  efficiency  is  the  use  of  adequate 
and  permanent  records.  The  rapid  increase  in  the  manufacture  and  use 
of  recording  pyrometers  is  a  proof  of  the  appreciation  of  efficiency  prin- 
ciples on  the  part  of  the  manufacturers  engaged  in  the  various  technical 
industries.  Where  recording  pyrometers  are  not  employed  in  an  industry 
in  which  temperature  measurements  are  necessary,  one  will  generally 
find  that  a  printed  form  is  used  upon  which  is  written,  periodically, 
the  temperatures  measured  by  an  indicating  instrument.  It  is  no  serious 
condemnation  of  the  workman  to  state  that  such  records  are  often 
"doctored, "  but  it  is  rather  a  reflection  upon  the  executive  who  puts  such 
temptation  in  the  way  of  a  workman.  One  of  the  well-known  "tricks 
of  the  trade"  is  to  "force"  a  furnace  so  that  it  will  be  at  the  proper  tem- 
perature during  the  periodic  trips  of  the  foreman  or  other  official. 

Indicating  instruments  and  recorders  may  be  used  together  to  great 
advantage.  The  recorder  furnishes  a  printed  record  and  a  check  upon 
the  operator  of  the  furnace,  and  a  record  of  value  in  correlating  properties 
of  the  finished  product  with  the  heat  treatment.  The  indicator  should 
be  of  assistance  to  the  operator  in  controlling  the  furnace  or  oven. 

Modern  practice  requires  a  temperature-recording  instrument  that 
is  as  simple  as  possible,  rugged,  reliable,  and  sufficiently  accurate.  Of 
these  qualifications  reliability  is  paramount,  particularly  in  cases  where 
the  recorder  is  used  for  controlling  the  temperature  automatically.  By 
a  reliable  instrument  is  meant  one  that  will  run  continuously  with  little 
attention  and  with  a  consistent  degree  of  accuracy.  That  is,  if  it  is  in 
error  by  5  per  cent,  one  day,  it  must  be  in  error  by  a  like  amount  on  any 
other  day,  and  not  by  2  per  cent,  or  10  per  cent.  The  earlier  forms  of 
recorders  were  complex  and  delicate,  or  mechanically  unsatisfactory  and 
inaccurate,  and  required  considerable  attention  to  keep  them  in  opera- 
tion. Within  the  past  few  years,  however,  the  development  in  the  manu- 
facture of  temperature-recording  devices  has  been  highly  satisfactory 
and  many  excellent  instruments  are  now  available. 

The  pyrometers  that  can  be  made  to  record  automatically  fall  under 


*  Associate  Physicist,  U.  S.  Bureau  of  Standards, 
t  Physicist,  U.  S.  Bureau  of  Standards. 


C.    O.    FAIRCHILD    AND    PAUL    D.    FOOTE  407 

the  following  classifications:  (1)  Gas,  saturated  vapor,  and  liquid 
thermometers;  (2)  resistance  thermometers;  (3)  thermoelectric  pyrom- 
eters; (4)  radiation  pyrometers. 

Of  these  four  types,  the  thermoelectric  pyrometer  recorder  has  the 
greatest  applicability,  especially  for  the  higher  temperatures  at  which 
the  first  two  named  are  not  suitable.  The  constant-volume,  industrial, 
gas  thermometer  is  successful  up  to  about  400°  C.  The  resistance  ther- 
mometer is  capable  of  very  high  accuracy  up  to  1000°  C.  At  such  high 
temperatures,  however,  thermocouples  are  more  serviceable  since  deterio- 
ration of  the  wire  from  continual  heating  does  not  so  seriously  alter  the 
electromotive  force  developed  by  a  couple  as  it  does  the  resistance  of  a 
resistance  thermometer.  Base-metal  couples  serve  satisfactorily  up 
to  1100°  C.;  and  platinum  platinum-rhodium  couples  up  to  1500°  C., 
although  above  1400°  C.  it  becomes  very  difficult  to  protect  the  couple 
from  contamination  by  the  furnace  gases  and  vapors.  Radiation 
pyrometers  are  useful  at  the  highest  attainable  temperatures,  but 
processes  in  which  temperatures  greater  than  1600°  C.  are  used  are  not  in 
general  subjected  to  very  precise  temperature  control. 

FORMS  OF  TEMPERATURE  RECORDS 

The  most  usual  form  of  temperature  record  is  that  in  which  tempera- 
ture appears  as  one  coordinate  and  time  as  the  other  coordinate.  The 
temperature-time  curve  drawn  on  such  a  record  has  been  called  an  auto- 
graphic record.  This  type  of  curve  is  the  most  easily  obtained  mechanic- 
ally and  is  valuable  a.s  a  continuous  record  of  the  temperature  of  a 
furnace  over  a  prolonged  run.  It  is  also  occasionally  used  to  detect 
transformation  points  in  steel,  which  appear  as  flexures  or  indentations 
on  the  plot  when  the  furnace  containing  the  sample  is  uniformly  heated 
or  cooled.  For  this  latter  work,  however,  the  "differential  temperature" 
curve  recorder  is  especially  adapted  and  will  be  described  later.  Other 
types  of  curves  obtained  with  special  recorders,  and  used  mainly  for 
laboratory  work,  are  the  temperature-rate  curve,  the  inverse-rate  curve, 
and  the  derived  differential  curve.  The  various  special  methods  have 
been  discussed  by  Burgess1  and  will  not  be  considered  in  the  present 
paper. 

GENERAL  TYPES  OF  THERMOCOUPLE  RECORDERS   FOR  TEMPERATURE- 
TIME  CURVES 

A  recorder  for  obtaining  a  temperature-time  curve  consists  essentially 
of  an  electrical  measuring  instrument  with  a  mechanism  for  periodically 


U.  S.  Bureau  of  Standards  Sci.  Paper  99. 


408  RECORDING    PYROMETRY 

recording  its  indications  upon  a  chart  that  moves  with  a  uniform  speed. 
As  in  the  case  of  simple  indicators,  there  are  two  general  types  of  recorders, 
one  operating  on  the  galvanometric  principle  and  one  operating  on  the 
potentiometric  principle."  Also,  as  in  the  case  of  indicating  instruments, 
the  potentiometric  principle  while  somewhat  more  complicated  has 
the  especially  desirable  feature  that  its  indications  are  independent  of 
the  thermocouple  resistance.  The  readings  of  a  recorder  operating  on 
the  galvanometric  principle  depend  on  the  variations  in  the  resistance  of 
the  external  electric  circuit,  although  the  effect  of  these  variations  can 
be  reduced  by  using  an  instrument  of  high  internal  resistance  or,  with  less 
satisfaction,  by  keeping  the  resistance  of  the  external  circuit  very  low. 
In  many  of  the  recorders  now  obtainable  the  resistance  is  sufficiently 
high  so  that  these  effects  become  of  little  practical  importance.2 

RECORD  CHARTS 

There  are  three  types  of  record  paper  in  general  use,  the  roll  charts 
and  drum  charts  shown  in  Fig.  1,  and  the  disk  or  circular  charts,  shown 
in  Fig.  2.  The  roll  chart  may  contain  enough  paper  to  last  a  month  or 
more  while  usually  the  drum  or  circular  charts  are  renewed  every  24  hr. 
For  single-point  recorders,  all  of  these  forms  are  employed ;  but,  with 
one  or  two  exceptions,  multiple-point  recorders  use  record  paper  in  the 
roll  form. 

Upon  circular  charts,  the  lines  of  equal  temperature  (time  coordinates) 
are  represented  by  concentric  circles,  and  lines  of  equal  time  (temperature 
coordinates)  by  arcs  following  the  course  of  the  galvanometer  pointer. 
The  distorted  appearance  of  such  a  record  is  at  first  somewhat  confusing 
but,  with  practice,  the  record  is  easily  read  even  when  the  complete  curve 
is  allowed  to  extend  around  the  chart  for  several  revolutions.  If  an 
extended  and  open  temperature  scale  is  required,  the  disk  record  be- 
comes somewhat  unsatisfactory  on  account  of  its  size,  since  the  diameter 
of  the  circular  sheet  must  be  more  than  twice  the  width  of  the  temperature 
scale.  The  necessity  of  opening  the  recorder  case,  usually  every  24  hr., 
to  mount  a  new  record  sheet  means  greater  liability  for  an  accumulation 
of  dust  which,  in  the  cheaper  forms  of  recorders,  becomes  a  really  serious 
factor. 

In  filing  records  for  future  reference,  the  circular  and  drum  type  one- 
day  records  offer  some  advantage  over  the  continuous  roll  records, 
although  the  latter  may  be  cut  to  any  convenient  length  when  removed 
from  the  recorder.  With  the  drum  and  roll  records,  the  coordinates  can 
be  made  rectangular.  In  some  cases,  however,  the  temperature  coordi- 


1  The  requirement  of  high  resistance  is  discussed  by  Foote,  Harrison  and  Fairchild : 
Met.  &  Chem.  Engng.  (1918)  18,  404. 


C.  O.  FAIRCHILD  AND  PAUL  D.  FOOTE 


409 


nates  are  parts  of  a  circle,  as  shown  in  Fig.  1  where  the  radius  of  the 
circle  is  the  length  of  the  galvanometer  needle.     The  roll  record  is 


BffT 

flu 


63C 


1800 


3000 


FIG-.  1. — ROLL  OR  DRUM  CHART. 


particalarly  adapted  to  multiple-point  recording  and  for  adjusting  to 
very  slow  or  very  rapid  record  speeds. 

The  methods  by  which  the  printing  of  the  record  is  accomplished 
will  be  described  in  the  discussion  of  the  various  instruments.     The  record 


FIG.  2. — DISK  CHART. 

may  be  obtained  by  pen  and  ink,  stylus  and  inked  ribbon,  inked  thread, 
carbon  paper  or  coated  paper;  by  puncturing  the  record  paper  by  means 


410  RECORDING   PYROMETRY 

of  an  electric  spark;  or  by  stamping  upon  the  record  sheet  some  imprint 
from  a  stencil.  The  difficulty  in  making  several  distinct  records 
upon  the  same  chart,  required  by  a  multiple-point  recorder,  has 
been  met  in  various  ways,  but  there  is  still  opportunity  for  development 
and  improvement  in  this  regard.  The  advantage  of  a  multiple-point 
recorder  for  extensive  installations  is  obvious  in  that,  for  a  small  additional 
cost,  the  instrument  will  do  the  work  of  several  single-point  recorders. 

THERMOCOUPLE  RECORDERS  OPERATING  ON  THE  GALVANOMETRIC 

PRINCIPLE 

The  recording  millivoltmeter,  or  galvanometer,  may  employ  the  same 
galvanometer  system  used  in  the  portable  indicating  instruments.  A 
more  rugged  instrument  is  desired,  however;  this  ruggedness  is  obtained 
by  increasing  the  strength  of  the  springs,  boom,  coil,  and  jewel  bearings 
of  a  pivot  instrument,  etc.  To  compensate  for  the  resulting  decrease  in 
sensitivity,  the  density  of  magnetic  flux  through  the  coil  may  be  increased, 
the  number  of  turns  of  wire  on  the  moving  coil  may  be  increased,  and  the 
so-called  swamping  resistance  in  series  with  the  moving  coil  may  be  de- 
creased. Most  indicators  have  a  single  magnet  while  recorders  have  as 
many  as  four  or  six  magnets.  A 'portable  instrument  would  become  too 
heavy  with  so  many  magnets.  In  general,  the  resistance  of  a  recorder 
is  less  than  that  of  an  indicator  of  the  same  type.  On  account  of  the  very 
small  electromotive  forces  developed  by  thermocouples  and  on  account 
of  the  necessity  of  using  a  comparatively  high  resistance  in  series  with 
the  galvanometer  coil  to  minimize  the  effect  of  a  varying  resistance,  the 
torque  that  can  be  produced  on  the  moving  coil  is  small.  Pivots,  sus- 
pensions, etc.  must  be  very  carefully  made  and  the  moving  coil  must  be 
accurately  balanced  and  so  mounted  that  it  swings  perfectly  free. 

The  galvanometer  pointer  cannot  be  used  to'  trace  a  legible  record 
directly  since  the  friction  between  the  paper  and  the  pointer  would  entirely 
alter  the  readings.  One  common  method  for  obtaining  the  record  is 
illustrated  by  the  Siemens  and  Halske  recorder,  shown  in  Fig.  3.  The 
paper  is  unwound  by  clockwork  at  a  uniform  speed  from  a  roll.  An 
inked  ribbon  lies  below  the  paper  and  above  a  metal  plate.  At  periodic 
intervals  the  chopper  bar  B  falls  pressing  a  stylus  on  the  end  of  the  gal- 
vanometer boom  N  into  contact  with  the  paper  and  against  the  ribbon 
and  metal  plate  underneath.  This  makes  a  small  dot  on  the  under  side 
of  the  thin  record  paper,  which  shows  through  from  the  top  as  illustrated. 
The  paper  is  ruled  with  the  proper  time  and  temperature  coordinates  and 
the  row  of  dots  obtained  by  continuous  operation  constitutes  the  required 
temperature-time  curve.  The  maximum  frequency  with  which  the  dots 
can  be  recorded  depends  on  the  natural  period  of  vibration  of  the  mov- 
ing coil.  In  general  practice,  the  dots  appear  at  intervals  of  10  to 
30  seconds. 


C.    O.    FAIRCIIILD    AND    PAUL   D.    FOOTE 


411 


This  principle  is  employed  in  many  instruments  of  American  make. 
The  chopper  bar  may  be  operated  by  an  electric  motor,  clockwork, 
or  electromagnet,  and  the  design  must  be  such  that  the  galvanometer 
boom  swings  clear  of  the  bar,  between  the  intervals  of  depression,  and  such 
that  the  depression  of  the  bar  against  the  boom  in  no  way  damages  the 
coil  mounting.  These  conditions  may  be  satisfied  in  pivot,  suspension, 
and  combination,  pivot-suspension  systems. 

Recorders  may  be  classified  according  to  the  type  of  support  em- 
ployed in  mounting  the  moving  coil.  The  double-pivot  support  is  the 
most  common  and  was  developed  for  its  greater  ruggedness  and  the 
constancy  of  sensitivity  obtained.  The  outstanding  fault  of  the  double- 
pivot  instruments  is  the  tendency  to  stick  and  the  failure  of  the  manu- 


FIG.  3. — PRINCIPLE  OP  AUTOGRAPHIC  RECORDER. 

facturers  to  make  them  sufficiently  dust-proof.  An  indicator  of  the 
double-pivot  type  may  have  its  moving  coil  so  mounted  that  the  pivots 
do  not  center  in  the  jewel  bearings  but  roll  around  in  a  small  arc  over  the 
cup-shaped  surface  of  the  bearings.  This  reduces  the  friction  consider- 
ably, but  the  method  is  not  applicable  for  recorders,  in  which  the  pointer 
is  periodically  struck  by  a  bar  with  sufficient  force  to  move  the  pivots  in 
their  bearings.  Unipivot  galvanometers  are  not  used  for  recorders  be- 
cause the  moving  coil  is  free  to  swing  in  a  plane  at  right  angles  to  the 
scale  and  the  action  of  the  chopper  bar  would  be  to  keep  the  coil  in  con- 
tinual vibration. 

Unipivot-suspension  instruments  are  not  common  and  are  not  widely 
known.  The  suspension  can  be  made  very  long  and  hence  comparatively 
heavy  and  strong.  By  crimping  the  suspension  or  by  supporting  it  on 
a  spring,  a  definite  fraction  of  the  weight  of  the  moving  coil  may  be  taken 


412  RECORDING    PYROMETRY 

from  the  pivot.  The  coil  requires  clamping  during  shipment  and  must 
be  mounted  in  such  a  way  that  it  will  always  seat  properly,  if  jarred  out 
of  the  bearing.  There  is,  of  course,  very  little  danger  of  the  suspension 
breaking  and  this  type  of  construction  can  be  followed  in  making  an  in- 
strument of  good  constancy  and  sensitivity  at  relatively  low  cost. 

All  the  advantages  of  the  foregoing  type  of  galvanometer  construction 
are  found  in  the  double  suspension,  and  with  the  latter  a  still  greater 
sensitivity  and  constancy  are  possible.  The  coil  is  mounted  so  that 
jarring  can  move  it  a  very  small  distance  and  the  spring  supports  of  the 
suspensions  are  so  made  as  to  allow  this  amount  of  motion  without  undue 
tension.  The  coil  is  usually  clamped  when  the  instrument  is  not  in  use. 
This  construction  is  somewhat  better  in  case  the  recorder  is  unavoidably 
subject  to  jar,  the  characteristic  vibration  of  the  moving  part  being  of 
shorter  period  with  greater  damping.  It  is  still  necessary  to  keep  the 
recorder  quite  free  from  vibration,  a  precaution  that  should  always  be 
taken. 

Recorders  may  also  be  classified  according  to  their  motive  power.  For 
most  precise  movement  of  the  chart,  a  clock  must  be  used  either  alone 
or  in  conjunction  with  an  electromagnet,  which  may  be  applied  to  do 
most  of  the  work.  The  clock  alone  must  be  large,  and  have  plenty  of 
power,  particularly  for  multiple-point  recorders.  The  commutators  of 
multiple-point  recorders  must  operate  with  considerable  friction;  this  is 
best  overcome  by  means  of  an  electromagnet  operated  by  the  clock. 
The  magnet  is  also  used  in  some  instruments  to  operate  the  chopper  bar. 
By  having  the  magnet  lift  a  weight  with  each  stroke,  it  can  be  used  to 
furnish  power  for  driving  the  chart;  this  is  done  in  one  of  the  instruments 
described  below.  A  motor  may  be  used  for  the  driving  power,  and  by 
attaching  a  governor,  very  satisfactory  control  of  the  chart  motion  is 
obtained.  The  motor  requires  somewhat  more  attention  than  a  clock, 
but  in  some  recorders  is  indispensable. 

INDUSTRIAL  TYPES  OF  RECORDERS 

Foreign  designs  will  not  be  described,  for  the  writers  believe  that  no 
foreign-made  recorders  can  compare  favorably  with  the  American  makes 
in  simplicity  of  construction,  ruggedness,  and  general  applicability  for 
industrial  use.  The  American  manufacturers  first  in  the  field  supplied 
a  need  by  making  a  fairly  cheap  form  of  single-point  recorder,  simply 
made,  and  sufficiently  accurate  to  be  well  worth  its  use.  This  was  done 
at  a  time  when  the  imported  instruments  were  expensive,  complicated, 
and  only  accurate  when  cared  for  by  an  expert. 

THE  CIRCULAR-CHART  SINGLE-POINT  RECORDER 

The  circular-chart,  single-point  recorders  are  made  by  Beighlee 
Electric  Co.,  the  Bristol  Co.,  the  Brown  Instrument  Co.,  and  the  Hoskins 


C.   O.    FAIRCHILD    AND    PAUL   D.    FOOTE  413 


FIG.  4. — BEIGHLEE  RECORDER. 


FIG.  5. — BRISTOL  RECORDER. 


414 


RECORDING    PYROMETRY 


Mfg.  Co.  Figs.  4  to  6  illustrate  the  various  forms  of  this  type  of  instru- 
ment. With  the  exception  of  the  Beighlee  recorder,  Fig.  4,  this  type  is 
not  made  for  multiple  records.  Attention  might  be  called  to  the  fact 
that  the  time  coordinates  of  the  circular-chart  recorders  are  relatively 
short  and  thus  these  charts  are  unsuited  to  the  measurement  of  rapidly 
changing  temperatures.  They  are  best  suited  for  furnace  or  oven  opera- 
tion where  a  fixed  temperature  is  maintained  or  desired,  or  a  slow  rise 
or  fall,  as  met  with  in  the  ceramic  industry  and  in  annealing  ovens. 

Fig.  6  shows  the  simple  construction  of  this  recorder.  The  milli- 
voltmeter  is  mounted  on  the  door  so  that  charts  may  be  renewed  without 
danger  of  injuring  the  milli voltmeter  pointer.  The  presser  frame  is  also 


FIG.  6. — BROWN  RECORDER. 

mounted  on  the  door  and  is  operated  by  the  reciprocating  arm  shown  at 
the  upper  left-hand  side  of  the  chart.  This  arm  is  slowly  drawn  back  by 
the  clock  (hidden  by  chart)  against  the  tension  of  a  spring.  Its  release 
throws  the  presser  frame  against  the  pointer  and  a  dot  is  registered  on 
the  chart.  The  pointer  is  made  flexible;  that  of  the  Bristol  instrument 
has  a  flattened  part,  which  results  in  a  slight  rubbing  action  on  the  chart, 
necessary  in  the  use  of  smoked  paper.  In  Fig.  6  is  seen  a  metal  guide 
extending  across  the  paper  just  below  the  point  where  the  record  is  made. 
A  heavy  paper  having  a  slot  along  this  guide  is  placed  underneath  the 
thin  paper  chart  and  over  the  carbon  paper  so  that  in  changing  records 
the  paper  is  not  easily  soiled.  The  small  spool  seen  in  the  upper  right- 
hand  corner  is  a  resistance  in  series  with  the  moving  coil  of  the  meter. 
One  of  its  purposes  is  for  calibrating  the  instrument  or  changing  the 


C.    O.    FAIRCHILD    AND    PAUL   D.    FOOTE  415 

temperature  range,  e.g.,  to  change  a  range  0  to  1000°  to  0  to  1500°  the 
total  resistance  of  the  instrument  would  be  increased  by  approxi- 
mately 50  per  cent.  This  should  not  be  done  with  low  resistance  instru- 
ments without  also  correcting  the  lead  resistance;  i.e.  the  correction  for 
range  should  apply  to  the  entire  electric  circuit. 

The  Hoskins  millivoltmeter  is  unique  in  the  fact  that  the  meter  is 
behind  the  record  and  the  glass  window  closes  flat  against  the  chart, 
sustaining  the  pressure  of  the  pointer  when  the  latter  is  raised  by  the 
pressure  bar.  This  construction  gives  greater  convenience  in  changing 
records. 

The  Beighlee  recorder  is  much  less  simple  in  construction  than  the 
foregoing  types.  Its  distinctive  features  are:  a  somewhat  more  open 
scale  (larger  chart),  a  unipivot-suspension  meter,  and  a  unique  method 
of  making  the  record.  The  pointer  is  not  periodically  depressed  but 
swings  free  with  its  tip  close  to  the  record.  Every  30  sec.  an  electric 
spark  passes  from  the  pointer  to  the  chart  plate  puncturing  the  paper. 
The  record  is  a  series  of  holes  with  seared  edges,  which  are  easily  seen. 
There  is  a  tendency  of  the  spark  to  jump  the  gap  at  an  angle,  causing  a 
slight  error,  which,  however,  is  not  serious.  This  instrument  is  not  de- 
signed particularly  for  installation  near  the  furnace,  but  in  an  office  or 
central  pyrometer  station.  Its  high  resistance  makes  it  possible  to 
use  very  long  leads  of  small  wire.  The  clock  is  electrically  wound  and 
requires  no  attention  except  regulation.  To  accomplish  multiple  record- 
ing with  this  instrument,  the  chart  is  divided  into  sectors,  and  if  six 
records  are  to  be  made,  the  chart  moves  forward  slightly  more  than  a 
sixth  of  a  revolution  each  %  min.  The  time  coordinates  are  very  short 
and  the  instrument  is  adapted  in  this  form  only  to  large  furnaces  with 
constant  or  slowly  changing  temperatures.  It  should  be  used  combined 
with  an  indicator  for  controlling  temperatures. 

The  only  drum-chart  recorder  is  made  by  the  Stupakoff  laboratories. 
It  is  the  double-pivot  type  with  a  high  resistance  and  ordinary  scale  length. 
The  drum  is  rotated  once  in  a  day  or  a  week.  This  company  also  makes 
a  single-point  recorder  of  the  roll-chart  type.  The  galvanometer  is  the 
double-suspension  type  with  300  ohms  resistance  for  a  15-millivolt 
range.  In  the  manufacture  of  these  instruments  an  effort  is  made  to 
get  the  greatest  accuracy  consistent  with  the  length  of  scale,  and  not 
to  furnish  an  instrument  that  will  stand  hard  knocks.  These  instru- 
ments should  be  mounted  under  cover  in  a  place  free  from  vibration. 

ROLL-CHART  RECORDERS 

Roll-chart  recorders  are  made  by  The  Brown  Instrument  Co.,  Charles 
Engelhard,  Hoskins  Mfg.  Co.,  S.  H.  Stupakoff,  Taylor  Instrument  Co., 
Thwing  Instrument  Co.,  Wilson-Maeulen  Co.  The  general  advantages 


416 


RECORDING    PYROMETRY 


of  the  roll  chart  are  legibility  (coordinates  are  parallel  and  may  be  made 
rectangular),  width  of  scale,  and  adaptability  to  rapidly  changing  tem- 
peratures, and  the  recording  of  more  than  one  record  on  a  single  chart. 
With  a  paper  speed  of  1  in.  an  hr.  a  20-yd.  roll  will  last  1  mo.  Rolls 
are  usually  furnished  in  20-yd.  lengths  and  longer. 

Fig.  7  illustrates  the  Brown  continuous  recording  pyrometer.  The 
fundamental  principles  of  its  operation  are  the  same  as  those  of  the 
circular-chart  recorders.  The  scale  is  nearly  twice  the  width  of  that  on 
the  circular  chart.  An  inked  ribbon  is  extended  across  the  paper  under 
the  meter  pointer  and  is  slowly  changed  by  the  clock.  The  scale  seen 
above  the  chart  is  mounted  on  the  presser  bar. 


FIG.  7. — BROWN   ROLL-CHART   RECORDER. 

The  instrument  is  not  made  to  record  for  more  than  one  thermocouple. 
The  Brown  multiple  recording  pyrometer  is  a  multiple-galvanometer 
double-scale  instrument.  For  taking  two  to  eight  records,  two  millivolt- 
meters  are  mounted  side  by  side  and  two  temperature  scales  are  printed 
on  one  width  of  paper.  This  results  in  a  very  narrow  chart,  limiting  the 
usefulness  of  the  instrument  and  the  accuracy  with  which  the  record  can 
be  read.  It  differs  from  the  single-record  instrument  in  having  a  commu- 
tating  switch  and  a  multicolored  ribbon,  both  of  which  are  operated  by 
the  clock.  The  latter  instrument  is  sometimes  constructed  with  a  galva- 
nometer having  a  double  coil  on  the  one  moving  system,  and  is  applied  for 
detecting  transformation  points  in  steels.  This  is  discussed  more  in 
detail  in  the  section  on  transformation-point  indicators. 


C.    O.    FAIRCHILD    AND    PAUL    D.    FOOTE 


417 


Charles  Engelhard  has  heretofore  handled  imported  instruments,  but 
has  recently  developed  a  recorder  of  the  high-resistance  double-suspen- 
sion type.  The  chart  has  rectangular  coordinates,  and  a  graduated 
width  of  4^  in.  (11.4  cm.).  The  resistance  is  upward  of  700  ohms  for 
rare-metal  thermocouples. 

The  Hoskins  multiple  recording  pyrometer,  which  has  been  recently 
developed,  is  shown  with  cover  and  chart  removed  in  Fig.  8.  It  is  made 
for  ten  records  of  base-metal  thermocouples.  The  scale  is  over  7  in. 
(17  cm.)  wide,  the  highest  range  instrument  reading  to  2500°  F.  (1371  °  C.) . 
The  essential  features  are  an  electric  drive  (C  is  a  solenoid  arm)  and  a 
clock  operating  only  the  electric  contact  device  D.  The  motion  of  the 


FIG.  8. — HOSKINS  RECORDER. 

solenoid  arm  raises  the  frame  I  (which  is  depressed  in  other  instruments), 
changes  the  commutator  or  rotary  selective  switch  H,  and  moves  the 
chart  a  step  forward  every  third  contact.  Records  are  made  every  20 
sec.,  the  change  from  one  couple  to  the  next  occurring  every  minute. 
Instead  of  using  different  colors  or  symbols  for  different  couples,  the 
chart  is  divided  by  parallel  lines,  between  any  two  of  which  only  one 
couple  records.  The  spaces  are  numbered  consecutively  and  the  records 
are  distinguished  by  referring  to  the  parts  of  the  chart  in  which  they  fall. 
This  is  more  clearly  seen  by  pointing  out  that  the  selective  switch  is 
fastened  directly  on  the  shaft  G,  which  holds  the  toothed  wheels  engaging 
perforations  in  the  paper  chart.  A  plug  switchboard  LK  mounted 

27 


418 


RECORDING   PYROMETRY 


within  the  case  is  so  made  that  it  is  possible  to  arrange  the  couples  in 
any  order,  or  to  put  any  couple  on  more  than  one  number.  The  resist- 
ance of  the  milli voltmeter  is  150  to  300  ohms,  and  it  is  intended  to 
operate  with  a  fixed  lead  resistance  adjusted  to  50  ohms.  The  high 
external  resistance  is  selected  to  take  care  of  compensating  leads,  the 
resistance  of  which  is  about  0.13  ohm  per  foot.  Calibration  can,  of 
course,  be  made  for  any  external  resistance  met  with  in  practice. 

Fig.  9  illustrates  the  thread  recorder  for  single  records,  made  by  the 
Taylor  Instrument  Co.     Using  an  inked  thread  gives  rectangular  coor- 


FIG.  9. — THREAD  RECORDER. 

dinates.  The  illustration  shows  the  ease  with  which  replacements  are 
made  over  the  five  small  pulleys,  one  of  which  keeps  the  thread  taut. 
The  large  clock  has  two  parts,  one  of  which  moves  the  chart  while  the 
other  lifts  the  depressor  frame.  The  depressor  frame  or  bar  is  lifted  by 
the  clock  through  a  pawl,  which  engages  a  slowly  turning  ratchet.  The 
pawl  is  tripped  out  of  engagement,  allowing  the  bar  to  fall  pressing  the 
pointer  against  the  thread  and  record  paper.  The  small  resistance 
plug  is  a  range  control  series  resistance,  which  allows  the  employment  of 
two  sensitivities. 

Figs.  10,  11,  and  12  are  three  views  of  the  multipyrograph,  a  thread 
recorder  for  multiple  records.     The  mechanism  is  built  on  a  heavy  metal 


C.    O.    FAIRCHILD    AND    PAUL   D.    FOOTE 


419 


frame  that  slides  in  grooves,  so  that  the  entire  works  may  be  pulled  for- 
ward for  examination  and  cleaning,  etc.  The  principal  additions  to  the 
thread  recorder  are  an  electromagnet,  commutating  switch,  and  index 
dial  showing  numbers  through  the  small  opening  in  the  left-hand  box. 
The  commutating  switch  is  inclosed  in  the  rectangular  box  below  and  is 
connected  through  the  sprocket  chain,  which  is  turned  in  steps  by  the 
electromagnet.  The  switch  consists  of  a  row  of  long  flat  phosphor- 
bronze  springs  with  platinum-rhodium  contacts.  One  spring  at  a  time 
is  pressed  by  a  cam,  which  is  properly  synchronized  with  the  numbers  and 


FIG.  10. — MULTIPLE-POINT  THREAD  RECORDER. 

colors  of  the  threads.  A  wiping  action  is  given  the  contacts  by  an  offset 
arrangement,  assuring  good  contact.  This  principle  of  a  wiping,  or 
sliding,  action  of  contacts  is  applied  quite  universally,  though  only  in  the 
better  class  of  recorders  are  rare-metal  contacts  used.  A  high-resistance 
(500  ohms  and  more)  galvanometer.would  render  unnecessary  the  assured 
perfection  of  contact  which  is  afforded  by  a  wiping  action  and  platinum 
points. 

The  clock  of  this  recorder  does  not  require  winding,  but  only  provides 
an  escapement  for  the  gradual  release  of  power  supplied  by  the  electro- 


420 


RECORDING   PYROMETRY 


magnet.  The  solenoid  coil  raises  a  heavy  bar,  which  is  retained  in  the 
upper  position  by  a  pawl  catching  on  a  ratchet.  The  latter  is  allowed  to 
turn  at  a  uniform  rate  by  the  escapement.  The  motion  of  the  magnet 
turns  the  switch  cam  shaft  and  operates  the  depressor  arm.  The  chart  is 
run  through  the  power  supplied  to  the  ratchet  by  the  fall  of  the  lifted 
parts.  Electric  contact  is  made  every  %  min.,  the  interval  being  gov- 
erned by  the  position  of  the  pawl,  that  is  the  number  of  ratchet  teeth 
passed  during  the  rise  of  the  magnet. 

Three  inked  threads  of  different  colors  are  mounted  on  a  frame  as 
shown  in  Fig.  11,  in  which  the  chart  rolls  are  shown  lowered  into  the  posi- 


FIG.  11. — MULTIPLE-POINT  THREAD  RECORDER. 

tion  for  replacement.  The  threads  are  brought  alternately  into  positions 
governed  by  the  commutating  mechanism.  Six  records  are  obtained 
by  changing  the  order  of  records  in  such  a  way  that  three  records  appear 
as  a  line  of  uniformly  spaced  dots,  and  three  appear  as  lines  of  dots 
spaced  in  pairs.  The  proper  selection  of  numbers  will  usually  keep 
records  of  the  same  color  well  separated.  The  moving  coil  of  the 
millivoltmeter  is  given  additional  protection  from  dust  by  the  casing 
shown  in  Fig.  11. 

To  make  this  recorder  operate  on  one  thermocouple,  the  couple  is  con- 
nected to  the  six  terminals  in  parallel.     If  a  record  of  only  one  color  is 


C.    O.    FAIRCHILD   AND   PAUL   D.    FOOTE 


421 


desired  the  threads  may  be  replaced  by  three  of  like  color.  The  scale 
of  the  chart  is  4^  in.  (11.4  cm.)  wide  and  may  be  graduated  in  various 
ranges  of  degrees  Fahrenheit  and  centigrade. 

The  Thwing  recording  pyrometer  is  illustrated  in  Fig.  13,  which  shows 
a  type  making  six  records  with  two  galvanometers.  Records  are  distin- 
guished on  this  instrument  by  making  one  record  of  3-min.  dashes,  the 
second  of  1^-min.  dashes,  and  the  third  with  dots.  The  cycle  requires 
7  min.,  since  both  galvanometer  pointers  are  depressed  at  once.  After 
the  third  record,  the  thermocouple  circuit  is  opened  and  the  pointer  re- 
turns to  zero  where  it  is  pressed  upon  an  inking  pad.  This  method  of 


FIG.  12. — MULTIPLE-POINT  THREAD  RECORDER. 

making  easily  distinguished  records  is  simple  and  results  in  no  confusion. 
The  Thwing  recorders  are  also  made  with  three  or  four  galvanometers 
giving  nine  or  twelve  records.  The  width  of  (scale  decreases  with  the 
number  of  records,  and  for  twelve  records  is  but  2^  in.  (6.3  cm.)  wide. 
The  mechanism  of  the  instrument  is  very  simple  and  can  be  made  so 
even  with  considerable  friction  in  operation,  because  the  clock  is  very 
powerful,  and  is  wound  once  daily.  The  clock  does  all  the  work,  turning 
the  paper  roll,  depressing  the  needle,  and  closing  and  opening  the  thermo- 
couple circuits.  The  commutator  contacts  are  of  silver  and  should  give 
good  results  with  a  slight  wiping  action,  which  is  obtained.  This  recorder 
is  often  furnished  to  operate  on  combinations  of  thermocouples  and  one 
or  more  radiation  pyrometers,  which  is  frequently  very  convenient. 


422  RECORDING   PYROMETRY 

The  galvanometers  used  are  double-pivot  instruments  of  a  standard 
Thwing  type,  which  has  been  described  by  the  writers  in  a  previous 
article.  The  resistance  can  be  made  fairly  high,  but  varies  widely  with 
different  instruments,  remaining  high  enough  to  give  considerable  lati- 
tude in  length  of  leads  (copper)  used. 

The  Tapalog,  a  single-  or  multiple-point  recorder  made  by  the  Wilson- 
Maeulen  Co.,  is  illustrated  in  Figs.  14  and  15.     The  figures  illustrate  a 


FIG.  13. — THWING  RECORDER. 

four-record  instrument  which  is  the  most  common  form.  The  very 
heavy  and  strong  construction  of  this  instrument  is  easily  recognized 
in  the  illustrations.  The  case  is  heavy  enameled  cast  iron  with  %-in. 
plate-glass  windows.  The  works  are  mounted  on  an  iron  casting  sup- 
ported on  a  shelf.  There  is  no  need  of  an  extra  dust-proof  cabinet  and 
this  case  is  bolted  directly  to  a  wall. 

Rectangular  coordinates  on  the  chart  are  obtained  by  pressing  the 


C.    O.    FAIRCHILD    AND    PAUL    D.    FOOTE 


423 


galvanometer  pointer  against  a  nearly  sharp  straightedge  extending 
across  and  underneath  the  record  paper.  In  Fig.  15,  the  chart  carriage 
is  lowered  into  position  for  change  of  paper  and  inked  ribbon,  which  here 
have  been  removed.  The  sharp  straightedge  is  visible  midway  between 
the  sprocket  roller  W  and  the  sprocket  drum  D.  The  front  of  the  depres- 
sor frame,  sometimes  called  the  chopper  bar,  is  straight  and  in  line  with 


FIG.  14. — WILSON-MAETJLEN  RECORDED. 

the  sharp  straightedge.  The  pointer  is  thus  caught  between  two 
edges,  giving  a  dot  under  the  paper  which  is  thin  and  translucent.  The 
records  are  distinguished  by  different  colors.  Dots  are  made  at  10-sec. 
intervals,  eight  dots  to  a  thermocouple,  and  a  change  of  couples  every 
80  sec.  The  recorder  may  be  made  to  persist  in  indicating  the  tempera- 
ture of-  one  couple  by  moving  the  lever  to  "Single,"  which  disconnects 
the  commutator  within  the  cylindrical  box  on  the  left. 

Power  is  obtained  from  three  dry  cells  placed  within  the  case,  and  the 


424 


RECORDING    PYROMETRY 


speed  of  recording  operations  is  regulated  by  the  clock  in  the  box  at  the 
upper  right-hand  side.  The  clock  case  is  the  background  in  Figs.  16 
and  17.  It  is  a  powerful  eight-day  clock  with  two  springs.  The  escape- 


FIG.  15. — WILSON-MAEULEN  RECORDER. 


FIG.  16. — CONTACT  MECHANISM  OF  TAPALOG. 

ment  is  mechanically  linked  with  the  recorder  parts  through  the  sprocket 
E,  Fig.  16,  which  is  engaged  by  the  pawl  D.  The  chart  drum  is  driven 
by  the  pair  of  gear  wheels  seen  in  the  lower  right-hand  corner.  The  fol- 
owing  description  explains  the  operation  of  the  contact  mechanism  and 


C.    O.   FAIRCHILD   AND   PAUL   D.    FOOTE  425 

the  method  of  taking  the  load  off  the  clock.  The  depressor  frame  B 
is  held  up  by  the  overbalancing  weight  C  and  is  allowed  to  rise  slowly 
by  the  slow  motion  of  the  escapement  sprocket  E,  which  is  engaged  by 
the  pawl  D  and  its  spring  0.  In  Fig.  17,  the  corner  of  B  has  been  cut  away 
to  show  parts  P  and  W.  When  electric  contact  (battery  circuit)  is  made, 
the  electromagnet  forces  the  frame  B  down  with  a  sharp  blow.  This  lifts 
the  weight  C.  The  part  P  bolted  to  B  engages  the  lug  W  on  the  arm  H, 
forcing  the  latter  to  the  left  until  it  is  caught  by  a  lug  entering  the  notch 
K  in  the  arm  G.  As  soon  as  the  frame  B  is  depressed,  it  starts  rising 
slowly  with  the  motion  of  E.  When  the  frame  B  reaches  a  certain  point, 
it  lifts  the  arm  G  (P  strikes  L)  disengaging  it  from  the  lug  on  H,  allow- 
ing H  to  be  pulled  into  its  original  position  by  the  spring  T.  The  latter 


-'^-.T-~ 


.FiG.  17. — CONTACT  MECHANISM  OF  TAPALOG. 

operation  is  sudden,  so  that  the  lower  end  of  H  strikes  the  spirally  wound 
wire  V  on  M,  making  electric  contact  in  the  battery  circuit.  Thus  the 
cycle  is  complete.  In  this  cycle,  energy  is  intermittently  stored  up  in 
the  spring  T  and  the  weight  C.  The  contact  on  H  and  the  wire  V  are 
platinum-rhodium  and  platinum,  so  that  good  contact  is  always  made. 

Eight  strokes  of  an  electromagnet  store  up  sufficient  energy  to  shift 
the  commutator.  This  is  not  shown  in  detail  as  the  parts  are  so  small 
and  superimposed  as  to  make  it  difficult  to  show  their  operation  photo- 
graphically. The  commutator  is  thoroughly  tested  before  incorporation 
in  the  recorder,  by  running  it  for  a  month  in  the  factory.  The  contacts 
are  of  rare  metal  and  a  combined  blow  and  wipe  contact  is  employed. 

Until  recently  the  galvanometer  of  this  recorder  was  made  only  with 
double  pivots  and  having  a  resistance — including  the  swamping  re- 
sistance— of  50  to  150  ohms.  In  1919,  the  manufacturers  began  furnish- 
ing the  instrument  with  a  monopivot  galvanometer,  and  were  able  to 


426 


RECORDING    PYROMETRY 


increase  the  total  internal  resistance  to  450  ohms  for  a  55-millivolt 
range.  The  armature  and  springs  of  the  galvanometer  have,  in  this 
case,  a  combined  resistance  of  46  ohms.  The  period  of  vibration  of 
the  moving  coil  about  a  horizontal  axis  "is  such  that  very  rarely  may 
trouble  be  expected  due  to  jarring  of  the  recorder.  The  galvanometer 
parts  are  particularly  strong  and  the  method  of  lowering  the  chart  carriage 
for  replacing  ribbon  and  paper  precludes  any  likelihood  of  injury  to  them. 

THERMOCOUPLE  RECORDER  OPERATING  ON  THE  POTENTIOMETRIC 

PRINCIPLE 

The  principle  of  operation  of  the  potentiometer  circuit  and  the  various 
arrangements,  as  it  is  applied  in  pyrometry,  has  been  described  by  the 
writers3  elsewhere.  For  the  reader's  convenience  the  diagram  of  the 
simple  potentiometer  circuit  is  given  again  in  Fig.  18.  The  three  steps 


BA 


FIG.  18. — SIMPLE  POTENTIOMETER  CIRCUIT. 

in  the  operation,  adjusting  the  current  through  DE,  connecting  the  ther- 
mocouple (the  galvanometer  is  deflected),  and  turning  the  point  G  until 
the  galvanometer  deflection  is  zero,  are  done  automatically  in  the  multi- 
pie-point  potentiometer  recorder. 

The  curve-drawing  recorder  made  by  the  Leeds  &  Northrup  Co.  is  a 
potentiometer  recorder  for  one  couple  only.  The  ink  pen  remains  in 
contact  with  the  paper  chart  at  all  times  and  a  continuous  terraced  curve 
is  drawn.  In  this  single-point  recorder,  the  current  through  the  slide 
wire  is  set  by  hand,  since  a  commutator  would  be  required  to  do  this 
automatically,  and  the  addition  of  a  commutator  converts  the  instru- 
ment into  a  multiple-point  recorder,  as  illustrated  in  Figs.  19  and  20. 
The  potentiometer  recorder  consists  essentially  of  a  potentiometer  with 
the  mechanical  device  shown  in  Fig..  21  for  automatically  changing  the 

3  See  paper  by  Foote,  Harrison  and  Fairchild  on  Thermoelectric  Pyrometry,  this 
volume. 


C.    O.    FAIRCHILD    AND    PAUL    D.    FOOTE  427 

slide-wire  contact  and  moving  the  pen  across  the  chart.  It  is  the  per- 
fection of  this  simple  device  that  has  made  possible  the  application  of  the 
potentiometric  principle  to  the  autographic  recorder. 

Figs.  19  and  20  show  the  construction  of  this  instrument  in  some 
detail.  The  case  consists  of  a  cast-iron  back  with  an  oak  and  glass  cover, 
which  is  raised  in  Fig.  20.  On  the  left  of  the  case  are  shown  the  main 
switch,  the  thermocouple,  and  battery  binding  posts;  and  on  this  par- 
ticular instrument  an  automatic  cold-junction  compensator.4  On  the 
left,  within  the  case,  is  the  double-pole  commutator  with  five  sectors, 
one  of  which  is  for  the  standard-cell  connection.  The  motor  is  mounted 
above  the  box  containing  the  governor,  which  is  attached  to  the  end  of 
the  motor  shaft.  The  gears  and  cams  on  the  right  connect  the  main 


FIG.  19. — LEEDS  &  NORTHRUP  RECORDER. 

shaft  with  the  commutator,  print  wheel  (in  place  of  pen),  and  paper  chart. 
In  Fig.  21,  the  moving  coil  of  the  galvanometer  is  marked  7  and  this  is 
seen  in  Fig.  19  between  the  pole  pieces  of  the  horseshoe  magnet. 

The  chart  of  the  Leeds  &  Northrup  recorder  is  10  in.  (25  cm.)  wide 
and  has  rectangular  coordinates.  The  paper  has  been  removed  in  Fig. 
19  showing,  near  the  bottom  of  the  case,  the  empty  roll.  The  paper  goes 
from  this  roll  over  the  drum  and  out  of  the  case  through  a  slot  in  the 
bottom.  The  pen  in  single-point  recorders,  or  the  print  wheel  in  the 
instrument  illustrated,  travels  on  a  rod  above  the  drum  and  is  attached 
to  a  cord.  In  Fig.  19,  this  rod  is  turned  in  synchronism  with  the  commu- 

4  See  paper  on  Thermoelectric  Pyrometry. 


428 


RECORDING    PYROMETRY 


tator  so  that  the  proper  number  on  the  print  wheel  is  down.  The  rod  is 
kept  away  from  the  paper,  against  the  tension  of  two  helical  springs,  by 
arms  at  each  end,  which  are  pressed  against  the  peripheries  of  two  cams 
on  a  shaft  running  in  the  rear  of  the  frame  seen  in  Fig.  20  where  the  entire 
works  is  swung  outward,  on  a  heavy  cast  iron  frame. 

A  short  description  of  the  device  shown  in  Fig.  21  and  its  cycle  of 
operations  will  explain  how  the  deflection  of  the  galvanometer  results 
in  a  movement  of  the  slide  wire  and  pen  without  requiring  that  the  galva- 
nometer do  any  work.  The  disk  1  is  mounted  on  the  same  shaft  as  the 
slide  resistances  R  and  DE,  Fig.  18,  which  are  wound  on  the  circumfer- 
ences of  the  disks  visible  in  Fig.  20.  The  power  supplied  by  the  motor 


FIG.  20. — LEEDS  &  NORTHRTJP  RECORDER. 

enters  this  mechanical  system  through  the  shaft  6  carrying  the  large  cams 
6E  and  the  small  cams  6B  and  6C.  At  each  revolution  of  the  shaft  6, 
the  cams  6E  straighten  out  the  arm  2,  which  perchance  has  been  tilted  a 
moment  before,  and  in  doing  this  rotate  the  disk  1,  arm  2  being  pressed 
at  this  time  against  the  disk  1  by  the  spring  3.  The  arm  2  is  pivoted  on 
the  spring  8,  which  is  fast  to  the  frame  of  the  instrument.  When  the 
cams  6E  have  rotated  until  their  longest  radii  are  passing  the  extensions 
of  arm  2,  the  cam  6C  begins  to  raise  3,  lifting  2  away  from  the  disk. 
When  2  is  free,  the  cam  6B  raises  the  rocker-arm  6,  which,  in  case  the 
galvanometer  is  unbalanced,  catches  the  pointer  under  one  of  the  right- 
angle  levers  4L  and  4R  pivoted  at  24E.  The  lever  4L  or  4R  is  thus  made 


C.    O.    FAIRCHILD    AND    PAUL   D.    FOOTE 


429 


to  swing  the  arm  2  by  pressing  against  one  of  the  eccentrically  located 
lugs,  2C.  The  rocker-arm  5  is  then  immediately  lowered  to  allow  the 
galvanometer  to  swing  freely.  Cam  6C  is  so  shaped  and  fixed  on  the 
shaft  6  that  it  will  recede  from  the  spring  3,  allowing  3  to  press  2  against 
the  disk  just  before  the  cams  6E  begin  once  more  to  straighten  2. 

This  mechanism,  in  its  cycle  of  operations,  moves  the  contact  on  the 
slide  wire  whenever  the  potentiometer  is  out  of  balance  with  the  thermo- 
couple and  in  so  doing  operates  to  obtain  or  restore  the  balance.  The 
shaft  6  rotates  once  in  about  2  sec.,  which  is  slow  enough  to  allow  the 
galvanometer  time  to  come  to  rest  or  nearly  so.  This  design  is  such  that 


FIG.  21. — BALANCING  MECHANISM  OF  POTENTIOMETER  RECORDER,    a,   MECHANISM 
UNBALANCED;  b,  MECHANISM  BALANCED. 

the  amount  of  rotation  of  the  arm  2  increases  with  the  extent  of  the  galva- 
nometer deflection,  since  the  pointer  approaches  the  fulcrum  of  the  lever 
4L  or  4R  as  the  deflection  increases.  The  motion  of  5  is  adjusted  so 
that  the  rotation  of  2  will  correspond  to  a  rebalanceing  step  of  the  pen, 
of  ^4  in.  (19  mm.)  when  the  deflection  is  a  maximum,  and  decreases 
uniformly  to  about  ^o  in.  when  the  deflection  is  just  sufficient  to  catch 
the  boom  under  one  of  the  right-angle  levers.  This  gives  sufficient 
rapidity  of  the  various  actions  to  take  the  pen  the  width  of  the  scale  in 
somewhat  less  than  1  min.  A  record  is  made  once  a  minute  on  the 
multiple-point  recorders  of  standard  design.  The  position  of  the  pen, 


430  RECORDING    PYROMETRY 

when  a  balance  has  been  obtained  just  before  each  record,  corresponds 
to  a  definite  point  on  the  slide  wire,  for  the  pen  is  fixed  to  a  cord  fast  to 
the  slide-wire  disk  and  is  wound  or  unwound  with  the  rotation  of  the  disk. 

Once  during  a  revolution  of  the  commutator,  the  thermocouple  is 
disconnected  and  the  standard-cell  connection  made.  At  the  same  time 
the  potentiometer  slide  wire  is  let  loose  from  its  shaft  and  the  clutch 
engages  a  second  resistance  R,  Fig.  18.  Movements  of  the  disk  then 
result  in  changing  the  resistance  of  the  battery  circuit  and  the  current 
is  thus  set  to  its  proper  value.  The  pen  does  not  follow  this  adjustment 
and  no  record  is  made  of  variations  in  the  current.  With  batteries  in 
fair  condition,  the  current  is  easily  maintained  constant;  but  if  there 
arises  any  doubt  of  this  constancy,  the  recorder  may  be  watched  for  a 
few  minutes  and  when  the  standard-cell  connection  is  made,  the  first 
deflection  of  the  galvanometer  is  an  indication  of  the  change  in  the  current 
since  the  last  adjustment.  A  short-circuiting  contact  on  the  slide  wire 
carries  the  pen  to  zero  on  the  chart  when  the  battery  has  run  down,  thus 
providing  ample  warning  under  most  circumstances. 

The  scale  of  this  recorder  is  uniform  when  graduated  in  millivolts, 
and  departs  from  uniformity  for  a  temperature  graduation  according 
to  the  temperature-electromotive  force  relation  of  the  thermocouple. 
The  standard  galvanometer  is  sufficiently  sensitive  to  work  satisfactorily 
with  a  full-scale  range  of  10  millivolts,  which  gives  a  very  open  scale, 
particularly  for  base-metal  couples,  when  34o  in-  (2-5  mm.)  of  scale 
corresponds  to  2^4°  C.  This  recorder  is  used  with  resistance  ther- 
mometers, in  which  case  the  electrical  circuit  takes  the  form  of  a 
Wheatstone  bridge,  and  the  scale  can  be  opened  until  the  entire  range 
corresponds  to  as  little  as  2°  C.  The  zero  of  the  scale  can  be  adjusted  to 
correspond  to  any  fixed  electromotive  force,  so  that  the  scale  may  be  put 
within  any  range  of  temperature  desired.  The  great  adaptability  of  the 
instrument  is  readily  apparent.  Some  of  the  applications  will  be  con- 
sidered in  a  section  on  temperature  control.5 

TRANSFORMATION-POINT  INDICATORS  AND  RECORDERS 

Instruments  for  obtaining  transformation  or  critical  points  in  steels 
have  been  in  use  for  a  number  of  years,  and  are  fairly  well  known  in  the 
steel  industries.  The  simplest,  being  also  the  least  accurate  method  of 
measuring  the  temperature  at  critical  points,  is  to  record  or  plot,  from 
indicator  readings,  the  temperature-time  of  the  sample  of  steel  when 
placed  in  a  furnace  and  heated.  This  method  requires  a  very  steady 
rate  of  heating  and  the  sensitivity  is  ordinarily  only  sufficient  for  high- 
carbon  steels.  It  is  possible,  by  using  a  potentiometer  and  very  sensitive 
galvanometer  and  forcing  the  furnace  to  rise  in  temperature  at  a  fixed 

6  This  volume. 


. 

C.    O.    PAIRCHILD   AND   PAUL   D.    FOOTE  431 

rate,  to  obtain  a  temperature-time  curve  that  will  indicate  plainly  all  the 
transformations.  There  is  great  difficulty  in  keeping  the  rate  of  heating 
constant,  and  it  is  "not  necessary  to  control  this  rate  better  than  to  keep 
it  from  changing  rapidly.  However,  in  the  research  laboratory,  good 
heating  rate  control,  sufficient  sensitivity,  and  an  accurate  method  of 
measuring  time  intervals,  as  with  a  chronograph  or  stop  watch,  will  give 
all  the  necessary  data,  sometimes  requiring  for  a  complete  interpretation, 

the  replotting  of  the  data6  as  temperature  versus  inverse  rate,  —  • 

AT 

Burgess  has  described7  the  use  of  a  neutral  body  and  the  differential- 
couple  arrangement  (first  devised  by  Sir  Roberts- Austen),  which  tends 
to  avoid  to  a  large  extent  the  destruction  of  useful  data  by  variations  in 
heating  and  cooling  rates.  The  differential  couple  is  mounted  with  one 
hot  junction  in  the  test  piece  and  the  other  in  a  neutral  body  (one  with 
no  transformation  points). 

The  data  obtained  are  curves  of  temperature  of  test  piece  versus  tem- 
perature difference  between  test  piece  and  neutral  body.  These  are 
mounted  side  by  side  in  the  furnace  and  must  be  of  such  size,  specific 
heat,  emissivity,  etc.  as  to  heat  and  cool  at  nearly  the  same  rate. 

The  industrial  laboratory  requires  a  method  of  automatic,  or  at  least 
semi-automatic,  recording  of  cooling  curves.  Simplicity  of  the  apparatus 
implies  a  limited  scale  range,  incompatible  with  the  direct  6  versus  t 
method  and  requiring  the  use  of  the  differential  thermocouple.  To 
avoid  errors  due  to  improper  measurement  of  time  intervals,  the  records 
of  temperature  and  temperature  difference  must  be  on  one  chart  and  are 
best  obtained  from  a  single  galvanometer  connected  alternately  to  the 
thermocouple  in  the  test  piece  and  the  differential  couple.  A  curve  of 
temperature  versus  temperature  difference  may  be  obtained  simply,  in  a 
semi-automatic  recorder  using  two  galvanometers. 

The  Brown  Transformation  Point  Recorder. — Fig.  7  illustrates  the 
type  of  recorder  applied  for  this  purpose.  The  single  galvanometer 
of  this  instrument  has  two  windings  of  low  resistance,  one  of  which  has  in 
series  with  it  a  high  resistance  and  is  employed  to  measure  temperatures. 
This  coil  is  connected  permanently  to  the  thermocouple  in  the  test 
piece.  At  intervals,  a  record  is  made  of  its  temperature.  At  alternate 
intervals,  the  other  coil  is  connected  to  the  differential  couple  and  a 
record  made  of  its  temperature  difference.  This  difference  shows  on 
the  chart  as  the  distance  apart  of  the  two  curves  of  9  and  6-6'.  If 
the  first  coil  were  disconnected  during  the  time  the  other  is  connected 

6  G.  K.  Burgess:  On  Methods  of  Obtaining  Cooling  Curves.     U.  S.  Bureau  of 
Standards  Sci.  Paper  99. 

7  Loc.  cit. 


432 


RECORDING   PYROMETRY 


0-0'  would  be  measured  by  the  deflection  from  zero  on  the  chart. 
But  0-0'  may  be  positive  or  negative  and  this  arrangement  would  necessi- 
tate a  reversing  switch  and  offer  no  advantage.  To  obtain  sufficient 
sensitivity  base-metal  couples  and  rather  large  test  pieces  are  used.  The 
instrument  will  show  a  curve  with  a  very  marked  jog  at  the  eutectoid 
transformation  point,  and  by  close  observation  the  allotropic  transfor- 
mation point  As  may  be  detected  in  pure  iron.  There  is  not  an  excessive 
sensitivity,  but  practically  the  best  that  can  be  done  with  a  double- 
pivot  galvanometer. 

The  Leeds  &  Northrup  Transformation-point  Indicator. — This  instru- 
ment is  a  semi-automatic  recorder  giving  a  continuous  curve  of  0  vs. 
6-0'.  One  observer  is  required  and  no  time  measurements  are  made;  it 
is  illustrated,  in  part,  by  Fig.  22.  The  wall-type  reflecting  galvanometers 

(two)  are  not  shown.  The  complete  appa- 
ratus includes  the  two  galvanometers,  furnace, 
rheostat,  and  thermocouples. 

The  recorder  consists,  in  the  main,  of  a 
potentiometer  with  a  slide  wire  and  drum  chart 
on  the  same  shaft.  In  moving  the  slide  wire 
to  obtain  a  balance  of  the  potentiometer 
galvanometer,  the  chart,  with  ordinates 
graduated  in  degrees  (or  millivolts),  turns 
simultaneously  and  its  position  with  reference 
to  a  pen  held  in  contact  indicates  the  temper- 
ature of  the  test  piece.  The  pen  is  mounted 
above  the  chart  on  a  carriage,  which  may  be 
moved  by  turning  a  screw  across  the  width 
of  the  chart.  Upon  the  carriage  is  a  ground- 
glass  screen  with  a  central  vertical  mark.  A 
spot  of  light  from  the  differential  galvano- 
meter is  focussed  on  this  screen.  The  differen- 
tial couple  is  connected  directly  to  this  galvanometer  and  its  de- 
flections are  followed  by  turning  the  pen-carriage  screw  keeping  the 
mark  on  the  glass  coincident  with  the  light  beam  from  the  galvanometer. 
A  separate  glass  scale  is  provided  for  balancing  the  other  galvanometer. 
The  observer  has  two  motions  to  perform,  viz.,  turning  the  slide  wire 
(and  drum)  and  turning  the  pen-carriage  screw.  The  zero  temperature 
point  is  suppressed  and  the  end  of  the  slide  wire  is  usually  made  to  corre- 
spond to  2  millivolts  or  the  corresponding  temperature.  The  other  end 
of  the  slide  wire  corresponds  to  10  millivolts  giving  8  millivolts,  or  about 
800°  C.,  over  the  whole  chart  length  of  20  in.  (50.8  cm.).  This  is  a  suffi- 
ciently long  temperature  scale  for  all  practical  purposes. 

In  series  with  the  differential  galvanometer  is  a  resistance  that  may 
be  used  to  cut  down  the  sensitivity  of  the  galvanometer,  which  will  be 


FIG.  22. — LEEDS  &  NORTH- 
RUP TRANSFORMATION  POINT 
INDICATOR. 


DISCUSSION  433 

usually  deflected  off  the  scale  at  a  eutectoid  transformation  point.  The 
sensitivity  is  ample  for  detecting  all  the  transformation  points  ordinarily 
required  in  practice.  The  period  of  the  galvanometer  is  short  enough  to 
allow  the  heating  and  cooling  to  be  done  in  less  than  1  hr.  The  curve 
obtained  is  a  continuous  line,  slightly  ragged  due  to  manual  operation, 
and  is  easily  translated  into  metallurgical  terms.  It  can  be  replotted  into 


_   Qf\ 

a  curve  of  0  vs.      r^  —  -,  the  so-called  "Derived  Differential  Curve," 

aJS 

due  to  Rosenhain,8  especially  if  errors  are  suspected  due  to  considerable 
differences  in  the  cooling  curves  of  the  test  piece  and  the  neutral.  This 
curve  also  aids  in  the  interpretation  of  results  and  corresponds  to  the 
inverse-rate  curve  when  only  measurements  of  temperature  and  time  are 
made  without  the  use  of  a  neutral  body  and  differential  thermocouple. 

• 

DISCUSSION 

R.  W.  NEWCOMB,  New  York,  N.  Y.  (written  discussion*).  —  On 
page  417  mention  is  made  of  a  new  instrument  with  an  exceptionally 
high  resistance  that  has  been  developed  by  Charles  Engelhard.  All 
friction  and  wear  of  moving  parts  of  the  moving  system  has  been  elimi- 
nated by  replacing  the  hardened  steel  pivots  and  jeweled  bearings, 
commonly  used  on  other  instruments,  by  a  double  metallic  filament, 
one  at  the  top  and  one  at  the  bottom,  under  slight  tension.  Any  pos- 
sible distortion  of  the  moving  coil,  because  of  tension,  has  been  eliminated 
by  introducing  a  solid  spindle  as  the  axis  of  the  coil  between  the  points 
on  the  coil  where  the  metallic  filament  is  attached.  The  filaments  serve 
not  only  as  a  support  for  the  moving  coil,  but  also  to  lead  in  the  current 
from  the  binding  posts  to  the  moving  coil,  and  as  a  source  of  counter- 
torque;  instruments  so  constructed  do  not  require  leveling,  and  are 
mechanically  very  robust. 

The  clock  of  the  recorder  serves  only  to  drive  the  chart  at  its  specified 
rate  and  to  operate  a  small  contact-making  device;  i.e.,  there  is  no 
other  mechanical  load  on  the  clockworks.  The  contact-making  de- 
vice is  so  constructed  that  there  is  a  quick-make,  a  quick-break,  and 
a  wiping  effect  while  the  contact  is  being  made.  Contact  surfaces 
on  this  switch  are  of  platinum  platinum-iridium.  The  operation  of 
this  contact,  which  occurs  once  each  minute,  sends  a  current,  from  a 
6-volt  supply,  through  a  solenoid  magnet,  which  operates  the  depressor 
bar  mechanism  in  the  case  of  the  single-record  instrument;  in  the  case  of 
the  multiple-record  instrument,  it  operates  the  automatic  switch  and 
color-changing  features,  as  well  as  the  depressor  bar. 

A  new  method  of  inking  is  employed.     On  the  single-record  recorders 

8  Observations  on  Recalescence  Curves.     Proc.  Phys.  Soc.  Lond.  (1908)  21,  180. 
*  Received  Nov.  1,  1919. 

28 


434  RECORDING    PYROMETRY 

the  paper  passes  over  a  small  roller  about  Y±  in.  in  diameter,  which  is 
located  directly  across  the  instrument,  beneath  the  depressor  bar. 
The  paper  is  held  clear  from  the  roller  surface  by  small  hubs  located  at 
each  end.  The  roller  is  covered  with  a  fabric  tube,  impregnated  with 
the  inking  compound,  and  is  slowly  turned  by  the  passage  of  the  paper. 
The  pointer  swings  above  the  chart  and  below  the  depressor  bar.  When 
the  depressor  bar  falls,  the  position  of  the  pointer  at  its  intersection 
with  the  color-carrying  roll  underneath  the  chart  is  recorded.  On  the 
multiple-record  instruments,  there  are  as  many  rollers  as  the  capacity 
of  the  multiple-recorder  in  thermocouples,  i.e.,  on  a  four-point  recorder 
there  are  four  rollers,  on  a  six-point  recorder,  six  rollers,  etc. 

The  operation  of  the  automatic  switch  that  controls  the  rollers,  the 
depressor  bar,  and  the  color-changing  mechanism  is  accomplished  by 
the  solenoid  magnet;  the  up-and-down  motion  of  the  magnet  is  changed 
into  a  rotating  motion  for  the  operation  of  the  switch  and  color-changing 
mechanism,  by  a  double-acting  locking  pawl  engaged  with  a  pinion. 
When  the  small  contact  switch  on  clockwork  makes  contact,  the  armature 
of  the  magnet  is  drawn  down,  allowing  the  depressor  bar  to  record  the 
position  of  the  pointer  corresponding  to  the  temperature  of  the  thermo- 
couple. As  soon  as  the  contact  is  broken,  the  reacting  spring  on  the 
solenoid  magnet  turns  the  automatic  switch  and  color-changing  mechan- 
ism to  the  next  point. 


HIGH-TEMPERATURE    CONTROL  435 


High -temperature  Control 

BY  C.  O.  FAIRCHILD,*  B.  S.,  AND  PAUL  D.  FOOTE,f  PH.    D.,    WASHINGTON,  D.  C. 
(Chicago  Meeting,  September.  1919) 

THE  meaning  of  temperature  control  can  be  extended  to  cover 
not  only  the  control  of  temperatures  b'ut  also  the  control  of  processes 
through  a  knowledge  of  the  temperatures  involved.  In  this  sense  it 
has  a  very  wide  interest.  A  list  of  the  industries  in  which  temperature 
control  is  used  in  one  way  or  another  would  cover  nearly  the  entire 
industrial  field.  This  discussion  will  be  confined  to  the  field  of  high 
temperatures. 

In  practically  all  industries  involving  operations  at  high  temperatures, 
pyrometers  are  used  or  men  are  paid  for  their  ability  either  to  estimate 
temperatures  or  gage  an  operation  by  some  physical  or  chemical  change 
or  condition  dependent  on  temperatures.  As  progress  is  made  in  the  devel- 
opment of  instruments  and  of  methods  for  measuring  temperature,  some 
of  these  highly  skilled  artisans  are  learning  the  use  of  a  new  tool.  Many 
processes  have  been,  in  recent  years,  improved  by  means  of  exact  measure- 
ment thus  substituted  for  estimation,  but  in  a  great  many  industries  the 
pyrometer  has  had  practically  no  opportunity  to  demonstrate  its  useful- 
ness. The  perfecting  of  instruments  must  be  accompanied  with  a  dis- 
semination of  the  knowledge  that  new  instruments  are  available  and  can 
be  economically  used.  The  demand  at  present,  however,  is  ahead  of 
the  supply,  and  many  industries  are  handicapped  by  the  want  of 
pyrometers. 

One  of  the  highest  paid  skilled  tradesmen  of  the  present  time  is  the 
man  in  the  steel  rolling  mill  who  knows  when  proper  working  tempera- 
tures are  attained.  The  metallurgist  in  his  development  of  steels  is 
continually  demanding  closer  adherence  to  given  temperature  ranges  in 
the  processes,  and  the  pyrometer  is  rapidly  becoming  indispensable. 
Properties  of  the  finished  products  are  being  correlated  with  working 
temperatures  and  so  closely  that  in  some  cases  even  the  pyrometer  is 
taxed  to  give  the  required  accuracy.  The  greatest  progress  has  been 
made  in  those  industries  in  which  the  lower  temperatures  are  used, 
particularly  below  500°  C.  In  this  lower  range  automatic  control 
has  been  highly  developed. 

*  Associate  Physicist,  U.  S.  Bureau  of  Standards- 
t  Physicist,  U.  S.  Bureau  of  Standards. 


436  HIGH-TEMPERATURE  CONTROL 

GENERAL  DISCUSSION  OF  PROBLEM  OF  CONTROL 

Some  of  the  factors  that  increase  the  difficulty  of  the  regulation  of  fur- 
naces, ovens,  kilns,  tanks,  etc.,  are:  Inconstancy  of  heat  supply,  variation 
in  internal  absorption  or  generation  of  heat,  variation  of  heat  lost  by  radia- 
tion, etc .,  and  unsteady  supply  or  composition  of  material  to  be  heat  treated . 
As  each  of  these  items  is  intimately  associated  with  temperature  and  tem- 
perature variations,  there  is  little  room  for  doubt  that  furnace  control 
is  best  accomplished  with  and  through  a  knowledge  of  the  temperatures 
and  temperature  variations.  Further,  this  knowledge  becomes  increas- 
ingly important  at  high  temperatures,  finally  becoming  the  prime  req- 
uisite in  all  cases.  How  are  the  temperature  and  its  variations  to  be 
determined?  Where  (in  what  part  of  the  furnace,  kiln,  or  oven)  is  it  to 
be  determined?  What  is  the  best  way  in  which  the  temperature  may 
be  indicated  so  that  it  will  aid  in  control?  Or  can  automatic  temperature 
control  be  accomplished? 

There  is  generally  some  point,  or  perhaps  several,  in  a  furnace,  the 
temperature  of  which  is  more  intimately  connected  with  the  desired  con- 
ditions than  any  other.  In  a  hardening  furnace,  it  is  a  simple  matter  to 
put  a  thermocouple  at  some  point  and  find  the  temperature  in  the  immediate 
vicinity.  But  it  is  another  matter  to  determine  the  actual  temperature 
of  a  piece  of  steel  in  the  furnace.  This  particular  case  will  be  treated 
later.  In  open-hearth  furnaces  two  temperatures  are  particularly  desired, 
that  of  the  metal  and  that  of  the  roof.1  It  is  essential  to  tap  the  furnace 
when  the  metal  is  in  a  certain  temperature  range,  but  before  this  range  is 
reached  the  temperature  of  the  roof  is  considerably  more  useful  for  furnace 
control.  In  forcing  the  furnace  to  maximum  production,  the  maximum 
rate  of  heating  the  charge  is  limited  by  the  refractoriness  of  the  brick 
making  up  the  roof.  Other  factors,  of  course,  enter  but  this  serves  as 
an  example  of  the  necessity  for  a  study  of  where  to  install  the  pyrometer. 
Burgess  found  that  remarkably  uniform  results  are  obtained  without  the 
use  of  pyrometers,  at  least  with  the  temperatures  of  the  metal.  The  roof 
temperature  was  found  to  vary  over  wide  limits.  The  exceptionally  con- 
sistent results  obtained  in  open-hearth  practice  are  due  to  highly  skilled 
men  who  have  learned,  by  long  practice,  how  to  estimate  temperatures  and 
run  the  furnace  by  the  appearance  of  the  slag,  etc.  However,  suppose 
that  an  optical  pyrometer  is  used  to  measure  the  temperature  of  the  roof 
and  that  the  bricks  of  the  roof  melt  at  1710°  C.  At  this  temperature, 
then,  the  deterioration  of  the  roof  is  quite  rapid.  But  at  only  a  slightly 
lower  temperature,  possibly  1690°,  at  which  there  are  no  signs  of  melting 
to  aid  the  furnace  man,  the  life  of  the  roof  is  very  much  greater.  Only 

1 G.    K.    Burgess:   Temperature    Measurements   in   Bessemer   and   Open-hearth 
Practice.     U.  S.  Bureau  of  Standards  Tech.  Paper  91  (1917). 


C.    O.    FAIRCHILD    AND    PAUL    D.    FOOTE  437 

by  means  of  a  pyrometer  can  so  close  a  regulation  of  temperature  as  this 
be  attempted  and  production  forced  to  a  uniform  high  value  without  too 
rapidly  burning  down  the  roof. 

In  the  example  cited,  the  furnace  can  be  operated  quite  satisfactorily 
without  a  pyrometer,  because  a  physical  change  (the  melting  of  the  roof) 
begins  at  a  certain  temperature  but  does  not  at  this  temperature  attain 
a  disastrous  rate.  The  argument  is  not  affected  by  the  fact  that  numer- 
ous other  changes  occurring  serve  to  guide  the  furnace  man.  A  pyrites 
dead-roast  furnace  tends  to  become  too  hot  and  reach  a  temperature  at 
which  the  pyrites  softens.  This  is  a  danger  point  as  the  ore  begins  to 
•ball  up,  but  before  this  tendency  has  become  too  great  the  operator  has 
some  leeway  and  can  cool  the  furnace  by  prompt  action  without  coming 
to  disaster.  With  a  pyrometer  in  the  hottest  bed  of  the  furnace,  the 
temperature  can  be  kept  consistently  at  the  highest  safe  point,  without 
the  necessity  of  occasional  drastic  action  to  cool  the  furnace,  thus  lower- 
ing production. 

The  working  of  glass  is  an  excellent  example  of  an  operation  that  can 
be  carried  on  without  a  pyrometer  because  the  glass  grows  softer  very 
slowly  with  a  rise  in  temperature.  But  since  it  works  best  in  a  small 
range  of  temperature,  a  pyrometer  is  of  considerable  advantage,  and  glass 
manufacturers  are  speeding  up  production  and  increasing  the  quality 
and  uniformity  of  the  product  by  installing  pyrometers.  These  instru- 
ments have  been  found  practically  indispensable  in  the  lehrs,  where  it  is 
difficult  to  make  any  estimate  of  the  temperatures.  Here  it  is  necessary 
to  know  the  highest  temperature  reached  and  also  the  rate  of  cooling 
of  the  ware. 

In  order  to  control  a  furnace  with  a  pyrometer,  or  at  least  to  obtain 
the  most  help,  it  is  desirable  to  have  the  pyrometer  indicate  the 
temperature  where  the  widest  variations  occur  and  where  these  changes 
take  place  the  earliest.  In  industries  such  as  glass,  steel,  and  ceramics, 
in  which  very  high  temperatures  are  reached,  'this  generally  cannot  be 
done  unless  a  radiation  or  optical  pyrometer  is  used.  The  position  in 
which  a  thermocouple  is  placed  sometimes  depends  on  its  ability  to 
withstand  the  maximum  temperatures  attained  and  the  expense  involved 
in  using  thermocouples  at  high  temperatures.  Often  the  position  is  so 
chosen  as  to  protect  the  thermocouple  even  though  the  temperature  indi- 
cated is  not  the  one  it  is  most  desirable  to  know.  One  of  the  severest 
tests  of  permanently  installed  thermocouples  is  found  at  the  pyrex  glass 
tanks  of  the  Corning  Glass  "V^orks.  Platinum  platinum-rhodium  couples 
are  installed  in  the  walls  of  these  tanks  so  that  the  hot  junctions  do  not 
reach  into  the  inner  face  of  the  wall.  The  indicated  temperature,  1475- 
1500°  C.,  is  possibly  100°  below  the  actual  temperature  of  the  glass.  The 
couples  are  found  to  deteriorate  very  rapidly  and  means  are  provided 


438  HIGH-TEMPERATURE    CONTROL 

for  frequently  checking  them  by  inserting  a  test  couple  into  the  same 
protecting  tube. 

The  question  of  what  pyrometer  to  use  in  a  certain  case  is  always  a 
vexing  one,  unless  the  user  has  had  considerable  experience.  A  proper 
answer  to  the  question  would  be  a  table  of  industrial  operations  and 
descriptions  of  pyrometers. 

The  methods  of  rendering  temperature  information  useful,  that  is  the 
method  of  indicating  and  recording  temperatures  and  their  variations, 
are  very  numerous  and  must  be  adapted  to  special  needs  in  a  plant. 
The  simplest  way  is  to  put  the  pyrometer  indicator,  preferably  of  the  wall 
type,  in  a  conspicuous  place,  where  the  operator  can  read  it  with  the  least 
trouble;  special  means  must  often  be  employed  for  making  the  scale 
sufficiently  legible.  The  scale  must  be  as  open  as  possible,  so  that  small 
changes  of  temperature  can  be  readily  detected.  Some  indicators  are 
provided  with  two  pointers,  one  of  which  can  be  turned  to  the  tempera- 
ture desired.  This  second  pointer  may  be  made  a  single  line  or  a  double 
one,  the  latter  indicating  the  limits  of  variation  allowable.  In  the  ab- 
sence of  a  recorder,  the  second  pointer  is  nearly  indispensable  as  a  means 
of  judging  rates  of  change.  In  poorly  lighted  rooms,  illuminated  scales 
can  be  used.  If  cheap  labor  is  employed  and  the  operator  is  unable 
to  adequately  interpret  the  indicator  reading,  signaling  lights  or  alarms 
may  be  operated  either  manually  or  automatically.  In  larger  plants, 
the  indicators  may  be  situated  at  a  central  station,  where  the  pyrometer 
man  reads  them  and  transmits  the  proper  signal  to  the  operator.  Colored 
lights  are  best  used  for  this  purpose,  white  for  correct  temperature,  green 
for  too  low,  and  red  for  too  high,  and  white  with  red  or  green  for  small 
departures  from  the  correct  temperature.  The  light-ing  circuit  is,  of 
course,  completely  separated  from  the  pyrometer  circuit.  If  the  plant 
electrician  is  allowed  to  install  the  wiring,  he  must  be  convinced  that 
electrical  insulation  and  leakage  take  on  a  new  significance  in  electrical 
pyrometry.  The  use  of  these  colored  lights  or  a  signal  system  is  not 
restricted  to  control  at  a  fixed  temperature;  they  may  be  employed  for 
governing  the  rate  of  heating  up  a  kiln  for  instance.  In  the  ceramic 
industries,  pyrometers  are  particularly  useful  as  a  means  of  avoiding 
loss  of  time  and  curtailment  of  production  on  account  of  a  non-uniform 
rate  of  firing. 

The  operator,  in  controlling  a  furnace  to  reach  a  desired  temperature, 
bases  his  action  on  experience  and  judgment.  Possibly,  he  turns  a  gas  or 
oil  valve  a  certain  part  of  one  turn  according  to  the  change  just  observed 
in  the  condition  of  the  furnace.  Obviously  the  adjustment  can  best 
be  made  with  a  knowledge  of  the  changes  that  have  occurred  over  a  con- 
siderable interval  of  time  rather  than  by  watching  the  change  occurring 
from  moment  to  moment.  This  is  one  of  the  most  prominent  advantages 
of  a  recording  pyrometer;  and  when  it  is  possible  and  the  class  of  labor 


C.    O.    FAIRCHILD    AND    PAUL   D.    FOOTE  439 

employed  warrants  it,  the  operator  should  be  given  the  advantage  of 
inspecting  the  recorder  chart.  It  is  necessary  that  the  record  be  made 
with  a  frequency  greater  than  that  of  significant  changes  that  are  liable 
to  occur  in  the  furnace,  particularly  when  a  multiple  recorder  is  used. 
In  case  a  single-record  recorder  is  used,  or  a  multiple  galvanometer  re- 
corder in  which  each  galvanometer  is  always  connected  to  one  pyrometer, 
the  position  of  the  pointer  or  pen  will  always  show  the  change  that  is 
occurring.  The  recorder  should  be  so  constructed  that  the  record  is 
visible  to  the  last  minute. 

When  pyrometers  have  been  installed  where  they  have  not  been  used 
before,  the  immediate  result  is  usually  confusion.  Difficulty  is  encoun- 
tered in  correlating  the  indicated  temperatures  with  other  conditions, 
so  that  there  may  be  less  efficient  operation  than  before.  Perhaps  there 
exists  a  notion  that  the  pyrometer  will  run  the  furnace.  Too  much  atten- 
tion is  given  to  temperature  and  the  attempt  is  made  to  operate  without 
regard  to  other  necessary  factors,  or  the  new  knowledge  is  misinterpreted 
because  of  the  errors  of  previous  ideas.  The  logical  way,  of  course,  is 
to  observe  and  record  temperatures  and  related  phenomena  until  the  full 
significance  of  temperature  is  discovered.  The  pyrometer  may  fail  to 
indicate  correctly  or  consistently,  which  is  more  likely  to  be  the  fault  of 
the  user  than  of  the  manufacturer.  But  the  greatest  confusion  results 
from  previously  conceived  and  erroneous  notions  of  what  temperatures 
have  existed  in  former  practice  and  quite  frequently  of  what  conditions 
signify  a  rising  or  falling  temperature.  A  furnace  fired  with  some  fuel, 
as  gas  or  oil,  will  usually  have  a  damper  in  the  vicinity  of  the  waste- 
heat  flue  or  exit,  with  which  the  draft  is  or  can  be  controlled.  A  certain 
position  of  this  damper  corresponds  to  a  certain  temperature  gradient  in 
the  furnace.  As  the  damper  is  closed,  this  temperature  gradient  is 
gradually  changed,  either  increasing  or  decreasing,  depending  on  the 
draft  and  fuel  supply.  Some  point  is  easily  reached  at  which,  if  the  draft 
is  reduced  further,  the  temperatures  will  decrease.  There  is,  then,  this 
possible  condition:  With  the  damper  initially  wide  open,  there  will  be 
a  point  or  section  of  the  furnace  that  will  get  hotter  and  then  colder  as  the 
damper  is  gradually  closed.  So  it  is  conceivable  that  a  pyrometer  in- 
stalled at  a  certain  point  will  show  a  temperature  change  exactly  the 
reverse  of  what  a  most  experienced  furnace  operator  will  expect,  when 
certain  changes  in  draft  or  firing  are  'made.  Of  course  the  pyrometer  is 
not  the  only  available  means  of  learning  the  truth;  draft  and  carbon- 
dioxide  indicators  and  gas  analyses  properly  distributed  will  predict 
temperature  changes  correctly,  but  few  furnace  operators  are  able  to  do 
this  without  one  or  more  such  instruments.  In  some  industries  in  which 
the  heat  treatment  of  materials  is  essential  pyrometers  are  not  needed, 
for  the  heat  treatment  results  in  a  chemical  or  physical  change  that 
is  perfectly  definite  and  sufficient  for  control,  or  operating  conditions  can 


440  HIGH-TEMPERATURE    CONTROL 

be  maintained  so  constant,  for  example  in  certain  distillation  processes, 
that  a  pyrometer  will  do  nothing  more  than  indicate  a  fixed  temperature 
within  the  accuracy  of  the  instrument.  In  some  such  instances,  a  more 
sensitive  and  accurate  instrument  may  result  in  unexpected  improve- 
ments and  in  perfecting  a  process  to  a  degree  of  refinement  not  considered 
possible. 

In  general,  however,  a  furnace  is  kept  operating  as  near  to  desired 
conditions  as  possible  in  spite  of  the  persistent  and  often  perplexing 
effects  of  the  variables  mentioned.  Temperature  measurements,  where 
they  have  not  been  made  before,  have  no  known  relation  to  the  rates  of 
change  and  the  values  of  these  variables.  So,  at  first,  these  measure- 
ments are  almost  useless.  The  initial  attitude  of  the  operator  is  similar 
to  that  which  he  would  have  toward  the  introduction  of  a  new  variable 
for  him  to  worry  about,  when  as  a  matter  of  fact  he  is  gaining  exact 
knowledge  of  what  he  had  before  been  guessing.  It  is  evident  that  this 
is  only  a  transition  period  which  is  bridged  with  difficulty,  and  during 
which  there  must  be  a  little  faith  in  the  pyrometer  and  its  usefulness. 

The  employment  of  pyrometers  is  generally  least  in  those  industries 
in  which  operating  conditions  are  the  worst  because  of  the  great  difficulty 
met  with  in  maintaining  uniformity  of  conditions.  It  is  particularly 
true  of  such  an  industry  as  the  ceramic,  in  which  the  chemical  constitution 
of  raw  materials  is  of  prime  importance  and  difficult  to  control.  The  kiln 
operator  cannot  fire  his  kiln  according  to  any  certain  time-temperature 
relations  because  he  does  not  know  how,  not  because  it  is  impossible.  He 
will  never  know  how  so  long  as  pyrometers  are  not  installed  and  the  time- 
temperature  relations  are  associated  with  other  variables.  Pyrometers 
are  now  being  used  in  kilns  in  order  that  the  rate  of  heating  may  be  main- 
tained with  greater  certainty  and  a  loss  of  time  due  to  too  slow  firing  be 
avoided,  but  the  finishing  time  is  determined  with  Seger  cones.  It  is 
hoped  that  the  Seger  cones  will  eventually  be  supplanted  by  even  more 
useful  pyrometric  methods. 

Once  installed,  pyrometers  are  useful  in  so  far  as  they  have  been 
properly  selected  and  the  installation  properly  completed.  The  installa- 
tion of  thermocouples  is  discussed  elsewhere,  but  in  temperature  control 
it  is  necessary  not  only  to  attain  correct  temperature,  where  this  is 
desired,  but  also  to  attain  a  correct  temperature  at  some  other  point 
in  the  furnace,  variations  of  which  have  a  known  significance  and  rela- 
tion to  proper  operation.  That  is,  in  some  furnaces  it  is  possible  to  so 
place  a  thermocouple  that  should  the  furnace  tend  to  become  too  hot 
this  tendency  would  be  seen  in  the  pyrometer  indication  so  quickly  that 
the  cause  could  be  removed  and  a  cooling  action  initiated  before  any  vital 
part  of  the  furnace  would  be  overheated.  The  pyrometer  should  ob- 
viously function  in  a  like  manner  during  cooling.  The  maximum  tem- 
perature that  a  thermocouple  can  withstand  is  more  often  exceeded  when 


C.    O.    FAIRCHILD    AND    PAUL   D.    FOOTE  441 

thus  installed,  and  this  fact  is  directly  responsible  for  the  slow  develop- 
ment of  automatic  control  at  higher  temperatures. 

Control,  either  automatic  or  manual,  and  automatic  signaling  are 
always  accomplished  by  the  employment  of  a  method  of  forcing  the 
furnace  to  heat  or  cool  between  limits,  at  a  rate  and  through  a  range 
depending  on  the  process  and  constancy  required.  Ordinarily  there  is 
little  difficulty  in  obtaining  enough  sensitivity  of  the  instrument  for  either 
automatic  signaling  or  control,  except  in  the  research-laboratory  furnaces, 
where  at  times  the  limit  of  sensitivity  is  employed.  In  control  of  a 
furnace  through  pyrometric  measurement,  it  is  not  necessary  that  the 
pyrometer  be  accurately  calibrated.  It  must,  however,  be  particularly 
reliable  and  have  a  consistent  error,  as  has  been  stated  elsewhere. 

DEVICES   USED  FOR  CONTROL 

Automatic  Alarm. — An  ordinary  pyrometer  galvanometer  of  low  re- 
sistance may  be  fitted  with  two  contacts  on  pivoted  arms,  between  which 
the  meter  pointer  plays.  No  relay  is  necessary  for  the  small  current  and 
voltage  required  to  operate  a  bell.  The  automatic  alarm  is  seldom  used 
in  this  form  since  the  alarm  operates  for  too  large  a  part  of  the  time 
unless  the  contacts  are  placed  wide  apart,  when  their  usefulness  is  much 
lessened.  The  action  may  be  made  intermittent  on  more  complicated 
instruments.  The  alarm  should  properly  be  employed  only  when  the 
departure  from  a  certain  temperature  range  results  in  real  danger  to  life 
or  property. 

Manual  Signaling. — The  development  of  manual  signaling  has  taken 
place,  for  the  most  part,  in  the  larger  plants  having  extensive  pyrometric 
installations  of  such  scope  that  a  central  pyrometer  station  is  necessary. 
Fig.  1  illustrates  a  form  of  central  station  developed  in  part  by  Charles 
Engelhard.  With  such  an  installation,  only  the  thermocouples  and 
signal  lights  are  in  the  furnace  room  and  the  indicators  in  the  station  are 
connected  to  different  couples  by  means  of  push  buttons  on  switchboards. 
The  more  sensitive  types  of  double-suspension  or  unipivot-suspension 
millivoltmeters  or  thermocouple  potentiometers  are  used  and  can  indi- 
cate easily  temperature  variations  of  0.2  per  cent,  of  the  scale  range.  The 
substitution  of  manual  operation  for  automatic  is  necessary  to  attain  the 
closest  correlation  of  temperature  measurements  and  other  physical 
conditions  and  the  physical  and  chemical  properties  of  the  products. 

Signaling  is  accomplished  by  colored  lights,  using  three  at  each 
furnace.  Temperatures  are  read  at  frequent  intervals,  determined  by 
furnace  operation  and  the  operator's  interpretation  of  signals.  In  some 
plants,  pneumatic  tubes  are  installed  between  the  furnace  room  and  the 
station  for  transmitting  notes  concerning  the  process  or  measurements. 
The  furnace  man  can  signal  the  station  with  the  ordinary  enunciator, 


442 


HIGH-TEMPERATURE    CONTROL 


as  shown  in  Fig.  1  at  the  top  of  the  switchboard.  Such  a  signaling  system 
of  colored  lights,  enunciators,  and  pneumatic  tubes  is,  of  course,  not 
peculiar  to  pyrometry.  This  is,  however,  a  very  fitting  application  of  such 
means  of  communication. 

Automatic  Signaling. — The  descriptions  of  the  various  types  of  re- 
corders makes  it  clear  how  automatic  signaling  may  be  accomplished  with 
pyrometer  galvanometers.  In  a  similar  manner  to  the  way  in  which  auto- 
graphic records  are  made,  a  depressor  bar  or  frame,  in  its  rise  and  fall, 
closes  either  of  two  pairs  of  contacts  depending  on  the  position  of  the 
pointer.  The  device  may  be  attached  to  an  indicator  according  to  Fig. 
2,  or  a  recorder  may  be  made  to  do  both  recording  and  signaling.  The 
latter  arrangement  has  not  been  very  successfully  applied  to  the  galva- 


FIG.  1. — CENTRAL  PYROMETER  STATION  OF  CHARLES  ENGELHARD. 

nometer  type  of  recorder,  but  is  easily  adapted  to  the  potentiometer 
recorder  of  The  Leeds  &  Northrup  Co.  In  this  case  two  contacts  move 
with  the  slide  wire  on  its  shaft  and  the  third  is  stationary. 

The  galvanometric  instrument  illustrated  in  Fig.  2  has  two  platinum 
contacts  mounted  on  an  arm  which  swings  across  the  scale  to  adjust  the 
signaling  range.  The  signaling-circuit  current  does  not  flow  to  the 
galvanometer  pointer  but  each  platinum  contact  is  double,  one  above  the 
other,  and  the  two  are  brought  together  by  the  pressure  of  the  depressor 
arm  transmitted  through  the  pointer.  The  pointer  is,  in  this  case,  a 
selector.  The  arrangement  of  Bristol  differs  from  this  in  that  a  tipping 
device  restrained  by  springs  is  tipped  by  the  pointer  when  the  depressor 
arm  falls,  in  a  direction  determined  by  the  pointer.  If  the  latter  is  either 


C.    O.    FAIRCHILD    AND    PAUL   D.    FOOTE 


443 


side  of  the  apex  of  a  triangular  dividing  piece,  this  will  determine  the 
direction  of  tipping.  The  springs  are  arranged  to  snap  the  contact  when 
the  pressure  has  reached  a  certain  value;  by  this  means  the  selection  can 
be  made  to  take  place  over  a  smaller  interval  than  with  two  contacts 
placed  side  by  side  and  close  together. 

The  electromagnetic  switches  operated  by  closing  these  contacts 
hardly  need  description.  They  are  made  applicable  to  any  range  of 
voltage  and  require  a  small  fraction  of  an  ampere.  The  circuit  may  in- 
clude any  of  the  well-known  signaling  devices,  some  of  which  have  been 
mentioned. 

Figs.  3,  4,  and  5  illustrate,  diagrammatically,  the  arrangement  of  the 
curve-drawing  potentiometer  recorder  with  signaling  lamps  and  furnace 
indicator.  The  diagrams  are  self-explanatory  but  the  new  method  of 
supplying  power  to  the  indicator  may  be  mentioned.  This  is  not  taken 


THERMO- 
COUPLE 


FIG.  2. — AUTOMATIC  SIGNALING  PYROMETER. 

from  the  thermocouple  but  the  indicator  is  shown  in  a  Wheatstone  bridge 
attached  to  the  line,  preferably  110-volts  alternating  current.  The 
movement  of  the  disk  carrying  the  slide  wire  S  and  the  three  contacts 
connected  to  the  lamps  results  in  connecting  the  proper  lamp  and  in 
unbalancing  the  bridge  circuit.  The  meter  may  be  of  high  resistance  and 
its  indications  are  easily  checked  by  the  recorder  indication  and  correc- 
tion accomplished  with  the  adjustable  resistance  in  the  circuit.  It  is  only 
intended  to  show  the  magnitude  of  departures  from  correct  furnace 
temperatures  and  a  large  variation  in  line  voltage  would  not  be  serious. 
Fig.  5  shows  the  use  of  the  double  recorder,  or  curve-drawing  recorder, 
with  a  commutator  for  two  couples,  which  gives  the  form  of  record  shown 
in  Fig.  6.  This  instrument  is  also  used  in  the  automatic  control  arrange- 
ment shown  in  Fig.  9. 

The  advantges  of  automatic  signaling  are  not  always  well  defined  and 


444 


HIGH-TEMPERATURE   CONTROL 


Hide  Wires  8  and  8,  are  mounted  o 

the  same  shaft  and  are  adjustable  Vj  ' 

with  respect  io  eac-h  other 


AND  SIGNAL  LAMPS 

CIRCUITS  FOR  OPERATION  OP  FURNACE  INDICATOR 


Thermocouple 
Connection 


FIG.  3. 


Iron-Couatantu  Lead  Wires'* 

2  Dry  Cells  In  Parallel 
-^*4r         Sna]>  Switch 

^i,  Motor  Volta 

--'"'""               Red~La~"  -fet" 

i  Range  100*100               1   **¥** 
Thermocouple    ' 

Furnace 

—  \                                    / 

Us-  T 

I1—  1-|:      )•  Automatic  Cold- 
's^- -junction  Cojnpensating 
I.                     noil 

RECORDER  INSTALLED    IN  CONNECTION                                  Resistance  Units, 
WITH    THERMOCOUPLE     IN      FURANCE                         u   ^  —  -irrif,  —  «w£T                 ,    - 

AND  OPERAT  NG  FURNACE  INDICATOR                                                                                       15 
AND   3  GNAL  LAMPS 

Becorder 

^aa 

^^  /                      \ 

Snap  Switch 

FIG.  4. 

1)0-1-«,           No'z     ffi~ 

ermocouple  W   Thermocouple  "TT"     i          . 

2  Drj  Cell;  in  Parallel 

i  —  n^3_             Sn«P  Switch 

[ir^onnect  to 
Q  Motor  To! 

White     "^it-S 

Range  lOu-0-100          L  iWiW 

Furnace 

CURVE-DRAWINO  RECORDER  WITH 
COMMUTATOR,    READING    ALTER- 
NATELY ON  TWO  THERMOCOUPLES           LiDe  —  ^W^-Win  ,    : 

r**~f\>           t     / 

=«r^         Recorder 

AT  THE  FURNACE;  THE  GREEN  LAMP 

IN  CIRCUIT                                                                        j^-1 
Snap  Switch                         C»ld  J 
Compen 

pLJ/                          \ 

unction 
lating  Coll 

FIG.  5. 
FIGS.  3,  4,  5. — AUTOMATIC  SIGNALING  RECORDER,  POTENTIOMETER  TYPE. 


C.    O.    FAIRCHILD    AND    PAUL   D.    FOOTE 


445 


the  method  has  not  been  put  to  very  extensive  use.  In  cases  where  very 
exact  temperature  measurements  and  control  are  necessary  and  the  sensi- 
tivity demanded  results  in  an  instrument  that  must  be  removed  a  distance 
from  the  furnace  or  oven,  automatic  signaling  may  be  required;  but  in 
most  circumstances,  the  proper  placing  of  the  indicator  or  indicators 
will  make  automatic  signaling  unnecessary.  To  make  one  indicator  oper- 
ate many  separate  systems  of  signaling  would  require  a  complicated  and 
cumbersome  commutator.  Such  an  arrangement  might  take  the  place 
of  some  of  the  observers  in  the  central  pyrometer  station  of  a  large  plant, 
but  sensitivity  and  flexibility  would  be  sacrificed. 

Automatic  Temperature  Control. — Temperature  control  at  low  ranges 
is  termed  thermostating  and  is  ordinarily  obtained  by  means  of  the  move- 
ment of  bimetallic  springs,  or  the  thermal  expansion  of  rods  or  fluid  col- 
umns. At  high  temperatures  these  devices  are  utilized  only  with  great 


FIG.  6. — RECORD  OF  LEEDS  &  NORTHRUP  DOUBLE  CURVE-DRAWING  RECORDER. 


difficulty,  and  thermoelectric  instruments  are  substituted.  The  principle 
•of  operation  is  similar  to  that  used  in  automatic  signaling.  Electro- 
magnetic impulses  arising  in  the  pyrometer  indicator  or  controller  are 
utilized  to  operate  oil  or  gas  valves  or  electric  switches.  In  case  the  valve 
or  switch  is  large  and  the  electromagnetic  operation  requires  more  than 
a  few  hundred  watts,  electric  relays  are  incorporated  between  the  control- 
ler and  the  valve. 

Automatic  temperature  control  is  complicated  by  the  interrelation 
of  such  factors  as  thermal  lag  and  the  magnitude  of  the  corrective  change 
in  heat  supply  during  each  period  of  reversal.  The  sensitivity  of  the 
control  instrument  determines  the  range  of  temperature  that  must  be 
covered  by  the  thermocouple  (or  other  pyrometer)  during  each  reversal 
of  the  valve  or  switch.  The  range  of  temperature  covered  at  the  source 
of  heat  is  greater  than  this  and  the  difference  is  determined  by  the  type  of 


446  HIGH-TEMPERATURE    CONTROL 

furnace,  the  method  of  heating,  and  the  locating  of  the  thermocouple. 
For  closest  regulation,  which  is  best  accomplished  electrically,  the  couple 
must  be  very  near  if  not  in  contact  with  the  heater.  Considering  the 
cycle  of  operations,  if  a  furnace  is  cooling  the  current  will  be  increased 
when  the  couple  has  dropped  a  certain  amount,  the  heater  being  at  a  still 
lower  temperature.  The  current  may  be  increased  by  a  single  step  upon 
reaching  this  point  or  a  mechanical  arrangement  may  be  used,  which  will 
continue  to  increase  the  current  so  long  as  the  couple  has  not  reached  the 
upper  limit  of  temperature.  The  single  step  will  ordinarily  give  the  least 
over-correction  of  heat  supply  and  the  method  is  the  most  simple  to  apply. 
It  will  fail  to  give  the  desired  result,  however,  if  there  exists  a  condition 
of  wide  variation  in  the  amount  of  heat  absorbed  or  lost  in  the  furnace 
operation,  for  in  this  case  the  heating  current  change  during  a  reversal 
must  be  large.  A  mechanism  that  is  always  increasing  or  decreasing  the 
heat  supply  in  small  steps  may  often  be  advantageous  and  is  practically 
indispensable  in  the  arrangement  mentioned  below,  for  automatic  control 
of  heating  or  cooling  rates. 

In  industrial  equipment  only  the  single-step  method  has  been  applied. 
In  electric  heating,  the  heating  current  is  changed  by  opening  or  closing 
a  switch,  which  will  shunt  a  rheostat,  change  the  series  or  parallel  con- 
nections of  the  heater,  or  change  the  secondary  side  of  an  automatic 
transformer.  In  gas  or  oil  heating,  the  supply  pipe  is  by-passed  and  the 
control  valve  is  placed  in  this  shunting  line. 

Fig.  7  illustrates  the  Brown  automatic  temperature-control  pyrome- 
ter, showing  the  galvanometer,  motor  for  raising  and  lowering  the 
depressor  frame,  and  the  solenoid  switches.  The  galvanometer  is  the 
high-resistance  type  with  300  ohms  for  a  base-metal  couple  and  a  scale 
range  of  1100°  C.  The  control  is  between  limits  1  per  cent,  of  the  scale 
range  in  extent;  that  is,  it  will  control  to  about  10°  for  the  above  range. 
The  range  can  be  lowered  considerably  for  closer  regulation,  say  to  3°  C. 
without  serious  disadvantage.  By  using  a  method  for  manual  correc- 
tion for  resistance  and  a  low-resistance  meter  with  a  very  open  scale, 
the  control  may  be  easily  perfected  for  less  than  a  1°  variation.  Obvi- 
ously the  cold  junction  of  the  thermocouple  must  either  be  buried  deeply 
in  the  ground  or  placed  in  ice  or  a  thermostated  box.  For  temperatures 
below  425°  C.,  Brown  substitutes  a  nitrogen-gas  thermometer,  which 
makes  it  possible  to  control  to  0.2°  and  better,  with  a  scale  range  of  10 
or  15°  C.  Fig.  8  illustrates  the  form  of  electromagnetically  operated 
valve  that  has  been  developed.  The  Bristol  company  has  also  adapted 
the  instrument  described  for  signaling,  so  that  it  will  control  the  furnace 
temperature  automatically.  This  company  has  patented  a  valve  some- 
what different  from  the  one  illustrated,  but  designed  to  accomplish  the 
same  results. 

The  General  Electric  Co.,  in  conjunction  with  the  Leeds  &  Northrup 
Co.,  has  developed  an  apparatus  for  automatically  controlling  the  opera- 


C.    O.    FAIRCHILD    AND    PAUL    D.    FOOTE 


447 


tion  of  electric  furnaces  for  hardening  steels.  Fig.  9  is  a  wiring  diagram 
of  the  control  panel.  This  equipment  automatically  heats  a  hardening 
furnace  up  to  a  temperature,  say  1000°  C.,  somewhat  above  that  desired, 
say  900°  C.,  and  holds  it  there  until  the  piece  being  treated  reaches  900°, 
when  the  furnace  temperature  is  dropped  to  this  value  and  held  there. 
This  unique  operation  is  accomplished  by  placing  one  thermocouple, 


FIG.    7. — AUTOMATIC  TEMPERATURE- 
CONTROL  PYROMETER. 


FIG.  8. — ELECTROMAGNETICALLY 
OPERATED  GAS  OR  OIL  VALVE. 


called  the  contact  couple,  in  contact  with  the  piece,  and  another,  called  the 
air  couple,  in  the  furnace  near  the  wall.  The  air  couple  is  kept  hot  until 
the  contact  couple  reaches  the  proper  temperature  when  the  air  tempera- 
ture is  lowered  to  this  value.  The  wiring  diagram  of  Fig.  9  is  clear  upon 
close  study  of  the  connections  and  a  discussion  is  scarcely  necessary.2  The 

2  In  this  diagram  the  following  must  be  noted:  (1)  Control  relays  are  closed  when 
E  is  connected  to  6  and  Si  is  connected  to  7.  (2)  Connecting  Si  and  7  closes  the 
heating  .circuit  by  the  magnetic  switch  on  the  left.  (3)  Contact  thermocouple 
operates  through  contacts  marked  Si,  Ri,  and  R2,  and  air-couple  contacts  <S2,  Rs, 
RS,  and  E.  (4)  Si  and  Rt  are  connected  and  open  a  relay  when  contact-couple 
reaches  the  desired  temperature. 


448 


HIGH-TEMPERATURE    CONTROL 


contacts  that  operate  the  relays  are  opened  and  closed  by  the  automatic 
adjustment  of  the  potentiometer  slide  wire.  By  disconnecting  either 
of  these  thermocouples  and  one  of  the  control  relays,  the  instrument  may 
be  used  simply  to  hold  a  furnace  at  a  fixed  temperature.  Another  appli- 
cation of  the  potentiometer  type  of  automatic  controller  maintains  the 
temperature  of  the  box  ovens  used  in  japanning  at  the  proper  point. 


BACK  VIEW  OF 
PANELWIRING 


~T=—h 

Air|Thermocoini 

FIG.  9. — WIRING  DIAGRAM  OF  AUTOMATIC  CONTROL  PANEL  FOR  ELECTRIC  FURNACES. 

It  is  made  to  control  at  one  temperature  for  a  certain  interval  and  then 
automatically  changes  to  another  temperature  and  maintains  this. 

Still  another  operation  can  be  performed  with  this  instrument.  One 
of  the  writers  has  constructed  an  apparatus  that  will  heat  or  cool  a  fur- 
nace at  a  predetermined  rate;  the  rate  may  be  constant  or  a  function  of  the 
temperature.  This  apparatus  has  been  used  in  studying  the  annealing 


C.    O.    FAIRCHILD    AND    PAUL   D.    FOOTE  449 

of  glass,  especially  the  fine  annealing  of  optical  glass.  This  type  of  con- 
trol is  obtained  by  placing  one  contact  on  the  potentiometer  slide  wire 
and  the  other  two  contacts  upon  a  disk  that  is  made  to  turn  slowly  at  a 
chosen  rate.  The  disk  is  turned  by  the  controlling  recorder  itself,  so 
that  the  whole  apparatus  is  automatic.  This  principle  of  moving  con- 
tacts is  applicable  to  any  of  the  controllers  described,  but  at  present  such 
control  has  not  been  utilized  industrially. 

As  a  thermostat,  the  potentiometer  recorder-controller  can  be  made 
capable  of  operating  upon  0.01°  changes  in  the  temperature  of  a  resist- 
ance thermometer  and  upon  less  than  0.5°  with  base-metal  thermocouples. 
Very  rapid  progress  is  properly  expected  in  the  future  development  of 
instruments  for  automatic  temperature  control  at  higher  temperatures. 
The  field  is  a  comparatively  new  one  and  undoubtedly  offers  possibilities 
that  are  not  fully  appreciated  at  the  present  time. 

SUMMARY 

The  general  problem  of  the  control  of  furnaces,  kilns,  ovens,  tanks' 
etc.  operated  at  high  temperatures  is  intimately  associated  with  the 
measurement  of  temperature  and  its  variations.  A  knowledge  of  exist- 
ing temperatures  may  be  made  useful  by  the  proper  selection  and 
installation  of  pyrometers  and  a  careful  study  of  the  relation  of  tem- 
perature variations  to  other  factors  involved  in  control.  These  general 
considerations  have  been  discussed  and  a  short  description  has  been 
given  of  the  devices  used  in  high-temperature  control. 

DISCUSSION 

R.  W.  NEWCOMB,  New  York,  N.  Y.  (written  discussion*). — On  page 
446,  the  middle  paragraph  states  that,  in  industrial  equipment,  only  the 
single-step  method  of  automatic  regulation  has  been  applied.  Quite 
recently,  there  has  been  developed  an  automatic  temperature  regulator, 
operating  in  conjunction  with  a  pyrometer  of  the  thermoelectric  type, 
in  which  the  control  is  a  slow  regulation,  with  a  range  capable  of  regulat- 
ing valves,  dampers,  rheostats,  or  any  other  rotatable  member,  through 
one  or  more  complete  revolutions.  It  can  control  two  valves  at  the  same 
time,  with  a  fixed  definite  ratio  between  them. 

Aside  from  those  conditions  in  which  it  is  necessary  to  control  two 
valves  with  a  definite  ratio  one  to  the  other,  the  greatest  advantage  that 
this  slow,  even  control  will  have  over  those  controls  that  are  either  all 
on  or  all  off,  will  be  for  use  in  connection  with  processes  where  a  large 
temperature  variation  is  required,  extended  over  a  considerable  period 
of  time;  that  is,  where  the  temperature  must  be  regulated  along  an  in- 
creasing or  decreasing  time-temperature  curve. 

*  Received  Oct.  15,  1919. 

29 


450  RESISTANCE   THERMOMETRY 


Resistance  Thermometry 

BY    F.    W.    KOBINSON,*   M.    SC.,    NEWARK,    N.    J. 
(Chicago  Meeting,  September,  1919) 

THE  temperature  coefficient  of  electrical  resistance  of  pure  metals 
is  high  and  therefore  the  resistance  increases  rapidly  with  rising  tempera- 
ture. In  1871,  Siemens  suggested  the  use  of  this  property  as  an  accurate 
means  of  temperature  determination.  Owing  to  practical  difficulties, 
particularly  the  contamination  of  the  metal  and  consequent  permanent 
change  in  its  resistance  and  its  resistance-temperature  relation,  this 
method  fell  into  disrepute  as  a  practical  standard  until  revived  later  by 
Callendar  and  Griffiths,  and  subsequently  by  Holborn  and  Wien,  all  of 
whom  showed  that  the  earlier  difficulties  were  not  inherent  in  the  method 
but  incident  to  the  mode  of  protection  of  the  resistance  coils.  Following 
the  work  of  these  investigators,  the  problem  of  temperature  measure- 
ment by  this  means  has  been  the  subject  of  careful  study  and  has  now 
assumed  an  importance  second  only  in  practical  adoption  to  the  thermo- 
electric method. 

It  is  generally  recognized,  both  here  and  in  Europe,  that  the  standard 
temperature  scale  should  be  the  thermodynamic  as  it  permits  the  evalua- 
tion of  high  temperatures  on  the  basis  of  the  radiation  laws  of  Stefan 
and  Bolzmann,  of  Rayleigh,  and  of  Wien  and  Planck,  on  a  scale  consistent 
with  that  obtained  by  means  of  the  gas  thermometer  at  low  temperatures. 
Lord  Kelvin  showed  that  only  a  very  small  correction,  amounting  to 
about  +0.7°  at  1000°  C. — almost  within  the  limits  of  experimental  errors — 
is  necessary  to  adjust  the  constant-volume  nitrogen  thermometer  to  the 
ideal  thermodynamic  scale. 

On  this  basis  the  Bureau  of  Standards,  Washington,  and  the  National 
Physical  Laboratory,  London,  have  established  a  fixed-point  scale  giving 
fixed-point  temperatures  up  to  1083°  C.,  the  melting  point  of  copper  in  a 
reducing  atmosphere.  This  scale  has  now  been  generally  adopted  in 
this  country  and  in  England,  but  progress  in  this  important  subject  was 
undoubtedly  hindered  by  the  war.  A  most  significant  conference  of  the 
Bureau  of  Standards  and  the  National  Physical  Laboratory  was  to  have 
been  held  in  Berlin  in  September,  1914,  with  the  German  authorities  of 
the  Reichsanstalt.  The  loss  to  science  and  the  industries  dependent  on 
pyrometry  through  the  enforced  cancellation  of  the  meeting  has  unques- 
tionably been  great. 

*  Manager,  Electrical  Dept.,  Hanovia  Chem.  &  Mfg.  Co. 


F.    W.    ROBINSON  451 

For  the  standardization  of  the  resistance  thermometer,  we  are  limited, 
for  all  practical  purposes,  to  the  fixed  points  of  the  standard  scale  falling 
within  the  range  of  practical  usefulness  of  the  resistance  thermometer. 
In  the  case  of  the  platinum  resistance  thermometer,  these  are  the  freezing 
point  of  mercury  at  —38.88°  C.,  the  melting  point  of  ice  0°,  the  trans- 
formation point  of  sodium  sulfate  32.384°,  the  vapor  of  water  boiling 
under  atmospheric  pressure  100°,  the  boiling  point  of  naphthalene  217.96°, 
the  boiling  point  of  benzophenone  305.9°,  and  the  boiling  point  of  sulfur 
444.5°. 

The  temperature  on  the  international  scale  t  is  then  deduced  from  the 
formula 


/    n  D     » 

Where  pt  =  100  X   (B~~          ^-)and  #,  #Q,  and  RK>O  are  the  measured 
Ytiioo  —  /to/ 

resistances  of  the  thermometer  at  temperature,  Z°,  0°,  100°.  Over  the  limits 
of  this  scale,  the  deviations  of  the  platinum  resistance  thermometer  from 
the  hydrogen  scale  of  the  International  Bureau  lie  within  the  limits  of 
experimental  error  and  for  most  practical  purposes  the  correspondence 
is  sufficiently  close  down  to'  the  boiling  point  of  oxygen  —182.9°  C.  and 
up  to  the  boiling  point  of  copper  1083°  (in  reducing  atmosphere).  The 
accuracy  within  these  wide  limits,  as  determined  by  various  experimenters, 
has  shown  somewhat  varying  results  and  might  well  be  made  the  subject 
of  a  careful  and  thorough  investigation.  For  use  in  making  resistance 
thermometers,  the  platinum  should  be  of  such  purity  that  the  value  of 

D 

5  in  the  above  equation  is  not  greater  than  1.52,  and  -j~-  should  not  be 
less  than  1.386. 

ADVANTAGES  OP  ELECTRIC  RESISTANCE  METHOD 

The  resistance  method  of  temperature  determination  possesses  for 
the  practical  range  of  the  instrument,  as  indicated  in  the  foregoing,  several 
very  important  advantages  over  all  other  methods  of  determination. 
The  temperature  coefficient  of  resistance  of  pure  platinum  is  such  that  for 
an  increase  of  temperature  of  300°  from  0°  C.  the  resistance  of  the  spiral 
is  more  than  doubled  and  this  increase  is  maintained  at  practically  the 
same  rate  up  to  the  highest  temperatures.  Knowing  the  high  degree  of 
accuracy  with  which  electrical  resistance  may  be  determined  by  relatively 
simple  apparatus,  the  great  sensitiveness  of  this  method  is  at  once  appar- 
ent. Using  the  usual  Wheatstone  bridge  method  in  one  or  other  of  its 
forms,  the  temperature  scale  on  commercial  instruments'  can  be  arranged 
for  any  desired  temperature  interval.  Taking  a  common  type  of  galva- 
nometer with  a  scale  5  in.  (12.7  cm.)  long,  the  instrument  may  be  graduated, 


452  RESISTANCE   THERMOMETRY 

in  degrees,  for  a  temperature  scale  beginning  at  600°  and  ending  at  700°. 
Such  a  scale  can  be  read  without  difficulty  to  one-fourth  of  1°.  In  com- 
parison with  the  thermoelectric  instrument  in  which  the  scale  must 
always  begin  at  zero,  the  increase  in  sensitiveness  of  the  electric  resistance 
method  is  very  great.  A  further  advantage  of  the  electric  resistance 
method  is  the  avoidance  of  cold-junction  errors  inherent  in  the  thermo- 
electric type. 

LIMITATION  OF  ELECTRIC  RESISTANCE  METHOD 

For  practical  purposes,  the  range  of  the  electric  resistance  thermometer 
covers  the  field  from  —  200°  C.  to  +900°  C.  For  the  measurement  of  tem- 
peratures by  this  method,  an  outside  source  of  current  is  essential;  and 
for  most  of  the  commercial  instruments  of  a  direct-reading  type,  a  storage 
battery  or  standard  cell  is  used  to  provide  this  current.  The  care  of  the 
storage  battery  under  circumstances  where  direct  current  is  not  available 
for  charging  is  one  disadvantage  of  the  method.  This  may  be  overcome 
by  the  use  of  dry  cells;  but  owing  to  the  inconstancy  of  the  dry-cell 
voltage,  the  remedy  is  rather  worse  than  the  disease. 

CONSTRUCTION  AND  PROTECTION  OF  RESISTANCE  SPIRALS 

Platinum  is  most  generally  used  as  the  resistance  metal.  It  can  be 
readily  obtained  in  a  chemically  pure  state  and  is  applicable  to  a  wide 
temperature  range.  Iridium,  palladium,  and  rhodium  have  all  been 
suggested,  but  are  apparently  not  in  commercial  use.  Nickel  is  some- 
times used,  but  is  not  recommended  for  temperatures  higher  than  250°  C., 
owing  to  the  change  in  the  temperature-resistance  curve  as  the  transi- 
tion temperature  of  nickel  is  approached  and  to  the  danger  of  oxidation 
of  the  metal  at  higher  temperatures.  According  to  Marvin,1  the  equation 
log  R  =  a  +  mt  holds  approximately  over  the  range  0-300°  C.  Molten 
tin  was  recommended  in  1916  by  E.  F.  Northrup  and  R.  C.  Sherwood,2 
who  find  that  the  resistance  temperature  relation  gives  a  straight-line 
curve  up  to  temperatures  between  1600°  and  1700°  C. 

One  common  method  of  mounting  the  resistance  wire  is  that  devised 
by  Callendar,  consisting  of  crossed  serrated  mica  plates  on  which  the 
platinum  wire  is  spirally  wound.  This  form  is  used  by  the  Leeds  & 
Northrup  Co.  and  by  the  Cambridge  Scientific  Instrument  Co.,  though 
in  some  instances  Leeds  &  Northrup  replace  the  mica  frame  by  steatite. 
With  this  form  of  spiral,  it  is  necessary,  in  order  to  protect  the  plati- 
num from  contamination,  to  mount  it  in  an  impervious  tube  such  as 


1  Electric  Resistance  of  Nickel  to  300°  C.     Phys.  Rev.  (1910)  30,  522. 

2  Jnl.  Frank.  Inst.  (1916)  182,  493. 


F.    W.    ROBINSON  453 

glazed  porcelain,  usually  protected  on  the  outside  by  a  tube  of  iron  or 
nickel.  For  very  high  temperatures,  the  Leeds  &  Northrup  Co.  uses  a 
form  of  potential  lead  thermometer.  Heavy  wire  is  used  in  the  coil  and 
is  freely  suspended  between  steatite  disks.  Owing  to  its  very  low 
resistance,  special  precautions  are  necessary  with  this  instrument  to 
obtain  a  satisfactory  degree  of  sensitivity. 

Another  type  of  spiral  is  that  manufactured  by  the  Hanovia  Chemical 
&  Mfg.  Co.,  in  which  the  platinum  spiral  is  wound  on  a  thin  tube  of 
transparent  quartz  with  an  outer  jacket  of  transparent  quartz  melted 
down  on  to  the  inner  core  so  that  the  platinum  wire  is  firmly  embedded 
in  the  quartz.  This  construction  gives  an  instrument  of  very  small 
volume  in  which  the  resistance  wire  is  thoroughly  protected  from  the 
contaminating  influence  of  dirt  and  reducing  gases.  Owing  to  the  small 
volume  of  the  instrument,  this  form  of  thermometer  follows  temperature 
changes  very  rapidly  and  each  spiral  can  be  accurately  adjusted  to  a 
standard  resistance  within  0.04  per  cent.  This  standardization  of  the 
resistance  coil  obviates  the  necessity  of  auxiliary  manganin  coils  in  the 
headpiece  used  .with  other  types  of  resistance  thermometers.  For  the 
most  accurate  calorimetric  work,  this  construction  is  not  recommended, 
owing  to  the  slight  change  in  the  constants  of  the  equation.  This  change, 
however,  is  not  of  sufficient  magnitude  to  impair  the  accuracy  for  ordinary 
laboratory  and  industrial  measurements.  As  mounted  for  industrial 
use  in  a  steel  or  copper  tube,  or  in  a  perforated  sheath  for  air  temperatures, 
the  quartz  resistance  thermometer  forms  a  very  rugged  and  convenient 
instrument. 

When  measuring  at  relatively  high  temperatures,  the  resistance  of 
the  thermometer  leads,  graduating  from  the  high  temperature  to  be 
measured  down  to  the  comparatively  cool  headpiece,  requires  special 
precautions  to  avoid  the  introduction  of  errors.  In  the  quartz  ther- 
mometers of  the  Hanovia  Chemical  &  Mfg.  Co.,  this  source  of  error  is 
obviated  by  terminating  the  leads  immediately  above  the  spiral  and  con- 
tinuing the  electrical  connection  with  the  headpiece  through  heavy 
metal  rods  of  low-temperature  coefficient.  The  method  originally  sug- 
gested by  Siemens  and  adopted  by  Leeds  &  Northrup  Co.  for  indus- 
trial use  consists  of  a  third  lead  of  the  same  wire  connected  on  the  upper 
end  of  the  thermometer  spiral  to  one  of  the  thermometer  leads  proper. 
This  loop  is  connected  in  series  with  the  balancing  resistance  of  the  bridge 
and  accurately  compensates  the  lead  resistance  where  a  zero-deflection 
instrument  is  used.  In  a  direct-reading  Wheatstone  bridge,  however, 
the  relation  of  the  thermometer  resistance  to  the  fixed  resistance  is 
slightly  altered  by  this  arrangement  and  a  small  error  remains  in  the 
reading.  Appended  is  a  table  showing  the  resistance,  up  to  900°  C., 
of  platinum  thermometers  having  a  resistance  25  or  50  ohms  at  the 
temperature  of  melting  ice. 


454 


RESISTANCE   THERMOMETRY 


TABLE  1. — Resistance  Values  for  Quartz  Resistance  Thermometers 

Rt  =  Ro  (1    +   at   +  #2) 


Degrees  C. 

50  J2  at  0  Degrees 

Difference 

Degrees  C. 

50  Jl  at  0  Degrees 

Difference 

-200 

9.50 

+400 

123.60 

1.72 

190 

11.58 

2.08 

410 

125.31 

1.71 

180 

13.66 

2.08 

420               127  02 

1.71 

170 

15.73 

2.07 

430               128.72 

1.70 

160 

17.79 

2.06 

440               130.42 

1.70 

150 

19.85 

2.06 

450               132.10 

1.68 

140 

21.90 

2.05 

460               133.78 

1.68 

130 

23.95 

2.05 

470               135.46 

1.68 

120 

25.99 

2.04 

480               137.13 

1.67 

110 

28.03 

2.04 

490               138.79 

1.66 

-100 

30.06 

2.03 

+500               140.45 

1.66 

90 

32.08                2.02 

510               142.10 

1.65 

80 

34.09                2.01 

520               143.75 

1.65 

70 

36.10 

2.01 

530               145.30 

1.64 

'  60 

38.11 

2.01 

540               147.02 

1.63 

50 

40.11 

2.00 

550               148.65 

1.63 

40 

42.10 

1.99 

560               150.27 

1.62 

30. 

44.08 

1.98 

570               151  .  88 

1.61 

20 

46.06 

1.98 

580               153.49 

1.61 

10 

48.03 

1.97 

590               155.09 

1.60 

+     0 

50.00 

1.97 

+600               156.69 

1.60 

10 

51.96 

1.96 

610 

158.28 

1.59 

20 

53.91 

1.95 

620 

159.86 

1.58 

30 

55.86 

1.95 

630 

161.44 

1.58 

40 

57.80 

1.94 

640 

163.01 

1.57 

50 

59.74 

1.94 

650 

164.57 

1.56 

60 

61.67 

1.93 

660 

166.13 

1.56 

70 

63.59 

1.92 

670 

167.68 

1.55 

80 

65.51 

1.92 

680 

169.23 

1.55 

90 

67.42 

1.91 

690 

170.77 

1.54 

+  100 

69.33 

1.91 

+  700 

172.30 

1.54 

110 

71.23 

1.90 

710 

173.83 

1.53 

120 

73.12 

1.89 

720 

175.35 

1.52 

130 

75.00 

1.88 

730 

176.86 

1.51 

140 

76.88 

1.88 

740 

178.37 

1.51 

150 

78.76 

1.88 

750 

179.88 

1.51 

160 

80.63 

1.87 

760 

181.38 

1.50 

170 

82.49 

1.86 

770 

182  .  87 

1.49 

180 

84.34 

1.85 

780 

184.35 

1.48 

190              86.19 

1.85 

790 

185.82 

1.48 

+200              88.03 

1.84 

+800 

187.30 

1.47 

F.    W.    ROBINSON 


455 


TABLE  1. — Resistance  Values  for  Quartz  Resistance  Thermometers 

(Continued) 


Degrees  C. 

50  a  at  0  Degrees 

Difference 

Degrees  C.       50  fl  at  0  Degrees        Difference 

210 

89.87 

1.84 

810 

188.77 

1.47 

220 

91.70 

1.83 

820 

190.23 

1.46 

230 

93.52 

1.82 

830 

191.68 

1.45 

240 

95.34 

1.82 

840 

193.13 

1.45 

250 

97.15 

1.81 

850 

194.57 

1.44 

.      260 

98.96 

1.81 

860 

196.01 

1.44 

270 

100.76 

1.80 

870 

197.44 

1.43 

280 

102.56 

1.80 

880 

198.86 

1-42 

290 

104.35 

1.79 

890              200.28                1.42 

+300 

106.13 

1.78 

+900     -         201.69                1.41 

310 

107.90 

1.77 

320 

109.67 

1.77 

330 

111.43 

1.76 

340 

113.19 

1.76 

350 

114.94 

1.75 

360 

116.68 

1.74 

370 

118.42 

1.74 

380 

120.15 

1.73 

390 

121.88 

1.73 

+400 

123.60 

1.72 

COMMON  TYPES  OF  MEASURING  APPARATUS 

Any  of  the  usual  methods  of  measuring  electrical  resistance  may  be 
applied  to  resistance  thermometers.  For  precision  work,  where  high 
laboratory  standards  of  accuracy  are  required,  either  the  Kelvin  double 
bridge  may  be  used  or  the  potential  drop  measured  across  the  terminals 
of  the  thermometer.  A  very  sensitive  arrangement  is  the  thermometer 
bridge  designed  by  the  National  Bureau  of  Standards  and  manufactured 
by  the  Leeds  &  Northrup  Co.  using  a  four-lead  thermometer  to  compensate 
for  the  temperature  rise  in  the  thermometer  leads. 

For  most  industrial  instruments,  however,  the  method  in  vogue 
is  almost  universally  some  modification  of  the  Wheatstone  bridge.  The 
method  of  the  Hanovia  Chemical  &  Mfg.  Co.  and  of  the  Cambridge 
Scientific  Instrument  Co.,  as  shown  in  Fig.  1,  gives  the  favorite  form  of 
direct  temperature  reading  instruments.  All  the  resistance  coils,  /, 
II,  and  ///  of  the  bridge  are  of  fixed  value  and  the  variations  in 
the  thermometer  temperature  are  graduated  on  the  galvanometer, 
in  temperature  degrees.  By  selecting  the  corresponding  resist- 


456 


RESISTANCE   THERMOMETRY 


a-ice  values  for  the  coil  777,  the  temperature  scale  is  so  arranged  as  to 
b^gin  at  any  desired  temperature. 

The  other  system  in  current  use  is  that  of  the  ohmmeter.  This 
method  is  used  by  the  Leeds  &  Northrup  Co.,  by  Paul  of  London,  by 
Carpentier,  and  others.  In  it  a  variable  resistance,  that  of  the  thermome- 
ter, is  balanced  against  a  known  resistance  by  means  of  a  zero-point 
galvanometer  reading.  The  temperature  scale  is  usually  indicated  by  a 
dial  and  pointer  on  the  resistance  box.  For  temperature  indicating  in 
shop  practice,  the  Leeds  &  Northrup  Co.  use  a  slide-wire  balancing 
resistance  marked  in  degrees  of  temperature  with  a  center  zero  voltmeter 
as  the  indicator.  The  slide- wire  resistance  is  adjusted  to  correspond  to 
the  temperature  desired  in  the  furnace  and  the  indicator  shows  a  deflec- 
tion +  or  —  according  as  the  furnace  temperature  is  higher  or  lower  than 
that  for  which  the  balancing  resistance  is  set.  When  the  indicator  needle 


— AAAAAAAA- 


FIG.  1. — COMMON  ARRANGEMENT  OF  RESISTANCES  OF  DIRECT  TEMPERATURE-READING 

INSTRUMENTS. 


is  in  the  center,  the  bridge  is  in  equilibrium  and  is  independent  of  the 
voltage  of  the  applied  current.  The  instrument  is  therefore  made  to 
use  lighting-circuit  current.  For  the  control  of  certain  types  of  furnace, 
this  arrangement  gives  a  convenient  and  satisfactory  instrument;  but 
for  the  measurement  of  unknown  temperatures,  the  constant  adjustment 
of  the  balancing  resistance  is  somewhat  annoying  and  the  temperature 
variations  +  or  --  are  vitiated  by  voltage  fluctuations,  which  always 
occur  in  commercial  lighting  circuits. 

In  the  direct-reading  type  of  measuring  instrument,  a  recording 
galvanometer  is  often  substituted  for  the  indicating  type  and  most 
manufacturers  put  out  such  instruments  either  for  recording  on  a  single 
thermometer,  or,  through  the  medium  of  an  automatic  switch  arrange- 
ment, for  recording  on  a  number  of  points  simultaneously  on  a  running 
paper  chart.  Records  of  the  individual  thermometers  are  either  num- 


F.    W.    ROBINSON  457 

bered  or  are  made  of  different  colors  to  identify  the  record  of  the  respec- 
tive thermometers. 

All  in  all,  the  platinum  resistance  thermometer  over  the  range  for 
which  it  is  applicable  (—  200°  to  +  900°  C).  provides  the  most  convenient 
and  reliable  method  of  temperature  determination  and  record,  particu- 
larly where  the  measurements  are  required  at  one  central  point  or 
at  some  distance  from  the  source  of  heat.  They  possess,  generally 
speaking,  a  much  greater  freedom  from  errors  liable  to  be  overlooked  in 
other  types  of  instruments.  With  reasonable  care  in  use,  they  are  little 
subject  to  disturbance  in  operation,  can  be  calibrated  for  any  tempera- 
ture scale,  and  with  suitable  protection  almost  any  degree  of  sensitiveness 
can  be  secured  for  any  desired  temperature  range. 

Some  of  the  more  important  industrial  applications  of  the  resistance 
thermometer  are:  Marine,  railway,  and  stationary  refrigerating  plants, 
both  for  control  of  the  operating  temperatures  of  the  refrigerating  plant 
and  of  the  cold  storage  rooms;  drying  ovens  of  various  types,  such  as 
photographic-film  drying  rooms,  core  ovens,  baking  ovens  for  enamel, 
metal,  and  leather,  and  wood-drying  kilns;  the  control  of  the  lehr  tem- 
peratures in  glass  annealing;  the  control  of  reaction  temperatures  in  the 
manufacture  of  sulfuric  acid  by  the  contact  process;  the  control  of  frac- 
tionating temperatures  in  oil  refineries;  for  boiler  testing,  flue  gases, 
feedwater,  and  superheater  temperatures  as 'well  as  the  temperature  of 
the  bearings  in  turbine  engines;  and  the  control  of  innumerable  chemical 
processes. 


458  RESISTANCE   THERMOMETRY    FOR    INDUSTRIAL    USE 


Resistance  Thermometry  for  Industrial  Use 

BY    CHARLES    P.    FREY,  *   PHILADELPHIA,    PA. 
(Chicago  Meeting,  September,  1919) 

THE  fundamental  principle  of  resistance  thermometry  lies  in  the 
determination  of  temperatures  by  the  measurement  of  an  electrical 
conductor  subjected  to  various  temperatures  and  the  translation  of  the 
resultant  changes,  in  ohms,  into  temperature  equivalents.  Such  re- 
sistance measurements  can  be  made  with  maximum  precision  by  the  use 
of  a  standard  Wheatstone  bridge  and  a  reflecting  galvanometer  of  high 
sensitivity.  Under  such  conditions,  results  are  obtainable  that  may  be 
accurate  within  0.1  or  even  0.01  per  cent.,  and  the  "idiosyncrasies" 
of  heated  electrical  conductors  can  be  studied  with  advantage  and  profit, 
even  if  the  latter  is  not  of  a  financial  nature. 

The  chief  requisites  for  producing  a  serviceable  commercial  instru- 
ment may  be  enumerated  as  follows :  First,  there  must  be  a  resistor  or 
"bulb"  that  can  stand  the  maximum  temperature  without  deterioration 
and  which  has  a  pronounced  temperature  coefficient.  Second,  there 
should  be  constructed  an  "even"  bridge,  having  two  fixed  arms  of  equal 
values,  in  ohms.  Third,  a  rheostat,  the  maximum  resistance  of  which 
will  be  equal  to  the  total  change  in  the  resistance  of  the  bulb,  between 
temperature  extremes,  is  required.  Fourth,  there  must  be  a  sensitive 
galvanometer  which  is  aperiodic,  or  nearly  so.  Finally,  there  should  be 
a  fairly  steady  source  of  direct  current. 

The  general  scheme  of  this  type  of  apparatus,  in  its  simplest  form,  can 
be  readily  understood  by  reference  to  Fig.  1.  Resistors  A  and  B  are 
constructed  of  manganin,  therlo,  or  some  other  resistance  wire  having  a 
negligible  temperature  coefficient.  The  sensitive  D'Arsonval  galva- 
nometer G  has  its  zero  in  the  center.  R  is  a  rheostat  and  X  is  a  resistor 
or  bulb  made  of  insulated  platinum  wire,  or  else  of  nickel,  copper,  or 
some  alloy,  according  to  requirements.  K  is  a  battery,  but  a  direct- 
current  service  line  is  often  used  instead,  with  sufficient  ballast  in  series 
to  properly  reduce  the  current. 

The  operation  of  the  instrument  is  very  simple.  All  that  is  necessary 
is  to  insert  the  bulb  in  a  source  of  heat  and  adjust  the  rheostat  until  the 
pointer  of  the  instrument  is  at  zero.  The  temperature  of  X  is  then  de- 
termined by  reading  a  scale  fastened  in  front  of  the  rheostat.  A  number 
of  bulbs  may  be  used  with  one  instrument,  placed  at  different  distances 


*  Chief  Electrical  Engineer,  The  Brown  Instrument  Co. 


CHARLES    P.    FREY 


459 


and  connected  by  means  of  leads  with  a  switch,  so  that  their  indications 
may  be  read  successively.  But  in  order  that  the  resistance  of  these 
leads  may  not  introduce  an  error  in  the  indications  of  the  instrument, 
the  arrangement  has  to  be  somewhat-modified,  as  shown  in  Fig.  2.  Leads 
L  and  L2  connect  the  bulb  with  the  instrument  and  L1  connects  the  gal- 
vanometer as  shown.  Leads  of  equal  resistance  are  hence  added  to 
R  and  X. 

But,  in  dealing  with  the  foregoing  conditions  in  the  construction  of 
resistance  thermometers  for  practical  purposes,  consideration  must  be 
given  to  the  fact  that  the  user  wants  an  instrument  that  is  direct  reading, 
accurate,  and  convenient.  The  first  question  that  arises  is:  "What 
constitutes  accuracy?"  If  the  manufacturer  can  guarantee  that  the 
indications  of  the  apparatus  are  correct,  such  a  statement  will  content 
most  prospective  purchasers.  But,  if  we  consider  construction  as  a  whole 


FIG.  1. 


from  the  technical  standpoint,  it  will  be  well  to  bear  in  mind  that  the 
maximum  degree  of  accuracy  attainable  depends,  from  the  very  outset, 
on  how  closely  the  thermometric  scale  of  the  indicator  may  be  read.  For 
instance,  if  the  scale  is  12  in.  (30.48  cm.)  long,  the  subdivisions  are  uni- 
form, and  each  subdivision  is  equivalent  to  ^f6  in.  (1.58  mm.),  the  limit 
of  accuracy  near  the  upper  end  of  the  scale  is  1  part  in  200.  Tempera- 
tures may,  therefore,  be  determined  within  0.5  per  cent.,  or  possibly 
0.25  per  cent.,  by  interpolation.  They  can  be  read  even  closer,  if  the 
galvanometer  is  calibrated.  But  in  any  event,  within  these  limitations 
we  need  not  concern  ourselves  unduly  about  Lord  Kelvin's  thermody- 
namic  scale,  or  its  relation  to  the  constant-volume  hydrogen  thermometer. 
Among  the  problems  assigned  to  our  experimental  department  during 


460  RESISTANCE   THERMOMETRY    FOR   INDUSTRIAL   USE 

the  past  12  mo.  were  two  relating  to  resistance  thermometry,  which  were 
of  interest  because  extreme  opposite  conditions  had  to  be  met.  The 
first  of  these  was  the  production  of  an  apparatus  having  a  range  of  0°  to 
850°  F.  (—18°  to  454° C.)  which  was  to  be  used  with  eight  interchangeable 
bulbs,  for  the  determination  of  flue  temperatures. 

The  main  consideration  was  the  selection  of  suitable  materials  for 
constructing  the  bulbs.  Platinum  wire  would  doubtless  have  been  best, 
but  owing  to  cost  and  war  conditions  it  was  practically  unprocurable. 
A  wire  said  to  be  pure  nickel,  but  which  actually  contained  some  slight 
impurities,  as  was  found  later,  was  used ;  but  these  impurities  were  not  a 
detriment  under  the  circumstances.  This  wire  was  wound  upon  threaded 
lava  insulators  about  1  in.  (25  mm.)  long,  and  ^  in.  diameter,  forming 
the  bulbs,  and  these  bulbs  were  then  adjusted  to  equal  values  at  32°  F. 
They  were  next  coated  with  a  thin  layer  of  cement,  consisting  chiefly 
of  quartz  and  carborundum.  After  being  provided  with  copper  leads  of 
large  diameter,  they  were  placed  in  a  calorimeter  and  heated  to  a  tempera- 
ture of  900°  F.  This  operation  was  repeated  several  times  until  it  was 
certain  that  they  had  become  "aged,"  and  that  their  respective  resist- 
ances were  fixed  at  any  temperature.  An  intercomparison  by  bridge 
measurements  established  the  fact  that  while  differences  in  their  indi- 
vidual resistances  and  temperature  coefficients  existed,  both  at  scale 
extremes  and  at  intermediate  stages,  such  differences  did  not  exceed 
0.1  per  cent.,  and,  consequently,  were  well  within  the  observable  limit 
of  accuracy  of  the  scale  calibration.  It  was  also  found  that  it  would  be 
comparatively  easy  to  produce  additional  bulbs  with  different  samples 
of  nickel,  by  the  well-known  method  of  adding  a  small  resistor  of  manganin, 
which  could  be  placed  most  conveniently  in  the  head  of  the  protecting 
tube  containing  the  bulb,  so  that  it  would  not  be  damaged  by  the  higher 
temperatures. 

It  should  be  added  that  the  subdivisions  of  the  scale  were  not  uniform 
but  progressively  larger  from  0°  to  500°,  after  which  they  diminished, 
being  of  about  the  same  width  at  800  °  as  at  100°  F.  Since  the  normal 
temperatures  to  be  measured  were  about  600°  F.  (315°C.)  the  scale  had  the 
widest  divisions  where  it  was  most  used,  which  was  an  almost  ideal  condi- 
tion. The  extreme  change  in  the  resistance  of  these  bulbs  was  approxi- 
mately 38  ohms  for  750°  F. ;  hence  it  was  comparatively  easy  to  construct 
a  finely  divided  helical  rheostat  to  counterbalance  these  changes. 

A  galvanometer  of  sufficient  sensitivity  to  respond  to  the  smallest 
observable  change  in  the  scale  index  was  produced  by  using  a  light  three- 
layer  copper-wire  coil.  The  resistance  of  this  coil  was  approximately 
50  ohms;  and  since  current  for  the  operation  of  this  apparatus  was  ob- 
tained from  a  110-volt,  direct-current,  service  line,  10,000  ohms  or  more 
of  ballast  resistance  was  used  in  series  so  that  the  galvanometer  had  a 


CHARLES  P.  FRET]  461 

negligible  temperature  coefficient.  This  apparatus  has  been  in  successful 
operation  for  nearly  10  months. 

Another  instrument,  designed  and  constructed  to  meet  unusual 
conditions,  was  a  precision  resistance  thermometer  having  a  range  of 
only  6°,  namely  from  —  1°  to  +5°  C.  We  were  informed  that  this  appa- 
ratus was  to  be  used  for  determining  "frazil"  temperatures.  Reference 
to  the  dictionary  revealed  the  fact  that  "frazil"  is  idiomatic  Canadian 
French,  and  is  used  in  referring  to  ice  under  water,  or  "anchor  ice." 
Now,  we  have  been,  and  still  are  under  the  impression  that  ice  floats, 
and  that  its  temperature  is  never  above  0°  C.,  so  we  assumed  that  the 
outfit  must  also  have  been  intended  to  test  water  near  the  freezing  point. 
The  specifications  called  for  a  bulb  protected  with  a  copper  tube  at  least 
10  ft.  (3  m.)  long  and  hermetically  sealed.  This  was  to  be  connected 
with  the  instrument  by  means  of  about  75  ft.  (22  m.)  of  highly  insulated 
triple-conductor  wire. 

The  main  obstacles  to  be  overcome  in  the  successful  construction  of 
this  apparatus  were  due  to  the  narrow  range  of  scale  and  the  problem  of 
insulation.  In  ordinary  cases,  when  the  scale  range  is  about  650°  C.  the 
average  change  in  the  resistance  of  a  standard  platinum  bulb  is  approxi- 
mately 0.068  ohm  per  degree  centigrade.  If  such  a  bulb  were  used  in  this 
instrument  it  would  give  a  total  change  in  its  resistance  of  only  about 
0.4  ohm  for  6°C.  Under  such  conditions,  the  rheostat  would  also  have  to 
have  a  resistance  of  approximately  0.4  ohm ;  and  in  order  to  obtain  suffi- 
cient galvanometric  sensitivity,  it  would  be  necessary  to  pass  an  excessive 
current  through  the  bulb,  producing  a  heating  effect  and  introducing  a 
"variable"  that  would  cause  a  radical  error  in  the  scale  reading. 

Another  problem  that  called  for  consideration  was  due  to  the  fact  that 
while,  theoretically,  it  is  easy  to  determine  temperatures  near  0°  C., 
this  cannot  be  done  empirically  with  accuracy,  without  setting  up  a 
rather  elaborate  outfit.  To  minimize  this  difficulty,  we  decided  to  use 
pure  electrolytic  copper  wire  specially  prepared  and  having  a  constant 
temperature  coefficient  of  0.00393°  C.  at  0°  C.  This  wire  was  wound 
upon  a  hollow  cylinder  of  very  thin  copper  tubing,  about  34  in.  (6  mm.) 
in  diameter  and  7  in.  (17  cm.)  long.  The  cylinder  was  first  given  a  thin 
coating  of  silk  and  insulating  varnish.  The  wire,  which  was  of  very  small 
diameter,  was  also  silk  covered.  After  the  bulb  had  been  constructed  in 
this  manner,  it  was  impregnated  with  insulating  compound  and  then 
alternately  baked  and  frozen,  until  it  was  "aged."  It  was  then  adjusted 
to  2600  ohms  at  0°  C.  Its  temperature  coefficient  was  next  determined 
between  20°  C.  and  — 1°C.  and  again  between  — l°and  +5°C.  Hence  the 
increase  in  resistance  at  any  temperature  between  —  1°  and  +  5°  C.  could 
be  determined  by  2600  X  [1  +  (0.00393  X  temperature)].  The  total 
change  in  resistance  between  —1°  and  +5°  C.  was  very  nearly  60  ohms. 
This  permitted  the  construction  of  a  rheostat  to  be  used  in  the  indicating 


462  RESISTANCE    THERMOMETRY    FOR    INDUSTRIAL    USE 

instrument  having  a  resistance  of  10  ohms  per  degree  centigrade  or  0.2 
ohm  per  scale  division.  Consequently  the  instrument  would  be  sensitive 
and  the  required  galvanometric  deflections  were  obtained  when  a  current 
of  less  than  5  milliamperes  was  passing  through  the  bulb.  The  maximum 
P.  D.  at  the  bulb  terminals  was  13  volts,  and  the  C2R  loss  was 0.065  watt, 
which  was  a  negligible  factor. 

But  since  the  scale  of  the  instrument  was  about  11  in.  (28  cm.)  in 
length,  and  was  subdivided  to  permit  the  determination  of  temperature 
to  Ho°  C.,  it  was  necessary  to  provide  an  extremely  sensitive  aperiodic 
galvanometer.  For  this  purpose  a  pivoted  movable  system  was  used,  the 
coil  of  which  was  made  of  aluminum  alloy  wire,  coated  with  specially 
prepared  enamel,  only  0.0003  in.  (0.007  mm.)  thick,  and  equipped  with 
delicate  springs  and  a  hollow  knife-edged  pointer,  also  of  aluminum. 
The  weight  of  this  movable  system  was  only  430  milligrams.  The  scale 
of  this  instrument  was  then  calibrated  on  a  Wheatstone  bridge  by  using 
the  bulb  resistance  equivalents  already  referred  to. 

The  final  operative  tests  were  made  as  follows:  The  bulb  terminals 
were  soldered  to  the  ends  of  the  triple  conductor  wire  and  it  was  lowered 
to  the  bottom  of  the  copper  protecting  tube,  which  was  provided  with  a 
covered  funnel-shaped  head.  Then  the  tube  and  funnel  were  filled  with 
an  insulating  compound  having  a  negligible  expansion  coefficient  below 
40°  C.  A  test  with  110- volt  direct  current  proved  that  the  insulation 
resistance  between  the  bulb  and  the  outer  casing  was  over  70  megohms. 
After  completion  the  apparatus  was  tested  by  placing  the  bulb  and  a  sensi- 
tive thermometer  in  a  freezing  mixture. 

Before  shipment,  when  the  temperature  of  the  outside  air  happened 
to  be  a  little  above  0°  C.  the  bulb  was  hung  out  of  a  window  and  the  tem- 
perature measured.  It  was  then  found  that  there  was  practically  no  lag 
in  the  response  of  the  bulb  to  temperature  changes,  since  if  the  tip  of  a 
finger  was  placed  on  the  lower  end  of  the  protecting  tube,  the  galva- 
nometer would  be  immediately  thrown  out  of  balance  and  would  almost 
instantly  return  to  normal  when  the  finger  was  removed. 

DISCUSSION 

G.  A.  ROUSH,*  South  Bethlehem,  Pa.  (written  discussionf). — Mr. 
Frey  is  correct  in  his  impression  that  ice  floats,  but  "frazil"  ice  happens 
to  be  the  exception  to  the  rule.  The  requirements  for  the  formation  of 
frazil  ice  seem  to  be  a  clear,  cold  night  and  water  on  a  bed  of  clean  rock. 
The  exact  causes  of  its  formation  are  not  definitely  known,  but  are  sup- 
posed to  be  somewhat  as  follows.  The  rock  bottom  has  a  greater  emissive 
power  for  radiant  heat  than  the  surface  of  the  water,  hence,  on  a  clear, 


Assistant  Professor  of  Metallurgy,  Lehigh  University .       f  Received  Oct.  18,  1919. 


DISCUSSION  463 

cold  night,  when  conditions  are  most  favorable  for  the  loss  of  heat 
from  the  surface  of  the  earth  by  radiation,  the  rock  cools  faster  than  the 
water  over  it,  due  to  the  greater  radiating  power  of  the  rock  and  the 
partial  transparency  of  the  overlying  water  to  the  radiant  heat.  This 
may  result  in  the  formation  of  a  film  of  ice  of  considerable  thickness 
forming  in  contact  with,  and  adhering  to  the  rock,  without  the  surface 
of  the  water  having  even  reached  the  freezing  point.  When  the  sun 
strikes  the  spot  the  next  morning,  a  reversal  of  the  action  takes  place 
and  the  rock  warms  up  faster  than  the  overlying  water,  with  the  result 
that  the  surface  of  the  rock  soon  becomes  warm  enough  to  melt  the  film 
of  ice  in  immediate  contact  with  the  rock,  and  the  whole  mass  then  floats 
to  the  surface. 


464  TIN:  AN  IDEAL  PYROMETRIC  SUBSTANCE] 

Tin:  An  Ideal  Pyrometric  Substance 

BY    E.    F.    NOBTHRUP,*   TRENTON,    N.    J. 
(Chicago  Meeting,  September,  1919) 

THESE  brief  notes  respecting  the  properties  of  pure  tin  that  make  it 
useful  as  a  pyrometric  substance  summarize  information  gathered  by 
the  writer  in  an  extensive  experimental  investigation  on  the  electrical 
properties  of  metals  in  the  molten  state. 

Tin  in  quantities  sufficient  for  pyrometric  purposes  may  be  obtained 
at  relatively  low  cost  and  in  a  state  of  high  purity.  The  metal  melts  at 
232°  C.  and,  according  to  determinations  made  by  Greenwood,  1909, 
does  not  begin  to  boil  until  a  temperature  of  2270°  C.  is  reached.  The 
writer  can  assert,  from  personal  observations  carefully  made,  that  tin 
shows  no  tendency  to  boil  at  a  temperature  of  1680°  C.  If  Greenwood's 
observations  are  correct  the  temperature  interval,  2038°  C.,  in  which  tin 
exists  as  a  liquid  under  atmospheric  pressure,  exceeds  that  of  any  other 
substance. 

It  has  never  been  observed,  as  far  as  the  writer  is  aware,  that  tin  forms 
any  chemical  union,  as  carbide,  with  carbon  at  the  highest  temperatures 
at  which  it  can  exist  as  a  liquid.  It  is  quite  certain  from  the  writer's 
personal  observation  that  tin  heated  in  Acheson  graphite  to  1680°  C. 
remains  chemically  uncontaminated. 

Wires  of  pure  tungsten  do  not  dissolve  in  molten  tin  at  temperatures 
at  least  as  high  as  1680°  C.  Tungsten  wires  or  rods  may,  therefore,  be 
used  as  electrodes  dipping  into  molten  tin  when  required  for  measuring 
the  resistance  of  the  molten  metal  at  very  high  temperatures. 

When  tin  is  raised  to  a  high  temperature  in  a  covered  graphite  con- 
tainer, the  CO  atmosphere  that  exists  above  its  surface  has  a  reducing 
action,  which  maintains  this  surface  of  mirror  brightness.  Incidentally, 
tin  maintained  molten  in  a  crucible  of  Acheson  graphite  makes  a  most 
excellent  bath  into  which  may  be  inserted  several  pyrometers  that  are 
to  be  intercompared  at  the  same  temperature. 

But  the  two  properties  of  tin  that,  in  its  molten  state,  make  it  par- 
ticularly valuable  as  a  pyrometric  substance,  are  the  strictly  linear 
character  of  the  increase  of  a  given  volume  in  resistivity  with  increase  in 
temperature  and  the  decrease  of  a  given  volume  in  density  with  increase 
in  temperature.  The  increase  in  the  resistivity  of  tin  in  the  molten  state 
has  been  studied  by  the  writer  with  very  great  care  and  he  can  assert 
positively  that  up  to  a  temperature  at  least  as  high  as  1680°  C.,  and  very 
probably  beyond  this  temperature,  the  resistivity  of  the  metal  increases 

*  President,  Pyrolectric  Instrument  Co. 


E.    F.    NORTHRUP  465 

linearly  with  increase  in  the  temperature.  The  same  can  be  asserted 
in  regard  to  the  decrease  in  the  density  with  increase  in  temperature. 

When  the  coefficients  have  once  been  accurately  determined,  as- 
suming that  suitable  methods  are  available  for  accurately  measuring 
either  the  resistivity  of  the  tin  or  the  expansion  of  a  given  volume  of  the 
tin,  one  can  in  the  former  case  deduce  the  absolute  temperature  and  in 
the  latter  case  the  change  in  the  absolute  temperature. 

Methods  for  accomplishing  these  results  in  a  practical  way  have 
been  described  by  the  writer  in  numerous  publications.  The  most 
refined  method  and  the  one  yielding  the  most  accurate  results  in  an 
exceedingly  simple  manner  for  determining  the  relation  between  tem- 
perature and  resistivity  has  been  given  in  a  paper  by  Northriip  and  Sher- 
wood.1 A  method  for  determining  the  expansion  of  tin  or  its  decrease 
in  density  with  increase  in  temperature  has  been  outlined  by  the  writer.2 

There  is  no  more  reason  why  one  should  go  back  to  the  volume  ex- 
pansion or  increase  in  pressure  of  a  given  quantity  of  gas  as  a  final 
standard  of  temperature  than  that  one  should  go  back  to  a  pure  element 
like  tin  as  a  final  standard,  provided  the  properties  of  this  latter  substance 
are  related  to  temperature  in  a  manner  as  simple  as  the  former.  It  is 
now,  at  least  in  the  writer's  mind,  quite  as  certain  that  the  resistivity 
of  a  given  volume  of  molten  tin  is  related  by  a  straight-line  law  to  tne 
absolute  temperature  as  is  the  pressure  of  a  given  volume  of  gas.  Gas 
thermometry,  for  practical  reasons,  ends  at  the  melting  temperature  of 
palladium,  1550°  C.,  while  there  are  no  practical  limitations  to  prevent 
the  accurate  determination  of  an  absolute  temperature  by  measuring 
the  resistivity  of  a  definite  volume  of  tin  when  the  temperature  is  at  least 
as  high  as  1680°  C.  and  probably  as  high  as  the  melting  point  of  platinum. 

If  at  the  time  the  writer's  researches  were  made  on  the  resistivity  of 
molten  metals  a  high  frequency  induction  furnace  had  been  available  in 
its  present  perfected  form  for  laboratory  use,  all  his  determinations 
could  have  been  made  with  much  greater  ease  and  rapidity  and  higher 
temperatures  could  have  been  obtained.  It  is  earnestly  hoped  that 
some  investigator  with  the  simple  means  now  at  his  disposal  will  make  a 
redetermination  of  the  melting  points  of  the  higher  melting  metals  in 
terms  of  the  known  resistivity  of  pure  tin. 

DISCUSSION 

P.  D.  FOOTE,  Washington,  D.  C. — Another  metal  very  similar  to  tin 
in  respect  to  the  long  temperature  range  for  the  liquid  phase  is  gallium. 
This  metal  is  liquid  at  room  temperature  and  has  a  boiling  point  probably 
near  2000°  C.  It  appears  much  like  mercury  except  that  it  wets  glass. 

1  New   Methods  for   Measuring   Resistivity  of   Molten   Materials:  Results  for 
Certain  Alloys.     Jnl.  Frank.  Inst.  (Oct.,  1916)  182,  477. 

2  Production  of  High  Temperature  and  Its  Measurement.     Trans.  Faraday  Soc. 
(1918)  13. 

30 


THERMOCOUPLE   INSTALLATION    IN    ANNEALING    KILNS 


Thermocouple  Installation  in  Annealing  Kilns  for  Optical  Glass 

BY   B.    D.    WILLIAMSON,*   M.    A.,   B.    SC.,    AND   H.    S.    ROBERTS,  *   WASHINGTON,    D.  C. 
(Chicago  Meeting,  September,  1919) 

DURING  the  wartime  rush  to  prepare  the  glass  necessary  for  the  needs 
of  our  army  and  navy,  the  problem  of  the  temperature  control  of  the  an- 
nealing kilns  became  most  serious.  The  narrow  limits  of  strain  allowable 
(all  the  specifications  called  for  less  internal  strain  than  the  average  of  the 
previously  used  German  glass)  made  necessary  a  very  exact  procedure 
and  the  methods  had  to  be  used  on  a  much  larger  scale  than  theretofore. 
At  the  Charleroi  plant  of  the  Pittsburgh  Plate  Glass  Co.,  the  members  of 
the  staff  of  the  Geophysical  Laboratory,  which  was  cooperating  with  the 
company,  had  occasion  to  study  the  methods  available,  and  reached  a 
number  of  conclusions  (many  of  them  by  no  means  new  except  in  their 
application)  that  may  interest  and  assist  those  who  have  to  meet  similar 
problems  in  other  fields.  The  following  is  a  short  account  of  the  essential 
features  of  the  system  of  temperature  control  evolved,  along  with  a  rather 
abridged  statement  of  the  reasons  that  led  to  the  adoption  or  rejection  of 
various  schemes. 


In  the  annealing  of  optical  glass,  temperatures  up  to  650°  C.  have  to  be 
measured  with  an  accuracy  of  better  than  5°  and  with  a  sensitivity  ex- 
ceeding this;  a  10°  error  will  practically  double1  the  time  necessary  for  the 
annealing  of  the  glass.  The  sensitivity  is  necessary  since  the  cooling  rate 
must  be  carefully  regulated  during  the  beginning1  of  the  cooling. 

The  conditions  at  once  limit  us  to  two  measuring  devices  for  the  tem- 
perature: the  thermocouple  and  the  resistance  thermometer.  Either 
device  will  fulfil  the  conditions,  and  we  chose  thermocouples  for  the  fol- 
lowing subsidiary  reasons :  (1)  Heavy  wires  may  be  used,  so  that  there  is 
no  danger  from  rough  handling.  (2)  Large  numbers  of  duplicate  couples 
may  be  prepared  without  the  need  of  any  skill,  while  the  preparation  of 
resistance  thermometers  calls  for  delicate  workmanship.  If  the  couples 
are  made  from  the  same  spool  of  wire,  the  calibration  curves  will  be 
sufficiently  close  to  render  calibration  of  individual  couples  unnecessary 
until  they  have  been  used  for  some  time  or  under  bad  conditions.  (3) 

*  Physical  Chemist,  Geophysical  Laboratory,  Carnegie  Institution  of  Washington. 
1  The  data  collected  on  these  points  will  be  published  very  shortly  by  L.  H.  Adams 
and  E.  D.  Williamson. 


E.    D.    WILLIAMSON    AND    H.    S.    ROBERTS  467 

The  installation  is  somewhat  cheaper.  (4)  A  large  variety  of  well- 
made  electrical  instruments  is  available  to  cover  most  of  the  possible 
requirements. 

CHOICE  OF  KIND  OF  WIRE 

Since  the  temperatures  to  be  measured  are  comparatively  low,  it  is 
unnecessary  to  use  the  expensive  rare-metal  couples  such  as  platinum- 
platin-rhodium  (Pt:  90  Pt  —  10  Rh),  which  is  the  combination  most 
used  in  scientific  high-temperature  work.  The  sensitivity  of  this  couple 
is  also  rather  low. 

There  are  two  main  factors  to  be  considered  in  addition  to  those  pre- 
viously mentioned:  reproducibility  and  length  of  service  under  given 
conditions.  The  first  of  these  gives  support  to  the  idea  of  using  pure 
metals.  There  are,  however,  very  few  combinations  of  this  kind  that 
give  the  necessary  sensitivity2  and  none  of  these  would  last  long  at  the 
required  temperature.  Nickel-iron3  gives  fair  service  if  the  atmosphere  is 
not  too  oxidizing.  By  using  alloys,  it  is  possible  to  get  the  required 
sensitivity  and  service  but,  of  course,  the  reproducibility  is  not  so  good, 
although  quite  good  enough  for  many  purposes. 

The  three  most  generally  useful  combinations4  are  copper-constantan, 
iron-constantan,  and  chromel-alumel.  The  first  is  good  only  at  tempera- 
tures up  to  300°  C.  or  thereabouts,  but  either  of  the  others  gives  good 
service  up  to  about  1000°  C.,  except  that  the  iron  must  not  be  in  an  oxi- 
dizing atmosphere  and  the  last  combination  should  not  be  used  in  a 
reducing  atmosphere.  For  our  particular  use  we  chose  chromel-alumel 
(supplied  by  the  Hoskins  Co.)  as  it  met  all  our  requirements  and  could  be 
put  to  use  in  some  other  furnaces  where  we  had  an  oxidizing  atmosphere 
quite  unsuited  for  an  element  with  iron  in  it. 

CHOICE  OF  ELECTRICAL  INSTRUMENT 

There  remains  the  question  of  the  type  of  measuring  instrument,  of 
which  three  must  be  considered:  Direct-reading  millivoltmeters,  poten- 
tiometers, and  instruments  on  the  same  principle  as  the  "Pyrovolter." 
The  direct  reader  may  be  neglected  where  accurate,  work  is  called  for. 
In  our  opinion  it  is  the  direct  cause  of  innumerable  troubles  and  costs, 
in  a  short  time,  far  more  than  the  difference  in  initial  cost  between  it 
and  a  better  type  of  instrument. 

The  potentiometer  has  one  drawback:  it  requires  that  a  constant, 
though  very  small,  current  be  kept  passing  through  its  coils  while  measure- 

2  See  paper  by  L.  H.  Adams,  this  volume. 

3  The  changes  in  the  slope  of  the  thermoelectric-temperature  curve  due  to  inversion 
points  in  the  metals  are  awkward  although  not  fatal. 

4  For  relative  sensitivities,  see  L.  H.  Adams,  op.  cit. 


468 


THERMOCOUPLE    INSTALLATION    IN    ANNEALING    KILNS 


ments  are  being  made.  It  is  sufficient  to  adjust  the  current  once  a  day 
by  balancing  the  battery  against  a  standard  cell.  On  the  other  hand, 
provided  the  standard  cell  is  kept  balanced,  there  is  no  possible  way  of 
reading  wrong  with  the  potentiometer,5  if  a  reading  is  got  at  all  it  must  be 
right.  This  constitutes  its  great  advantage.  We  found  it  easy  to  get 
the  average  American  workman  to  understand  the  reason  for  the  settings 
and  had  little  trouble  on  account  of  mistakes  by  the  temperature  readers. 
A  more  serious  objection  to  the  standard  cell  is  that  it  does  not  stand  exces- 
sive changes  of  temperature  and  may  easily  be  ruined  some  cold  winter 
night.  This  we  found  really  troublesome  in  the  case  of  the  portable 


-YWVWWW 


FIG.  1. — SET-UP  FOR  VARIABLE-CURRENT  TYPE  OF  POTENTIAL  MEASURER.     IN  THE 
"PYROVOLTER"  ONLY  ONE  ELECTRICAL  INSTRUMENT  is  USED.     AN  EXTRA  RESISTANCE 

AND    A   SWITCH    ARE    ADDED    SO   THAT   THE    RESISTANCE    AND    INSTRUMENT   CAN    TAKE 
EITHER  THE  POSITIONS  A  AND  G  OR  G  AND  A, 

instruments  during  the  winter  of  1917-18.  This  suggests  the  use  of  an 
instrument  which  does  not  need  a  standard  cell.  At  least  a  partial 
answer  is  found  in  instruments  on  the  same  principle  as  the  pyrovolter. 
An  instrument  of  this  kind  consists  of  three  parts:  a  fixed  resistance, 
a  galvanometer,  and  an  ammeter.  A  battery  is  also  needed  with  suitable 
resistances  to  cut  down  its  current.  The  connections  are  represented  in 
Fig.  1.  The  variable  resistance  R  is  varied  until  no  current  passes 

5  See  W.  P.  White  (this  volume)  for  a  discussion  of  the  potentiometer  used  in  this 
type  of  work. 


E.    D.    WILLIAMSON    AND    H.    S.    ROBERTS  469 

through  G.  The  fall  of  potential  through  C  (the  fixed  resistance)  must 
then  be  equal  to  the  thermo-electromotive  force  of  the  couple.  By  Ohm's 
law  this  is  equal  to  the  product  of  the  current  (as  read  at  A)  and  the  re- 
sistance C.  The  resistance  C  being  fixed,  the  current  immediately 
gives  a  reading  of  the  voltage  and  hence  of  the  current.  If  C  is  small  it 
is  not  necessary  to  read  very  small  currents;  e.g.,  if  C  is  0.01  ohm,  a 
current  of  1  milliampere,  which  is  well  within  the  sensitivity  of  possible 
measurement,  would  represent  a  voltage  of  0.01  millivolt  in  the  thermo- 
couple. This  type  of  instrument  is  capable  of  practically  unlimited 
sensitivity  and  will  probably  prove  very  popular  if  a  well-designed  in- 
strument is  put  on  the  market.  The  attempts  to  put  a  portable  instru- 
ment of  this  type  on  the  market  have  not,  in  our  opinion,  been  successful, 
owing  to  the  attempt  to  economize  more  than  was  possible.  It  is  useless 
to  try  to  use  one  and  the  same  electrical  instrument  both  as  ammeter 
and  galvanometer  and  expect  to  get  good  results,  since  the  very  features 

that  are  desirable  for  the  one  use  are  undesirable  for  the  other. 

« 

ARRANGEMENT  OF  LEADS  AND  OTHER  APPARATUS 

There  were  in  one  room  eleven  kilns,  in  a  second  twenty  kilns,  and  in 
a  third  four  kilns.  It  was,  however,  never  necessary  to  read  the  tempera- 
ture in  all  of  these  simultaneously.  We  immediately  determined  on  having 
a  central  booth  to  which  every  element  could  be  connected.  This  arrange- 
ment seemed  much  preferable  to  the  system  of  having  an  instrument 
in  each  room,  as  it  allows  all  the  readings  to  be  made  conveniently  by  one 
man  per  shift  and  recorded  by  him.  It  also  makes  possible  the  auto- 
matic recording  of  the  temperature  in  any  special  kilns.  It  necessitates, 
however,  a  large  amount  of  wiring  and  may  seem  a  little  complicated 
to  a  new  hand  who  has  not  seen  the  stages  of  development. 

In  considering  the  disposition  of  the  lead  wires,  it  is  necessary  to 
refer  for  a  moment  to  the  simplest  principles  of  the  thermocouple.  If  a 
circuit  consists  of  two  metals  as in  a,  Fig.  2,  and  there  is  a  difference  of  tem- 
perature between  the  junctions  of  the  metals  Ji  and  J0,  a  definite  elec- 
tromotive force  is  set  up  in  the  circuit.  If  a  cut  is  made  at  a  point  in  the 
circuit,  as  in  6,  and  a  potentiometer  is  inserted  to  read  the  potential, 
the  difference  in  temperature  between  the  hot  and  cold  ends  may  at  once 
be  measured.  In  practise,  one  junction  is  kept  at  a  constant  tempera- 
ture. The  only  necessary  precaution  is  to  see  that  the  .ends  A  and  A'  re- 
main at  the  same  temperature,  as  otherwise  additional  thermoelectric 
forces  may  be  generated  by  the  differences. 

When  a  number  of  couples  are  being  used,  it  is  convenient  to  use  a 
single  constant-temperature  junction  for  several  of  the  variable  junctions. 
This  necessitates  a  switch  or  plug  system  for  bringing  it  in  the  necessary 
circuit.  The  last  two  diagrams,  c  and  d,  show  two  of  the  schemes  used 


470 


THERMOCOUPLE    INSTALLATION    IN    ANNEALING    KILNS 


at  the  Charleroi  plant.  In  one  case  c,  the  lead  from  the  constant-tem- 
perature junction  J0  and  the  similar  leads  from  the  variables  Ji,  Jz,  Js 
were  led  to  terminals  A',  AI,  A2,  A3  enclosed  in  a  box,  so  that  all  were 
approximately  at  the  same  temperature,  and  copper  wires  were  used  from 


f— 

i 

A' 


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i 

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i 

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FIG.  2. — EVOLUTION  OF  LEAD-WIRE  DISTRIBUTION. 

there  to  the  potentiometer.  In  the  other  case  d,  the  lead  from  the  con- 
stant junction  J0  was  brought  close  to  the  terminals  of  those  from  the 
variables  J\,  J2,  Js  so  that  the  connections  to  the  copper  are  close  to- 
gether for  each  pair  AiA',  A2A'2,  A3A'3. 

The  first  set-up  was  generally  adopted  as  most  suitable  for  the  work 
in  hand.  There  were  banks  of  kilns  in  several  rooms  and  in  each  of  these 
we  put  a  box  in  which  the  connections  A',  A\,  A 2,  etc.,  were  enclosed, 


E.    D.    WILLIAMSON    AND    H.    S.    ROBERTS 


471 


and  from  this  box  ordinary  copper  wires  were  led  to  the  temperature 
station  where  the  potentiometer  was  kept.  The  lead  from  A\  was  con- 
nected to  the  similar  leads  from  the  other  rooms,  thence  to  one  terminal 
on  the  potentiometer.  The  leads  from  A\,  Az,  etc.,  were  led  to  a  switch- 
board so  that  any  one  of  the  junctions  might  be  connected  to  the  other 
terminal  of  the  potentiometer. 

The  ideal  way  to  handle  the  constant-temperature  junction  is  to  keep 
it  immersed  in  ice,  say  in  a  thermos  (vacuum)  flask.     If  this  is  imprac- 


1  "        I 

_, 

i              A 

tf 

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S       -f 

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t        ; 

i 

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fl  f< 

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V 

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l>{ 

Jt 

*j, 

FIQ.    3. — SPECIAL    EVOLUTION   OP  LEAD-WIRE  DISTBIBUTION  WHEN  COMPENSATING 

LEADS  ARE  USED. 

ticable,  it  may  simply  be  enclosed  in  any  fairly  constant  temperature 
container  and  a  correction  made  for  the  difference  between  this  and  melt- 
ing ice  temperature.  This  correction6  should  be  made  to  the  electrical 
reading  before  conversion  to  degrees.  In  practise,  it  is  often  useful  to 
run  the  wires  into  a  pipe  driven  10  or  12  ft  into  the  ground.  This 
provides  sufficient  constancy. 

A  distinct  simplification,  as  regards  the  disposition  of  the  leads,  is 
possible  if  one  of  the  couple  wires  is  copper,  as  by  making  the  points  A,  A', 


*  See  L.  H.  Adams  for  a  complete  discussion  of  this  point. 


472  THERMOCOUPLE    INSTALLATION    IN    ANNEALING    KILNS 

Fig.  2,  on  the  copper  side  the  necessity  of  their  being  at  the  same  tempera- 
ture vanishes,  because  the  leads  to  the  potentiometer  will  naturally  be 
of  copper  also,  leaving  no  chance  for  extraneous  thermoelectrics.  Even 
in  using  chromel-alumel  couples,  it  is  possible  to  take  advantage  of  this 
owing  to  the  fact  that  at  low  temperatures  the  junctions  chromel-alumel 
and  copper-constantan  are  almost  equivalent  as  regards  electromotive 
force.  It  is  necessary  in  this  case  to  have  the  junction  of  the  copper- 
constantan  to  the  chromel-alumel  at  a  temperature  not  exceeding  100°  C. 

Fig.  3  shows  the  evolution  of  a  scheme  for  using  such  leads.  At  a 
is  shown  simply  a  couple  with  leads  of  different  metals  from  the  metals 
of  the  couple.  In  this  set-up  there  are  two  pairs  of  points  AA'  and  BB' 
where  the  temperature  has  to  be  the  same  at  both  junctions  of  the  pair. 
At  BB',  this  is  generally  nearly  the  case,  as  normally  the  two  points  will 
be  close  together  because  the  junctions  will  be  between  double-twisted 
wires  and  will  be  very  close  together.  The  thermoelectromotive  force 
from  this  combination  will  be  the  sum  of  two:  that  generated  by  the 
couple  for  the  difference  in  temperature  between  the  hot  junction  «7i 
and  BB'  and  that  generated  by  the  combination  used  as  leads  between 
BB'  and  the  constant  junction  J0.  The  lead  wires  must,  therefore,  be 
chosen  to  give  the  same  electromotive  force  as  the  couple  combination 
for  the  range  of  temperature  through  which  BB'  is  likely  to  vary.  As 
already  stated,  the  combination  copper-constantan  gives  the  same  electro- 
motive force  at  low  temperatures  as  chromel-alumel.7  The  other  two 
diagrams  b  and  c  show  the  disposition  of  the  leads  as  we  used  them,  the 
arrowheads  indicating  that  the  copper  wires  are  carried  to  the  tempera- 
ture station.  This  use  of  compensating  leads,  one  of  which  is  copper, 
greatly  simplifies  the  wiring. 

If  it  is  desired  to  use  a  recording  instrument,  say  of  the  Leeds  &  North- 
rup  type,  it  is  necessary  to  have  some  convenient  type  of  switch  so  that 
one  can  read  on  the  potentiometer  any  of  the  couples,  which  are  also 
attached  to  the  recorder.  This  switch  should  be  of  a  double  push-button 
type  so  that  a  single  push  breaks  one  connection  and  makes  another.  It 
should  also  spring  back  to  the  old  position  when  released.  Such  a  switch 
is  used  in  telephone  work.  We  also  used  the  type  provided  by  the  Leeds 
&  Northrup  Co.  and  found  it  very  satisfactory.  We  found  an  automatic 
recorder  exceedingly  useful  as  a  check  to  take  a  record  of  a  certain  number 
of  the  kilns — especially  those  on  which  particularly  close  control  was  nec- 
essary. In  any  case  it  is  a  good  check  on  the  work  of  the  men  who  con- 
trol the  temperatures. 

GENERAL  REMARKS  ON  ANNEALING 

The  question  of  the  annealing  of  optical  glass  has  been  fully  discussed 
by  Adams  and  Williamson.8  They  have  concluded  that  optimum  results 

7  L.  H.  Adams,  op.  cit.  8  See  footnote  1. 


E.    D.    WILLIAMSON   AND   H.   S.    ROBERTS  473 

are  obtained  by  holding  the  glass  at  a  temperature  where  the  strain  takes 
about  6  hr.  to  disappear  and  then  cooling  it  at  a  rate  that  may  be  rapidly 
increased  as  the  temperature  decreases.  It  is  necessary  that  all  the  glass 
be  at  the  required  temperature,  and  it  is  therefore  almost  essential  to 
have  more  than  one  thermocouple  in  each  kiln  and  to  have  the  heating 
apparatus  arranged  so  that  different  adjustments  can  be  made  at  differ- 
ent parts.  A  difference  of  a  few  degrees  between  different  parts  of  the 
kiln  would  mean  that  one  part  of  the  lot  of  glass  would  not  be  annealed 
if  the  control  was  just  right  for  some  other  part.  If  any  trouble  is  found 
in  actual  practice,  one  of  the  first  things  to  do  is  to  test  the  variations  of 
temperature  by  placing  couples  in  different  positions  in  the  kiln.  The 
source  of  the  trouble  is  frequently  to  be  found  there. 

The  highest  temperature  necessary  is  less  than  600°  C.,  so  there  is  little 
trouble  in  this  work  due  to  corrosion  of  the  couple.  In  the  furnaces  where 
the  glass  was  softened  before  shaping  for  lenses,  etc.,  it  was  necessary  to 
measure  higher  temperatures,  but  by  using  heavy  wires  we  found  it  pos- 
sible to  read  to  1000°  C.  without  any  very  bad  effects.  With  the  set-up 
described,  there  is  no  difficulty  in  reading  accurately  to  within  2°  C.  The 
initial  rate  of  cooling  has  to  be  somewhat  carefully  watched,  but  the  diffi- 
culties involved  were  not  due  to  the  temperature  measurements  and  do 
not  concern  us  here. 

The  only  one  point  left  worthy  of  notice  is  the  care  of  the  instruments. 
The  recording  instrument  needs  occasional  oiling,  which  should  be  done 
by  some  one  who  knows  a  little  about  handling  delicate  implements.  New 
batteries  must  be  provided  periodically  for  all  types,  except  the  direct 
reader.  The  expense  connected  with  this  is  not  serious,  as  the  amount  of 
current  used  is  very  small  and  one  dry  cell  lasts  for  several  weeks  at  the 
very  least.  When  it  becomes  impossible  to  make  a  standard-cell  setting, 
this  fact  gives  immediate  notice  that  a  new  battery  is  required. 

The  lead  connections,  if  made  as  described,  will  probably  cause  no 
trouble  whatever  unless  (as  did  happen  in  one  or  two  instances)  the  work- 
man who  has  been  bricking  up*  a  kiln  gets  the  end  of  a  wire  completely 
covered  with  mortar  or  something  similar;  this,  however,  is  unlikely. 

SYNOPSIS  OF  OPERATIONS 

The  thermocouples  used  were  generally  about  4  or  5  ft.  (1.2  or  1.5  m.) 
long  and  were  made  of  No.  8  wire  (0.128  in.  or  3.3  mm.)  where  hard 
service  was  required;  otherwise  of  No.  14  (0.064  in.  or  1.63  mm.).  The 
junction  was  made  in  an  oxyacetylene  flame  using  borax  as  a  flux.  Short 
lead  wires  of  the  same  material  as  the  couple  wires  were  soldered  on  with 
ordinary  solder.  Before  tinning,  it  is  necessary  to  clean  the  ends  well; 
tinning  is  then  most  easily  accomplished  by  dipping  the  ends  after  heating 
into  a  small  crucible  of  molten  solder.  During  the  soldering  it  is  useful 


474  THERMOCOUPLE    INSTALLATION    IN    ANNEALING   KILNS 

to  keep  the  heavy  couple  wires  heated  with  a  small  flame.  Zinc  chloride 
makes  a  useful  flux.  The  other  end  of  these  leads  was  attached  to  the 
permanent  leads  by  small  double  connectors  (Fahnestock  connectors). 
The  couple  wires  were  insulated  from  each  other  by  means  of  the  double- 
holed  porcelain  insulators  furnished  by  the  Hoskins  Company. 

Lengths  of  ^-in.  (12.7  mm.)  iron  pipe  closed  at  one  end  were  used  to 
protect  the  couples  and  to  allow  of  ready  readings  at  different  spots  in 
the  kilns.  These  pipes  were  fixed  in  during  the  bricking-up  process  in 
the  muffle-type  furnaces,  and  were  fixed  permanently  in  the  other  fur- 
naces. This,  along  with  the  double  connectors  on  the  couples,  enabled 
us  to  insert  or  remove  couples  as  desired.  The  temperature  should 
always  be  taken  at  more  than  one  point  in  the  kiln. 

Any  kiln  that  required  especially  close  watching  had  the  temperature 
of  at  least  one  point  automatically  recorded  at  intervals  of  15  min., 
while  the  man  controlling  the  heating  and  cooling  recorded  all  tempera- 
tures at  least  once  an  hour  and  frequently  oftener,  using  a  potentiometer. 
We  found  no  difficulty  in  training  men  to  take  the  readings  properly  and 
mistakes  in  temperature  measurements  were  100  per  cent,  eliminated. 

It  is  also  recommended  that  a  consistent  scheme  of  testing  couples  be 
used.  The  couples  should  be  disconnected  in  rotation  and  sent  to  the 
laboratory  to  be  compared  with  a  standard  element.  In  a  well-equipped 
factory  it  should  be  a  simple  matter  to  test  each  couple  every  2  or  3  weeks 
by  having  a  number  of  extra  couples  ready  to  insert. 


ANNEALING    OF   GLASS'  475 


Annealing  of  Glass* 

BY   A.   Q.    TOOL,t   PH.   D.,    AND   J.   VALASEK, J  B.    S.,    WASHINGTON,    D.   C. 
(Chicago  Meeting,  September,  1919) 

THE  necessity  of  accurate  temperature  measurements  in  the  glass- 
making  industries  is  today  being  much  more  widely  appreciated  than  in 
the  past.  The  introduction  of  the  modern  simplified  and  perfected  pyro- 
metric  methods  in  connection  with  exact  regulation  of  furnace  tempera- 
tures has  caused  a  marked  improvement  in  the  glass  product  with  a 
quickened  rate  of  production.  An  example  of  the  processes  in  which 
much  improvement  has  been  and  still  can  be  made  is  that  of  annealing 
or  heat-treating  the  glass.  This  is  one  of  the  most  delicate  processes  in 
glass  manufacture  and  one  that  requires  a  most  careful  furnace  control. 

The  heat  treatment  is  undertaken  to  decrease  the  possibility  of  break- 
age and,  in  glass  for  fine  optical  instruments,  to  prevent  serious  warping 
of  accurately  ground  and  polished  surfaces  and  to  make  the  glass  more 
uniform  throughout  to  the  passage  of  light  waves.  These  objects  are 
accomplished  by  removing  all  the  harmful  stresses  that  exist  in  a  piece 
of  glass  when  it  has  been  cooled  too  quickly  or  unevenly.  Such  stresses 
exist,  for  example,  after  it  has  been  pressed,  cast,  or  otherwise  worked. 
Thus  the  process  of  annealing  consists  of  heating  the  glass  evenly  to  the 
temperature  at  which  it  softens  just  enough  to  relieve  these  stresses  in  a 
reasonable  time  and  then  cooling  slowly  and  uniformly  until  the  glass 
hardens  again.  Accordingly,  the  things  that  must  be  investigated  and 
determined  in  order  to  anneal  glass  without  loss  of  time  are :  the  annealing 
temperature,  the  time  that  the  glass  should  be  held  at  this  temperature 
while  the  stresses  relax,  and  the  quickest  cooling  procedure  that  will 
give  satisfactory  results.  After  these  characteristics  of  the  glass  are 
known  the  problem  is  one  of  pyrometry  and  temperature  control  entirely. 

In  cases  when  it  is  known  that  a  glass  article  will  be  required  to  with- 
stand some  definite  type  of  heat  or  mechanical  shock  it  is  often  possible, 
by  a  proper  heat  treatment,  to  produce  such  a  distribution  of  the  stresses 
in  the  glass  that  it  will  be  enabled  to  resist  these  shocks  more  effectively. 
This  process  is  usually  termed  "toughening"  or  "hardening"  the  glass, 
although  in  all  probability  it  consists  entirely  in  distributing  the  stresses 
properly.1  Although  this  article  will  deal  chiefly  with  the  applications 

*  This  is  a  general  synopsis  of  a  paper  to  appear  later  in  the  Bulletin  of  the  U.  S. 
Bureau  of  Standards. 

t  Associate  Physicist,  U.  S.  Bureau  of  Standards, 
t  Assistant  Physicist,  U.  S.  Bureau  of  Standards. 
'Lord  Rayleigh:  Phil  Mag.  [6]  (1901)  1,  178. 


476  ANNEALING   OF   GLASS 

of  pyrometry  to  the  determination  of  the  characteristics  of  glass  in  the 
annealing  range  and  to  furnace  control,  as  related  to  the  removal  of 
stresses,  much  that  may  be  said  applies  also  to  this  "toughening"  process. 
While  it  is  often  possible  to  obtain  fair  results  and,  accidentally,  even 
good  results  by  heating  to  an  indeterminate  temperature  and  then  cool- 
ing in  some  haphazard  way,  it  is  impossible  to  obtain  even  a  semblance 
of  efficiency  or  consistency  in  result  by  such  procedure.  In  fact,  to  carry 
out  the  process  with  the  least  loss  of  time  and  an  assurance  as  to  the  result, 
the  nature  of  the  specific  glass  to  be  annealed  must  be  well  known;  then 
after  the  proper  schedule  has  been  drawn,  the  problem  becomes  one  that 
demands  a  most  exact  control  of  the  temperature.  . 

ANNEALING  TEMPERATURE  RANGE 

The  first  step  in  determining  the  annealing  procedure  for  any  given 
glass  is  to  locate  the  most  satisfactory  temperature  at  which  to  remove 
the  stresses.  The  best  temperature  range  for  this  purpose  is  quite  nar- 
row, as  a  study  of  the  results  published  by  Twyman,2  Zschimmer  and 
Schulz3  and  others  will  make  clear.  The  total  range  does  not  in  any  case 
extend  over  more  than  100°  C.,  while  the  really  satisfactory  range  is 
probably  less  than  one-half  as  great.  The  upper  limit  of  this  range  is 
determined  by  the  danger  of  serious  deformation,  especially  in  the  case  of 
thin  articles,  and  also  by  the  fact  that  a  proper  cooling  is  difficult  when 
the  glass  is  soft.  The  lower  limit  is  fixed  by  the  lack  of  mobility  of  the 
glass  preventing  the  relaxation  of  the  stresses  in  a  reasonable  time. 
Within  certain  limits,  it  may  be  said  that  the  lower  annealing  tempera- 
tures will  prove  more  satisfactory  if  the  time  required  for  the  stresses 
to  relax  is  not  too  great.  This  arises  from  the  fact  that  the  cooling  then 
requires  a  less  exact  control  and  may  be  much  more  rapid.  However, 
when  the  lower  temperatures  are  employed,  especial  care  must  be  taken 
that  the  glass  assumes,  and  is  maintained  at,  the  proper  temperature  for 
the  required  time. 

It  is  somewhat  difficult  to  specify  definitely  the  lower  limit  but  the 
upper  limit  is  more  easily  determined,  since  there  are  a  number  of  phenom- 
ena appearing  at  those  temperatures  where  the  deformability  becomes 
too  great  for  annealing.  It  is  here  that  the  deformability  appears  to 
increase  with  extreme  rapidity,  speaking  comparatively,  as  the  tempera- 
ture is  raised.  This  region  may  be  located,  then,  by  observing  either  the 
deformation  of  strips  or  rods  of  the  glass  under  load  or  the  rapid  decrease 
in  the  restored  light,  shown  by  a  polariscopic  study  of  cylinders  that  are 
being  heated.  This  rapid  softening  is  also  accompanied  by  an  increased 
absorption  of  heat4  by  the  glass  and  a  marked  increase  in  the  thermal 

•  Tram.  Soc.  Glass  Tech.  (1917)  1.  »  Ann.  Phys.  (1913)  42,  345. 

4  M.  So:  Proc.  Tokyo  Math,  and  Phys.  Soc.  (Sept.,  1918). 
A.  Q.  Tool  and  J.  Valasek:  Phys.  Rev.  (Feb.,  1919)  13,  147. 


A.   Q.   TOOL   AND   J.   VALASEK  477 

expansion5  and  probably  other  effects.  These  changes  may  also  be 
used  to  locate  the  upper  limit.  The  requirements  in  the  use  of  any  of 
these  phenomena  for  such  determinations  are  standardized  methods  and 
careful  temperature  measurements. 

Methods  based  on  the  deformation  of  glass  used  in  a  number  of  plants 
to  determine  the  annealing  temperature  consist  usually  in  slowly  heating 
a  standard-sized  rod,  loaded  in  a  definite  way,  until  it  stretches,  bends, 
or  twists  (as  the  case  may  be)  at  a  certain  rate.  The  rate  of  heating  and 
other  conditions  under  which  the  test  is  performed  and  the  annealing 
temperature  determined  vary  according  to  the  experience  or  custom  of 
the  plant.  The  optical  method,  or  the  determination  of  the  annealing 
range  by  observing  the  temperatures  at  which  the  double  refraction  dis- 
appears, is  also  quite  generally  used.  This  double  refraction  results 
from  stresses  due  to,  or  at  least  modified  by,  the  heating.  In  using  this 
test,  the  amount  of  strain  should  be  small,  or  at  any  rate  constant. 
When  the  conditions  are  all  standardized,  the  results  become  compara- 
tively consistent.  It  is  also  possible  to  arrange  the  annealing  kiln  so  that 
such  observations  can  be  made  on  some  of  the  blocks  while  the  annealing 
is  being  carried  out.  Some  of  these  methods  were  tested  in  this  labora- 
tory and  gave  very  consistent  qualitative  results  when  the  proper  pre- 
cautions were  taken,  especially  in  regard  to  the  temperature  measure- 
ments. They  are  much  more  valuable,  however,  when  modified,  as 
described  later,  so  that  the  law  of  decrease  of  the  stresses  may  be  quanti- 
tatively determined. 

Results  that  are  more  satisfactory  in  many  ways  can  be  obtained,  for 
the  upper  limit  of  the  annealing  range,  by  finding  the  points  at  which  the 
heat  absorption  or  the  abnormal  expansion  begins  on  heating.  A  good 
differential  thermocouple  method  will  locate  a  relative  heat  absorption 
in  the  glass  on  heating  and  a  corresponding  evolution  on  cooling.  Al- 
though the  quantity  of  heat  involved  is  not  very  great,  the  effect  is  quite 
definite,  especially  on  heating.  Experiments  have  shown  that  the  begin- 
ning of  this  effect  should  be  regarded  as  the  upper  limit  of  the  annealing 
range.  Observations  of  this  sort  have  the  advantage  of  giving  a  definite, 
easily  located  temperature.  The  absorption,  at  least,  can  be  located 
with  a  simple  couple  if  the  Osmond  inverse  time  method  with  a  chrono- 
graph or  stop  watch  is  used. 

The  form  of  the  curve  obtained  on  heating  is  very  much  the  same  for 
most  glasses  and  shows  a  transformation  covering  a  range  of  30°  to  60° 
between  the  beginning  and  maximum.  One  type  of  borosilicate  glass, 
however,  shows  a  decidedly  peculiar  form  of  curve  with  a  very  long  trans- 
formation range.  The  temperatures  at  which  these  transformations 
begin  make  it  appear  that  the  effects  are  closely  connected  with  the 
softening  of  the  glass.  Glasses  of  varying  composition  were  tested. 

*  C.  G.  Peters:  Phys.  Rev.  (Feb.,  1919)  13,  147. 
W.  P.  White:  Am.  Jnl.  Sti.  (1919)  47,  1. 


478 


ANNEALING   OF   GLASS 


They  ranged  from  simple  boric-acid  glass,  which  shows  the  effect  at 
240°  C.,  to  heavy  barium  crown,  which  does  not  show  the  absorption  of 
heat  beginning  until  575°  C. 

TABLE  1. — Transformation  Temperatures  of  Various  Glasses 

Glass 


Beginning  of  Heat 

Number 

Name 

Absorption  on  Heating, 
Degrees  C. 

- 

Boric  acid  

240 

Borax                                              .        

450 

B.S.    76 

Dense  flint  

460 

B.S.  110 

Medium  flint  

445 

B.S.  188 

Light  flint  

485 

B.S.  145 

Barium  flint  

520 

B.S.    20 

Light  crown  

495 

B.S.    94 

Borosilicate  crown  

515 

K      266 

Borosilicate  crown  

545 

B.S.    87 

Light  barium  crown  

575 

B.S.  124 

Heavy  barium  crown  

575 

Pvrex  .  . 

520 

Within  the  errors  of  observation,  this  effect  begins  at  the  same  tem- 
perature as  the  increased  thermal  expansion  observed  by  C.  G.  Peters. 
The  exact  nature  of  the  transformation  causing  this  behavior  is  not  to  be 
determined  from  the  limited  data  now  available.  Further  data  on  these 
characteristics  should  throw  considerable  light  on  the  nature  of  glass 
and  the  relation  of  the  critical  range  to  the  composition.  An  immediate 
application  of  these  effects  lies  in  their  value  for  determining  the  upper 
limit  of  the  annealing  range. 

RELAXATION  TIME 

While  the  methods  described  appear  to  give  the  upper  limit  satisfac- 
torily, they  do  not  necessarily  determine  the  most  desirable  annealing 
temperature,  since  that  depends  to  a  great  extent  on  the  time  required  for 
the  stresses  to  relax.  This  time  can  be  most  easily  estimated  when 
Maxwell's6  relation  time  T  is  obtained.  This  constant  T  may  be  defined 
as  the  time  required,  while  the  strain  remains  constant,  for  the  stresses  in 

a  viscous  medium  to  decrease  to      times  their  original  value,  where  e  is 

6 

the  base  of  natural  logarithms.  The  time  T  is  most  easily  determined 
by  the  stretching  or  bending  methods  previously  mentioned.  In  such 
tests  the  temperature  of  the  glass  sample  should  be  kept  constant  and  uni- 
form to  a  fraction  of  a  degree  during  the  observations.  The  rate  of 

8  J.      axwell:  Phil  Mag.  [4]  (1868)  36- 


A.    Q.    TOOL   AND   J.   VALASEK  479 

deformation  ds/dt  is  measured  and  the  stress  F  calculated  from  the  load, 
and  the  dimensions  of  the  strip  or  rod  with  a  consideration  of  the  mode 
of  support.  From  these  data,  the  relaxation  time  may  be  obtained  by 
means  of  the  equation 


.  . 

when  E  the  elastic  coefficient  involved  is  known. 

At  a  steady  temperature,  T  remains  constant  for  a  considerable  range 
in  load.  When  the  temperature  6  is  changed,  it  will  be  found  that  T 
changes  very  nearly  according  to  Twyman's  empirical  relation7 


Tl  =  T0e    * 

Where  TI  and  T0  are  the  relaxation  times  at  the  temperature  6  and  00, 
and  k  is  a  constant,  depending  on  the  nature  of  the  glass.  From  tests 
on  various  glasses  it  would  appear  that  k  may  be  as  large  as  13  or  as 
small  as  10.  On  the  average,  however,  T  may  be  said  to  double  for  each 
8°  drop  in  temperature. 

The  relaxation  time  may  likewise  be  determined  from  observations 
on  the  changes  in  the  restored  light  in  a  polariscope.  This  method  can 
be  applied  in  the  proper  annealing  range  where  the  rate  of  deformation 
becomes  too  slow  to  be  measured  conveniently  by  a  mechanical  means. 
A  comparison  of  these  results  and  those  obtained  by  an  extrapolation 
of  the  bending  or  stretching  method  is  interesting.  The  relaxation  time, 
as  determined  by  the  optical  method,  changes  very  greatly  with  the  mag- 
nitude of  the  stress,  but  for  some  amount  of  double  refraction  usually 
has  the  same  value  as  that  determined  by  the  mechanical  method.  Thus 
larger  stresses  relax  more  quickly  and  small  stresses  more  -slowly  than 
would  be  expected  from  the  extrapolation.  These  observations  lead  to 
some  interesting  conclusions  as  to  the  manner  in  which  the  elastic  and 
viscous  properties8  of  glasses  change  when  they  harden.  In  the  case 
of  fine  annealing  proper  allowance  must  be  made  for  these  changes. 

It  was  found  through  experimental  annealing  that  the  best  annealing 
temperature  for  most  glasses  lies  within  20°  C.  of  the  beginning  of  the  crit- 
ical range.  For  certain  special  cases,  however,  lower  or  even  higher  tem- 
peratures may  be  desirable.  The  relaxation  time  T  and  also  k  should  be 
determined  in  this  range,  if  possible.  The  value  of  T  for  any  other  tem- 
perature may  be  calculated  from  Twyman's  formula.  If  the  initial 
stresses  F0  are  to  be  reduced  to  any  fraction  of  their  original  value,  so 
that  after  a  time  t  they  become  equal  to  F,  then 

t  =  T\ogeF0/F 
This  relation  is  derived  from  Maxwell's  formula  on  assuming  a  constant 

7  F.  Twyman:  Op.  cit.  •  Butcher:  Mess,  of  Math.  (1879)  8,  168. 


480  ANNEALING    OF   GLASS 

strain.  A  degree  of  annealing  such  that  F/F0  =  0.01  is  usually  sufficient 
for  glassware,  and  in  this  case  ta  =  4.6T,  which  may  be  considered  as  a 
satisfactory  annealing  time  for  most  purposes.  The  temperature  that 
appears  to  give  the  best  results  is  the  one  corresponding  to  a  value  of 
about  2  hr.  for  ta.  During  this  time  it  is  important  that  this  temperature 
is  taken  up  by  the  glass,  and  that  it  is  constant  and  uniform.  It  is  at  this 
point  that  modern  pyrometry  and  methods  of  temperature  control  be- 
come of  great  assistance,  in  obtaining  and  maintaining  the  required  con- 
ditions during  the  annealing  time. 

It  might  appear  that  the  choice  of  a  still  lower  temperature  would 
reduce  the  need  for  extreme  care  in  control  and  also  make  cooling  easier. 
Although  this  is  partly  true,  the  advantage  gained  by  the  employment 
of  a  temperature  low  enough  to  produce  a  marked  gain  would  be  offset 
by  the  increased  annealing  time  and  the  necessity  of  maintaining  a  con- 
stant temperature  over  a  longer  period.  That  is,  the  additional  time 
necessary  for  annealing  could  not  be  compensated  by  the  time  saved  by 
the  more  rapid  cooling. 

COOLING  PROCEDURE 

After  the  glass  has  been  annealed  sufficiently,  the  cooling  may  begin 
at  a  rate  that  depends  on  the  size,  the  physical  and  chemical  nature,  the 
relaxation  time,  thermal  expansion,  and  other  constants  of  the  glass. 
The  mode  of  packing  and  the  number  of  pieces  must  also  be  considered 
if  a  large  quantity  are  packed  together.  Unless  these  conditions  are 
known,  it  is  difficult  to  outline  any  definite  cooling  procedure.  More- 
over, any  general  or  theoretical  discussion  is  not  convincing  when  the 
number  of  variables  entering  the  problem  is  so  large. 

In  general,  the  cooling  should  proceed  in  such  a  manner  that  the  rate 
is  approximately  the  same  throughout  the  glass.  That  is,  the  rate  of 
cooling  should  not  be  changing  so  rapidly  that  the  inner  portions  lag  far 
behind  when  making  the  same  changes  in  rate  that  are  occurring  at  the 
surface.  For  small  or  thin  pieces,  like  lens  blanks  or  thin  bottles,  this 
does  not  preclude  a  rate  that  increases  rapidly  to  a  maximum,  which,  when 
the  expansion  coefficient  is  small  and  the  conductivity  and  relaxation 
time  are  large,  may  approximate  free  cooling  in  air,  starting  at  about 
250°  C. 

When  the  article  has  an  appreciable  thickness,  the  cooling  must  pro- 
ceed much  more  slowly  at  all  stages.  If  the  temperature  differences 
involved  are  not  too  great,  the  rate  at  any  stage  should  be,  roughly  speak- 
ing, inversely  proportional  to  the  square  of  the  thickness  unless  the  other 
dimensions  vary  too  greatly.  Tests  on  increasing  the  rate  as  the  tem- 
perature falls  show  that  the  procedure  suggested  by  Twyman  gives  good 
results.  In  this  case  the  rate  is  doubled,  for  a  time,  after  each  10°  C.  drop 
in  the  temperature.  This  doubling  must  cease  and  the  cooling  rate 


A.   Q.    TOOL   AND    J.   VALASEK  481 

become  linear  considerably  short  of  a  rate  that  would  produce  stresses 
approaching  the  breaking  limit.  The  condition  requiring  a  uniform 
temperature  over  the  surface  must,  in  every  case,  be  satisfied. 

It  is  physically  impossible  to  cool  large  pieces,  such  as  telescope  lens 
blanks,  very  rapidly  as  a  whole,  even  if  the  stresses  were  not  to  become 
dangerous.  Because  of  the  slow  cooling,  the  stresses  produced  by  the 
gradient  act  for  a  long  time  and  produce  their  full  effect  unless  the  relaxa- 
tion time  is  very  large.  Accordingly  it  is  desirable  to  anneal  large  pieces, 
if  possible,  at  lower  temperatures  than  are  employed  for  small  ones. 
Otherwise  the  cooling  would  have  to  be  extremely  slow  over  a  wide  range. 
For  fine  annealing,  it  is  extremely  important  to  maintain  surface-tem- 
perature uniformity. 

In  all  these  cases  some  permanent  strain  will  evidently  result  from 
the  cooling  and  it  is  necessary  to  adjust  the  rates  so  that  this  strain  is  too 
small  to  be  harmful.  Ultimately,  the  product  should  always  be  tested 
and,  if  its  condition  is  not  satisfactory,  readjustments  in  the  procedure 
must  be  made.  The  nature  of  these  readjustments  will  always  be  evident 
when  sufficient  data  of  the  sort  described  concerning  the  properties  of 
the  glass  are  at  hand.  A  temperature  record  of  the  heat  treatment  taken 
by  a  recording  pyrometer  will  be  found  of  great  assistance  to  determine 
whether  and  how  the  furnace  control  must  be  modified. 

Practically  the  only  method  for  testing  the  finished  product  for 
strain  is  to  examine  for  double  refraction  in  polarized  light.  This 
method  is  really  not  very  satisfactory  since,  as  Pockels9  has  shown,  the 
double  refraction  exhibited  by  the  various  glasses  under  the  same  stress 
varies  greatly  with  their  composition.  He  was  able,  in  fact,  to  calculate 
the  composition  of  glass  that  would  show  no  such  effect  and  later  to 
verify  the  result  by  experiment.  However,  there  are  not  sufficient  data 
available  on  this  point  to  be  of  aid  in  testing.  Moreover,  there  are 
no  reliable  specifications  as  to  the  magnitude  of  stresses  allowable, 
although  a  number  of  purely  arbitrary  standards  are  in  use.  When  the 
annealing  process  is  carried  out  in  a  scientific  and  efficient  manner,  it  is, 
as  a  rule,  not  difficult  to  eliminate  the  stresses  to  such  a  degree  that  the 
amount  of  double  refraction  is  very  small  if  not  negligible.  Consequently 
there  should  be  no  difficulty  in  meeting  any  reasonable  standard. 

SUMMARY 

In  summarizing  the  requirements  to  be  observed  in  annealing,  the 
following  points  are  to  be  noted: 

It  is  well  to  study  carefully  the  glasses  to  be  annealed  and  obtain  all 
the  information  bearing  on  the  choice  of  the  annealing  temperature  and 
time  and  mode  of  cooling.  If  the  temperature  of  the  glass  is  above  the 
annealing  temperature  (and  in  case  the  stresses  are  large,  it  may  be 

•  F.  Pockels:  Ann.  Phys.  (1902)  7,  745. 

31 


482  ANNEALING    OF   GLASS 

advisable  to  heat  it  somewhat  above)  the  glass  may  be  cooled  with  com- 
parative rapidity  to  this  steady  temperature  and  then  annealed  for  the 
required  time.  After  the  stresses  have  practically  vanished,  the  cooling 
must  proceed  at  a  rate  such  that  the  amount  of  strain  introduced  is 
negligible.  This  rate  may  be  increased  for  a  time.  This  time  and  the 
manner  of  increasing  the  rate  must  be  determined  from  the  nature  of  the 
glass  and  the  character  of  the  ware. 

In  order  to  profit  by  an  investigation  into  the  nature  of  the  glass  and 
by  former  experience  in  annealing,  it  is  necessary  to  have  a  complete 
pyrometric  record  and  control  of  the  entire  process.  The  subject  of 
temperature  control  and  recording  pyrometers  in  this  connection  is 
discussed  by  Fairchild  and  Foote  in  this  symposium.  An  automatic 
furnace  control  that  gave  good  service  in  this  laboratory  consisted  of  a 
General  Electric  control  panel  used  in  connection  with  aLeeds  &  Northrup 
recorder,  with  some  modifications  devised  by  C.  O.  Fairchild. 

It  must  be  emphasized  that  the  temperatures  of  interest  in  annealing 
are  those  that  are  taken  up  by  the  glass,  and  that  for  good  work  the  heat- 
ing must  be  uniform.  An  ideal  system  is  naturally  unattainable  but  a 
study  of  modern  methods  of  pyrometry  and  temperature  control  will 
make  it  clear  that  much  improvement  in  the  usual  plant  conditions  is 
possible. 

DISCUSSION 

E.  D.  WILLIAMSON,*  Washington,  D.  C.  (written  discussionf). — The 
paper  is  interesting  and  suggestive  and  the  work  described  should  be 
carried  to  completion,  as  it  presents  points  of  interest  quite  apart  from 
the  immediate  application  in  glass  manufacture.  It  ought  to  help,  for 
instance,  in  throwing  light  on  the  chemical  nature  of  glasses  and  on  all 
questions  of  viscous  flow. 

In  practice,  we  have  found  that  it  is  much  safer  to  use  temperatures 
some  40°or  50°  below  those  given  in  the  authors'  table,  especially  where  large 
muffles,  which  have  considerable  lag,  are  used.  It  is  exceedingly  difficult  in 
many  cases  to  regulate  the  initial  rate  of  cooling  closely  and  this  trouble 
can  be  completely  avoided  by  working  at  a  point  where  the  strain  takes 
several  hours  to  vanish. 

Results  based  on  Maxwell's  equation  are  of  doubtful  significance  as 
this  equation  is  made  up  on  a  definite  assumption  as  regards  the  relation 
of  stress  to  strain,  which  does  not  hold  in  the  case  under  consideration. 

A  point  that  is  insufficiently  emphasized  is  that  the  exceedingly  small 
conductivity  of  glass  causes  a  large  temperature  difference  between  the 
outside  and  the  center  while  heating  or  cooling.  This  means  that  for 
larger  pieces  the  initial  rate  of  cooling  must  not  only  be  less  but  the  point 
at  which  the  cooling  is  speeded  up  must  be  lower. 

*  Physical  Chemist,  Geophysical  Laboratory,  Carnegie  Institute  of  Washington, 
t  Received  Sept.  18,  1919. 


483 


Pyrometry  Applied  to  Bottle-glass  Manufacture 

BY   R.    L.    FRINK,    LANCASTER,    OHIO 
/ 

(Chicago  Meeting,  September,  1919) 

I  FEAR  that  my  treatment  of  this  subject  may  not,  in  all  instances, 
meet  the  approval  of  those  who  read  my  opinion  as  to  the  utility  and 
efficiency  of  pyrometers  in  the  making  of  glass,  or  bottle-glass.  It  may 
be  superfluous  for  me  to  add  that  this  opening  statement  is  based  on  over 
15  years'  experience  in  an  endeavor  to  successfully  apply  pyrometers, 
or  heat-measuring  instruments,  to  glass-melting  furnaces,  particularly  the 
type  known  as  tank-furnaces,  and  that  such  endeavors  have  proved  more 
or  less  futile.  It  is  my  desire,  therefore,  to  herein  set  forth  the  problems 
encountered,  hoping  thereby  to  stimulate  further  effort  in  the  successful 
application  of  pyrometers.  It  is  my  desire,  also,  to  invite  criticism  of  the 
methods  used  and  suggestions  from  those  who  may  have  a  wider  experi- 
ence, or  who  may  have  better  methods. 


FIG.  1. — GLASS  TANK  FURNACE. 

In  general,  I  believe  that  the  inefficient  application  of  pyrometry  to 
the  melting  of  glass  in  tank  furnaces  is  not  due  as  much  to  the  instruments 
as  to  the  conditions  under  which  they  must  be  used  and  the  character  of 
the  medium  in  which  they  must  reside.  Most  of  those  present  no  doubt, 
are  familiar,  at  least  in  a  general  way,  with  the  methods  and  apparatus  used 
in  the  melting  and  making  of  bottle-glass,  or  glass  that  is  formed  into 
containers  of  various  shapes  and  designs.  Most  of  this  glass  is  made  in 
what  is  known  as  a  tank  furnace.  These  furnaces  vary  widely  in  design 
and  construction.  In  general,  they  consist  of  a  rectangular  tank-like 
form,  built  up  of  fireclay  blocks,  and  range  from  2  by  4  ft.  (0.6  by  1.2  m.) 
to  24  by  140  ft.  (7.3  by  42.6  m.)  with  a  depth  of  glass  from  2  to  5  ft. ;  or  a 
capacity  of  from  approximately  2  to  700  tons.  Fig.  1  shows  the  general 


484  PYROMETRY  APPLIED   TO  BOTTLE-GLASS  MANUFACTURE 

design.  The  material  is  charged  in  at  A  A.  It  is  pushed  into  the  furnace 
by  means  of  an  iron  tool  and  floats  on  the  molten  glass  in  mounds,  or 
what  might  be  relatively  termed  "bergs"  of  batch  materials.  Here  it 
comes  into  contact  with  the  fire  and  temperature,  which  effects  a  chemical 
combination  of  the  batch  material.  As  it  melts  the  material  flows  for- 
ward and  passes  downward  through  the  throat  in  the  bridge-wall.  This 
throat  is  a  comparatively  small  opening,  usually  not  exceeding  24  in. 
(61  cm.)  in  width  by  18  in.  (45  cm.)  in  height.  The  glass  then  enters 
what  is  known  as  "the  refining  chamber,"  or  "working  end"  of  the 
furnace.  This  chamber,  above  the  bridge-wall,  is  in  open  communication 
with  the  melting  end. 

As  no  fire  is  introduced  into  the  working  end  of  the  furnace,  obviously 
its  temperature  is  considerably  lower  than  that  of  the  melting  end.  Con- 
sequently the  temperature  of  the  glass  decreases  as  the  glass  flows  toward 
the  ring  holes,  or  working  positions.  If  more  glass  is  taken  from  any 
one  working  position,  or  ring  hole,  than  from  the  opposite  side,  there  will 
flow  to  this  point  a  greater  quantity  of  the  hotter  glass  issuing  from  the 
throat.  As  a  result,  the  workman  working  this  glass  will  be  required 
to  regulate  his  gathering  or  the  machine  he  is  operating  accordingly. 
Should  he  for  any  reason  cease  to  operate  his  machine,  or  gathering,  and 
the  flow  of  glass  to  this  point  is  checked,  a  change  in  the  temperature  of 
the  glass  will  ensue.  This  necessitates  a  change  in  the  working  conditions 
when  again  starting  up,  also  a  further  change  when  operations  have  been 
resumed  until  the  normal  flow  has  been  produced. 

It  would  seem  that  a  simple  answer  to  this  would  be  to  introduce 
into  the  glass  at  this  point  some  form  of  temperature-measuring  apparatus, 
or  to  sight  upon  the  glass  at  this  point  a  pyrometer  working  upon  the 
optical  principle,  or  the  disappearing-filament  principle.  However,  none 
of  these  have  been  found  to  be  satisfactory.  The  thermoelectric  pyrome- 
ter is  absolutely  unsuitable  for  this  purpose,  because  we  have  not  as  yet 
been  able  to  construct  a  sheathing  or  protecting  tube  for  the  element  that 
will  withstand  the  erosive  action  of  the  glass.  The  optical  pyrometer  is 
likewise  unsuitable  because  the  glass  has  not  only  light  and  heat  trans- 
mitting properties,  but  is  highly  reflecting.  As  a  consequence,  the  glass 
temperature  at  the  surface  is  not  measured  but  measurements  are  ob- 
tained of  the  underlying  glass  if  it  should  be  hotter  than  the  surface,  or 
possibly  the  temperature  of  the  crown  or  side  walls,  or  of  the  flame,  which 
is  reflected  from  the  surface. 

At  the  melting  end  of  the  furnace,  the  batch  materials,  which  consti- 
tute the  glass,  are  composed  mainly  of  sand,  soda  ash,  or  salt  cake,  burned 
or  raw  lime,  with  possibly  the  addition  of  small  amounts  of  borax,  arsenic, 
antimony,  nitrate  of  soda.  In  some  rare  instances,  barium  carbonate, 
zinc  oxide,  etc.  are  being  introduced  in  varying  proportions  and  in  a 
dry  state.  As  they  are  injected  into  the  furnace,  the  charges  float  on 


R.   L.   FRINK  485 

the  surface  of  the  glass  and  come  into  contact  with  the  fire,  which  plays 
across  the  furnace.  As  a  result  more  or  less  of  these  materials  is  entrained 
in  the  gases  that  pass  across  and  carried  forward  impinging  upon 
the  side  wall,  into  the  ports,  checkers,  and  flues.  To  some  extent  this 
pervades  the  whole  atmosphere  of  the  furnace,  producing  a  severe  erosive 
action  upon  the  whole  interior  lining,  combining  with  the  material 
forming  the  side  walls,  which  effects  a  glaze  that  is  more  or  less  light 
reflecting. 

THERMOELECTRIC  PRINCIPLE 

If  pyrometers  are  introduced  either  through  the  crown  at  point  A, 
Fig.  1,  through  the  back  wall  at  B,  or  the  side  walls  at  C,  the  protecting 
or  sheathing  tubes  of  the  elements  (if  it  is  a  thermoelectric  equipment), 
are  attacked  by  the  entrained  and  volatilized  alkali.  It  only  requires  a 
short  time,  in  some  instances  a  week  or  possibly  a  month  or  two,  for  this 
alkali  to  dissolve  and  erode  these  protecting  tubes  sufficiently  to  ex- 
pose and  destroy  the  element  itself.  Therefore  it  is  not  only  expensive, 
but  extremely  difficult,  to  keep  these  elements  in  an  operative  condition: 

In  order  to  minimize  this  erosive  action,  perforated  silica  block  has 
been  used  as  a  protection  tube,  particularly  where  the  element  has  been 
introduced  through  the  crown,  as  at  A.  While  this  was  satisfactory  so  far 
as  reducing  the  erosive  action  and  destruction  of  the  element  is  concerned, 
it  was  not  entirely  satisfactory  because  it  was  necessary  to  make  these 
blocks  rather  large  in  order  that  they  would  have  sufficient  mechanical 
strength.  This  results  in  there  being  considerable  heat  conducted 
through  the  block  into  the  crown,  consequently  lowering  the  temperature 
readings  below  the  actual  temperature  of  the  furnace,  and  also  decreasing 
the  sensitiveness  of  the  instrument. 

What  has  been  said  of  the  elements  introduced  at  A  is  also  true  of 
those  introduced  at  B  and  C.  Those  located  at  the  two  latter  points, 
even  where  they  are  protected  by  a  silica  block,  do  not  resist  the 
erosive  action  nearly  as  well  as  those  at  A . 

Another  great  drawback  to  the  use  of  pyrometers  as  a  control  medium 
for  governing  melting  conditions  when  located  in  the  melting  end  is  the 
fact  that  they  do  not  give  readings  that  truly  represent  the  temperature 
or  fire  conditions  which  perform  effective  work  on  melting  the  materials. 
To  illustrate,  a  pyrometer  element  located  at  A,  Fig.  1,  reads  only  the 
temperature  produced  at  A,  which  is  perhaps  3  in.  (7.6  cm.)  below  the 
crown.  Conditions  can  be  produced  in  the  furnace  whereby,  with  a  high 
stack  draft  and  with  gas  and  air  valves  adjusted,  a  perfect  combustion 
can  be  obtained.  A  higher  temperature  can  then  be  produced  at  the 
surface  of  the  glass,  or  impinged  upon  the  batch  materials,  than  will  be 
produced  at  the  thermal  element  at  A ;  but  if  the  stack  damper  should  be 
lowered,  with  other  conditions  remaining  the  same,  the  travel  of  the  fire 


486  PYROMETRY  APPLIED   TO  BOTTLE-GLASS   MANUFACTURE 

across  the  furnace  will  be  retarded  and  a  greater  quantity  will  be  forced 
up  to  the  crown.  As  a  result,  the  temperature  at  A  will  be  increased 
while  the  temperature  of  the  furnace  may  be  lowered,  and  at  the  surface 
of  the  glass  will  be  very  much  lower  than  is  found  at  A,  or  at  least,  melting 
will  be  retarded.  This,  result  I  have  been  able  to  produce  many  times. 
For  this  reason  it  is  my  candid  opinion  that  a  pyrometer  in  the  melting 
end  of  a  tank  furnace  really  does  more  harm  than  good,  for  where  such 
conditions  can  be  obtained  the  instrument  must  be  misleading  in  its 
efficacy  as  a  means  for  furnace  control. 

This  is  true  to  a  greater  extent  when  the  elements  are  located  at  B 
and  C,  for  not  only  do  the  fire  conditions  have  a  pronounced  effect  on 
the  element  at  B,  but  the  quantity  and  proximity  of  the  batch  piles  also 
affect  it.  While  an  element  at  C  not  only  suffers  from  the  two  conditions 
mentioned,  it  also  has  the  disadvantage  that  when  the  fire  is  traveling 
away  from  this  side  of  the  furnace,  it  receives  that  temperature  only 
arising  from  the  imperfect  combustion  as  the  stream  of  gas  and  air 
issues  from  the  ports;  while  if  the  fire  is  approaching  the  element,  the 
latter  receives  the  greatest  intensity  of  heat  because  of  the  impingement 
of  the  gases  of  final  combustion. 

A  thermoelectric  element  introduced  at  D,  Fig.  1,  or  in  the  refining 
end  of  the  furnace,  under  certain  conditions  when  correctly  understood, 
is  of  considerable  utility  and  value  to  the  furnace  operator.  This  element 
not  only  reads  the  temperature  radiated  from  the  glass  immediately 
below,  but  it  also  indicates  the  temperature  of  the  gases  that  surround 
the  glass  in  the  refining  end  of  the  furnace.  This  results  indirectly  in 
its  being  an  .indicator  of  the  stack  damper  and  air-valve  settings  that  con- 
trol the  quantity  of  gases  of  combustion  forced  to  this  end,  or  the  influx 
of  air  into  the  furnace.  However,  it  gives  but  a  secular  indication  of 
temperature,  and  cannot  be  accepted  as  any  true  guide  of  the  tempera- 
ture of  the  glass. 

An  element  located  at  E,  Fig.  1,  is  more  efficacious,  when  properly 
installed,  in  giving  the  temperature  of  the  glass  immediately  adjacent 
to  it  but,  as  pointed  out,  this  may  be  at  a  point  where  the  glass  is  sub- 
stantially quiescent,  and  consequently  cooler  than  at  any  other  point. 
If  located  between  two  ring  holes  from  which  relatively  large  quantities 
of  glass  are  being  taken,  it  will  indicate  a  higher  temperature  than  at 
other  working  points  of  the  furnace.  • 

As  a  means  of  furnace  control,  a  pyrometer  introduced  in  the  flue, 
between  the  stack  damper  and  air  valve,  and  at  D,  in  conjunction  with 
an  efficient  type  of  draft  gage,  provides  as  good  a  means  of  regulating 
the  furnace  conditions  and  the  requirements  of  adjustment  as  it  is  possible 
to  obtain.  However,  so  far  as  furnace  control  as  related  to  the  melting 
conditions  is  concerned,  I  have  never  found  anything  that  affords  as  good 
a  criterion  as  the  appearance  of  the  melting  batch  piles,  the  flux  line, 


R.    L.    FRINK  487 

and  the  surface  of  the  glass  between  the  batch  piles  and  the  bridge-wall. 
Here  we  have  indications  that  are  directly  the  result  of  quantity,  quality, 
and  intensity  of  fire  affecting  the  material  to  be  acted  upon. 

OPTICAL  PRINCIPLE 

•  Some  furnace  men  and  operators  have  found  that  the  optical  pyro- 
meter meets  all  their  requirements  in  furnace  control  by  using  it  as  a 
criterion  upon  which  to  regulate  their  fire  conditions,  or  the  degree  and 
speed  of  combustion.  This  is  done  by  sighting  through  an  opening  in 
the  rear  wall  of  the  furnace,  at  some  convenient  point,  whereby  the  in- 
strument may  be  focused  upon  some  given  point  in  the  flame  and  the 
temperature  read  at  this  point.  The  readings  are  then  empirically 
applied  to  adjust  or  regulate  valves  and  damper  settings. 

I  do  not  question  but  that  after  careful  study  and  correlation  of 
readings  with  the  valve  and  damper  settings,  the  optical  pyrometer 
can  be  utilized  to  give  valuable  aid  to  the  operator  in  governing  furnace 
adjustments.  However,  I  have  never  been  able  to  obtain  results  that 
are  as  satisfactory  as  other  means.  Further,  I  have  never  been  able 
to  use  an  optical  pyrometer  of  the  Wanner  or  disappearing-filament 
type,  and  obtain  any  satisfactory  results  in  controlling  temperature  of 
the  glass,  for,  as  before  pointed  out,  the  emissivity  of  the  glass  is  de- 
pendent entirely  on  its  composition,  homogeneity,  uniformity  of  tem- 
perature, and  freedom  from  reflections  of  hotter  or  colder  bodies. 

As  an  example,  if  one  will  take  out  of  the  furnace  in  any  convenient 
manner,  a  mass  of  glass  of,  say,  100  lb.,  having  but  a  small  part  of  its 
area  exposed  to  radiation,  and  then  attempt  to  measure  the  temperature 
of  the  surface,  he  will  find  that  this  surface  temperature  is  very  much 
lower,  in  some  instances,  200°  to  300°,  than  will  be  registered  by  the 
optical  pyrometer. 

RADIATION  AND  DISAPPEARING-FILAMENT  PRINCIPLE 

What  has  been  said  of  the  optical  principle  is,  to  a  great  extent,  true 
of  the  radiation  and  disappearing-filament  types  of  pyrometer.  In  the 
disappearing-filament  and  optical  types,  we  also  have  the  calibration 
factor  to  contend  with,  while  in  the  radiation  type  we  have  the  perf ectness 
of  focus  and  reflecting  surface  to  maintain  in  order  to  obtain  anything 
like  accuracy. 

It  is  indeed  regrettable  that  some  one  cannot  devise  a  means  or  material 
whereby  temperatures  of  glass  can  be  accurately  measured.  However, 
it  is  not  strictly  essential  and,  in  fact,  I  believe  it  is  not  wise  to  attempt 
to  measure  the  temperature  of  the  melting  end  of  these  furnaces,  for  the 
reason  that  the  position  at  which  such  temperatures  are  measured  is  so 


488 


PYROMETRY   APPLIED   TO  BOTTLE-GLASS   MANUFACTURE 


small  in  proportion  to  the  volume  and  areas  involved  that  such  meas- 
urements do  not  give  information  of  any  practical  value.  If  we  could 
devise  a  means  whereby  the  temperature  of  the  glass  as  it  is  being  worked 
could  be  continuously  and  accurately  measured,  we  would  solve  a  problem 
that  would  be  of  inestimable  value  to  the  manufacturer. 

In  regard  to  this  statement,  let  us  consider  some  of  the  conditions 
wherein  this  temperature  plays  such  an  important  part.  Fig.  2  is  a 
diagrammatic  view  of  what  is  known  as  the  Owens  revolving  pot,  which 
consists  of  a  furnace  and  a  revolving  clay  vessel  situated  adjacent  to  the 
tank  furnace,  as  shown  in  Fig.  1.  A  spout  introduced  in  the  refining 
end  of  the  furnace,  some  inches  below  the  surface  of  the  glass,  has  a  gate 
member  arranged  so  that  by  adjusting  the  gate  a  quantity  of  glass  flowing 
into  the  revolving  pot  marked  AB  can  be  regulated  so  as  to  maintain  a 
constant  level,  or  the  gate  can  be  lowered  so  as  to  completely  shut  off 


FIG.  2. — OWENS  REVOLVING  POT. 

the  flow  of  glass  to  the  pot.  A  heating  chamber  surrounds  this  revolving 
pot  and  is  entirely  separate  from  the  melting  furnace  and  its  refining 
chamber,  and  is  supplied  with  auxiliary  means  for  obtaining  the  required 
temperature  therein;  this  is  done  by  means  of  oil,  natural  gas,  or  producer- 
gas  fuel.  The  glass  that  flows  into  this  pot  is  formed  into  various  articles, 
such  as  bottles  of  all  sizes,  fruit  jars,  packers'  ware,  etc.,  by  means  of 
what  is  known  as  the  Owens  bottle  machine,  the  fundamental  principle 
of  which,  briefly  stated,  is  as  follows: 

A  parison  mold  is  lowered  so  that  its  opening  and  under  surface  just 
comes  into  contact  with  the  surface  of  the  glass;  as  a  vacuum  is  created 
in  this  mold,  the  glass  is  forced  up  into  it  and  around  a  plunger,  which 
produces  the  required  cavity  within  the  parison  blank.  Subsequently, 
this  parison  mold  is  opened  and  removed  from  the  blank,  and  the  blow 
mold,  or  the  mold  that  governs  the  shape  of  the  article,  is  closed  around 
the  blank,  the  plunger  is  removed,  the  opening  closed  where  the  plunger 
passed  through,  compressed  air  is  admitted  into  the  cavity  in  the  blank, 


B.   L.    FRINK  489 

and  the  blank  distended  to  fill  the  contour  of  the  mold,  after 
which  the  mold  is  opened  and  the  article  is  ejected  upon  a  suitable  con- 
veyor, or  is  removed  manually. 

The  perfectness  of  the  article  depends,  primarily,  on  the  temperature 
of  the  glass  as  it  is  drawn  into  the  parison  mold.  It  is  a  serious  and 
perplexing  problem  as  to  what  this  temperature  should  be,  and  after  the 
temperature  has  once  been  determined,  for  a  given  size  and  weight  of 
ware,  its  effect  must  also  be  determined  as  related  to  the  condition  of 
the  molds,  speed  of  operation,  distribution  of  glass  in  the  article,  vacuum 
and  air  pressures,  etc.  and  should  be  maintained  uniform  and  constant. 
Numerous  ways  have  been  suggested,  and  I  have  tried  nearly  every 
suggestion,  but  I  have  not  found  a  method  for  measuring  the  tempera- 
ture of  the  glass  at  exactly  the  location  where  the  mold  takes  up  the  glass, 
that  has  been  entirely  satisfactory  or  successful. 

A  pyrometer  element  introduced,  at  C,  Fig.  2,  is  subject  to  the  same 
criticism  as  the  one  introduced  at  A,  Fig.  1,  in  the  melting  end  of  the 
furnace,  as  its  registrations  are  susceptible  to  all  the  variations  in  tem- 
perature that  may  be  produced  by  varying  fire  conditions,  draft,  and 
glass  temperature.  The  same  is  true  of  an  element  introduced  at  B, 
Fig.  2.  However,  there  is  no  flying  flux  or  alkali  to  attack  the  sheath- 
ing tube  or  element.  An  element  introduced,  at  A,  Fig.  2,  as  near  to 
the  glass  as  is  possible,  gives  about  as  effective  results  and  as  satisfactory 
as  any,  by  introducing  'the  element  through  an  open-end  protecting 
tube  so  that  possibly  from  %  to  1  in.  of  the  element  proper,  with  just  a 
thin  coating  of  clay  over  the  element  wires,  protrudes  beyond  the  open 
end  of  the  sheathing  tube.  This  will  bring  the  element  about  2  in. 
above  the  surface  of  the  glass.  Over  this  I  construct  a  clay  tile  shield 
about  2  in.  thick  and  6  in.  long,  to  protect  the  element  as  much  as  is 
possible  from  radiation  above  and  the  direct  flame  or  fire  in  the  fur- 
nace. In  this  manner,  a  registration  of  the  temperature  radiated  from 
the  glass  in  the  pot  is  obtained;  and  while  the  results  are  fairly  satis- 
factory, they  are  far  from  what  might  be  desired,  for  the  temperature 
of  the  glass  must  be  measured  to  insure  the  best  results. 

We  have  made  numerous  attempts  to  use  optical  and  radiation  pyro- 
meters to  obtain  the  temperature  of  the  glass  in  this  pot  at  the  required 
point,  but  these  attempts  have  proved  utter  failures. 

What  has  been  said  as  to  the  difficulties  encountered  in  applying 
pyrometers  to  the  Owens  pots  is  true  of  other  modes  of  gathering  or 
working  the  glass.  In  the  Hartford-Fairmont  process,  the  glass  flows 
into  a  channel,  is  then  paddled  over  a  weir,  discharged  through  an 
orifice,  cut  off  by  means  of  shears,  and  discharged  into  an  open  parison 
mold.  Here  it  is  essential,  in  order  that  a  given  weight  of  glass  shall  be 
discharged  into  the  mold  each  time,  that  the  glass  be  maintained  at  a 
definite  and  uniform  temperature.  In  the  Tucker-Reeves  method,  the 


490 


PYROMETRY   APPLIED   TO  BOTTLE-GLASS   MANUFACTURE 


FlG.   3. 


glass  flows  through  a  refractory  channel  to  an  orifice  (as  shown  in  Fig.  3)  of 
a  predetermined  size  and  then  through  controllable  periods  of  time  of  such 
flow  is  sheared  off  so  as  to  discharge  predetermined  weights  and  quantities 
of  glass  to  the  molds.  In  the  Brookes  device,  wherein  there  is  no  means 
for  regulating,  with  precision,  the  quantity  of  glass  flowing  through  the 
orifice  and  the  time  period  of  shearing,  by  a  gate  in  the  channel ,  obviously 
the  amount  discharged  is  directly  related  to  the  temperature. 

In  all  of  these  processes,  i.e.,  Hartford-Fairmont,  Tucker-Reeves,  and 
Brooks,  numerous  attempts  have  been  made  to  use  pyrometers  as  a  means 

of  controlling  the  temperature  of  the  glass, 
but  so  far  all  attempts  have  been  unsatis- 
factory. It  has  been  found  that  to  intro- 
duce the  thermoelectric  element  in  any  part 
of  this  apparatus,  it  is  necessary  to  keep  the 
same  out  of  contact  with  the  glass.  There- 
fore, the  elements  are  usually  introduced  at 
some  point  approximating  the  position 
marked  A,  as  shown  in  Fig.  3.  Although  I 
have  used,  in  some  experimental  work,  an 
element  introduced  at  B  and  into  the 
glass  stream  with  satisfactory  results,  so  far 
as  indicating  the  true  temperature  of  the  glass  is  concerned,  it  is  sub- 
stantially impossible,  from  a  practical  standpoint,  to  use  this  method  of 
installation,  because  of  the  high  erosive  action  of  the  glass  upon  the 
sheathing  tubes,  which  necessitates  the  shutting  down  of  the  machine  and 
serious  damage  to  the  spout  if  a  new  element  is  to  be  installed. 

CONCLUSIONS 

My  conclusions  on  this  subject  are  as  follows: 

That  pyrometers  applied  to  the  melting  of  glass  in  tank  furnaces  per- 
form no  useful  function  in  determining  the  regulations  or  control  of  fire 
conditions.  To  a  minor  degree,  they  do  assist  in  keeping  a  check  upon  the 
furnace  operation,  and,  more  effectively,  function  to  effect  a  psychological 
stimulus  for  the  operator. 

That  they  are  woefully  inadequate  as  a  means  for  controlling  the 
temperature  of  the  glass  at  the  working  or  refining  end  of  the  furnace, 
although  they  are  of  great  value  in  controlling  the  conditions  at  this 
point. 

That  the  thermoelectric  type  is  greatly  superior  to  the  radiation, 
optical,  or  disappearing-filament  types,  except  in  possibly  special  or 
isolated  cases. 

That  the  invention  or  design  of  a  protecting  tube  that  will  withstand 
the  erosive  action  of  the  glass  will  give  to  the  glass  manufacturer  a 
means  whereby  he  will  be  able  to  utilize  a  pyrometer  in  such  a  manner 
as  to  make  it  indispensable  and  of  inestimable  value. 


PYKOMETRY    IN    THE    MANUFACTURE    OF    OPTICAL    GLASS  491 


Pyrometry  in  the  Manufacture  of  Optical  Glass 

BY   ALBERT  J.    WALCOTT,*   ROCHESTER,    N.    Y. 
(Chicago  Meeting,  September,  1919) 

THE  success  of  various  operations  in  the  manufacture  of  optical  glass 
depends,  in  a  large  measure,  on  the  ability  to  maintain  proper  heat 
control.  A  good  pyrometer  system  is,  therefore,  a  very  necessary  part  of 
the  equipment  needed.  Men  of  long  experience  in  dealing  with  high  tem- 
peratures, who  have  acquired  through  constant  practice  the  ability  to 
judge  approximate  temperatures  by  means  of  color,  are  undoubtedly 
valuable,  but  even  the  most  competent  are  not  as  reliable  as  a  good 
pyrometer  system.  They  admit  that  their  ability  to  judge  accurately 
varies  from  day  to  day  and  that  their  judgment  is  affected  considerably 
by  the  condition  of  the  sky,  whether  it  is  clear  or  cloudy.  Besides,  where 
it  is  a  matter  of  only  a  few  weeks  to  train  a  man  to  operate  a  pyrometer 
system,  it  takes  about  as  many  years  to  train  him  to  judge  temperatures 
by  the  state  of  incandescence. 

The  pyrometer  outfit  in  use  in  the  glass  plant  of  the  Bausch  &  Lomb 
Optical  Co.  consists  of  the  following :  Platinum  elements,  optical  pyrome- 
ters, base-metal  elements,  indicators,  potentiometers,  and  recording 
pyrometers. 

Platinum  Elements  and  Optical  Pyrometers. — Temperature  control  in 
the  melting  operation  is  of  such  great  importance  that  it  is  very  desirable 
to  know  the  exact  temperatures,  relative  temperatures  being  of  little 
value.  The  use  of  the  platinum  element  is  to  determine  the  rate  of 
heating  or  cooling  of  the  furnace.  The  platinum  couple  is  installed  in 
either  the  crown  or  the  wall  of  the  furnace.  It  is  placed  first  in  a  porce- 
lain protecting  tube  and  this  is  placed  in  a  permanently  fixed  fireclay 
tube  projecting  from  1^  to  2  in.  into  the  fire-chamber.  It  has  been 
found  impractical  to  use  the  platinum  couple  for  determining  exact 
temperatures  of  the  melting  furnace,  for  the  platinum  element  deteriorates 
rather  rapidly  at  the  high  temperatures  used,  2500°  to  2700°  F.  (1371° 
to  1482°  C.).  When  protected  and  installed  as  described,  though, 
the  temperature  determined  is  local  and  varies  from  150°  to  200°  F. 
(83°  to  111°  C.)  from  the  actual  temperatures  of  the  glass  in  the  furnace. 

For  the  greater  part,  the  platinum  element  is  used  for  general  control. 
When  the  same  temperature  schedule  is  followed  day  after  day  the  plati- 


*  Research  Physicist,  Bausch  &  Lomb  Optical  Co. 


492  PYROMETRY   IN   THE   MANUFACTURE    OF    OPTICAL   GLASS 

num  element  might  be  used  satisfactorily  for  exact  temperature  control 
by  making  necessary  corrections  for  deviations  from  actualjtemperatures. 
Temperatures  are  read  by  means  of  a  direct-reading  indicator. 

For  determining  actual  temperatures  in  the  melting  operation  the 
more  satisfactory  way  is  by  means  of  an  optical  pyrometer.  The  Leeds 
&  Northrup  optical  pyrometer  has  been  found  quite  satisfactory.  The 
instruments  are  frequently  checked  against  each  other  and  also  against 
a  standard  reserve  lamp.  A  water-cooled  platinum  element  has  also 
been  used  as  a  check.  At  temperatures  from  2500°  to  2600°  F.,  the  optical 
pyrometer  has  been  found  to  be  correct  within  5  to  10°  F.  Errors  are 
likely  to  be  introduced  in  the  use  of  the  optical  pyrometer  by  not  focusing 
on  the  proper  places  in  the  furnace.  Parts  of  the  furnace  coming  in  actual 
contact  with  the  flame  are  considerably  hotter  than  the  pot  and  should 
therefore  be  avoided.  By  focusing  on  the  surface  of  the  glass  while  flame 
is  in  the  furnace,  errors  as  high  as  90°  F.  are  introduced,  caused  by  the 
reflection  of  the  flame  from  the  glass.  Fairly  satisfactory  results  have 
been  obtained  by  focusing  on  the  rim  of  the  pot. 

After  the  flame  has  been  turned  off,  temperature  determinations 
may  be  made  from  the  surface  of  the  glass.  When  this  is  done  it  must  be 
understood,  however,  that  the  temperature  determined  is  that  of  the 
bottom  of  the  pot  and  not  of  the  surface  of  the  glass,  the  color  from  the 
bottom  of  the  pot  being  transmitted  through  the  glass. 

Base-metal  Couples. — A  very  important  application  of  pyrometry  in 
optical  glass  industries  is  the  use  of  base-metal  couples  for  maintaining 
temperature  control  in  annealing  optical  glass.  Considerable  literature 
has  been  published  recently  on  various  phases  of  the  optical-glass  manu- 
facture, including  annealing,  so  that  it  will  not  be  necessary  to  give 
details  here.  All  optical  glass,  before  it  can  be  prepared  by  the  Precisions 
Optics  Department  to  be  put  into  work,  must  be  comparatively  free  from 
internal  stresses,  so  that  very  careful  annealing  is  necessary.  A  large 
amount  of  optical  glass  is  pressed  into  prisms  and  lenses,  varying  in  size 
from  a  few  cubic  inches  to  35  and  40  cu.  in.,  and  then  annealed;  some  glass 
is  annealed  in  the  form  of  large  cast  sheets.  The  kilns  and  lehrs  used 
for  annealing  in  the  glass  plant  of  the  Bausch  &  Lomb  Optical  Co.  are 
gas-fired;  the  kilns  are  well  insulated,  thus  giving  a  fairly  uniform  dis- 
tribution of  heat. 

A  3-ft.,  heavy-type,  base-metal  couple  of  Hoskins  make,  placed  in  a 
chromel  protecting  tube,  is  inserted  through  the  crown  of  the  kiln.  An 
immersion  of  about  18  in.  in  most  cases  brings  the  end  of  the  protecting 
tube  near  the  center  of  the  chamber  in  which  the  glass  is  placed.  The 
kilns  are  grouped  in  batteries  containing  from  six  to  ten.  All  the  couples 
of  a  battery  are  connected  to  a  cold-junction  well,  8  to  10  ft.  deep,  near 
the  battery,  and  to  a  junction  box  immediately  above  the  well  by  means 


ALBERT   J.    WALCOTT  493 

of  compensating  wire.  From  the  junction  box,  copper  lead  wires,  con- 
ducted through  conduits,  are  brought  to  a  selective  switch,  arranged  for 
50  couples,  in  a  special  pyrometer  room.  Temperatures  are  read  by  means 
of  a  Leeds  &  Northrup  portable  potentiometer.  By  means  of  a  double- 
throw  switch,  it  is  possible  to  connect  with  a  direct-reading,  high-resist- 
ance, Taylor  indicator.  The  potentiometer  has  been  found  the  more 
satisfactory;  hence  the  indicator  is  used  only  in  cases  of  emergency. 
A  man  is  employed  to  make  temperature  readings,  make  a  careful  record 
of  temperatures  and  regulate  the  burners,  a  satisfactory  automatic 
temperature  control  having  as  yet  not  been  found.  A  reading  of  each 
kiln  is  taken  every  15  minutes. 

The  couples  in  use  are  frequently  checked  against  a  standard  plati- 
num platinum-rhodium  couple.  All  checks  are  made  in  a  small  auxil- 
iary furnace  and  done  with  considerable  care.  Besides  these  checks  the 
couples  are  checked,  while  in  place  in  the  kiln,  every  other  time  the  kiln 
is  brought  to  a  maximum  temperature.  Such  checks  are  made  by  insert- 
ing a  "master"  base-metal  couple  through  an  extra  aperture  about  4  in. 
away  from  the  regular  couple.  Careful  records  are  kept  of  all  checks 
made.  Temperature  observations  of  cold-junction  wells  are  made  once 
a  month. 

Besides  the  installation  of  base-metal  couples  used  for  fine  annealing, 
a  separate  installation  of  nine  couples  is  used  in  connection  with  a  large 
lehr.  Here  also  a  special  man  is  employed  to  watch  the  action  of 
the  couples  and  regulate  the  burners  of  the  lehr.  The  highest  tempera- 
ture used  for  any  part  of  the  annealing  work  is  about  1200°  F.  (648°  C.). 
There  is,  therefore,  no  great  danger  of  rapid  deterioration  of  the 
couples. 

Since  the  maximum  temperature  to  which  a  kiln  is  heated  is  a  matter 
of  considerable  importance,  where  several  different  kilns  may  be  used  for 
annealing  the  same  class  of  work,  the  couples  should  agree  so  closely  as 
to  be  interchangeable.  Whatever  variation  from  the  standard  does 
exist  in  any  of  the  couples  is  recorded  and  taken  into  account  in  bringing 
kilns  to  maximum  temperatures.  A  couple  whose  millivoltage  for  any 
given  temperature  varies  from  day  to  day  is  very  undesirable  for  fine 
annealing  work.  The  greatest  variation  of  any  of  the  couples  in  use  at 
the  Bausch  &  Lomb  plant,  as  determined  there,  is  10°  F.  at  1000°  F. 
(5.6°  C.  at  538°  C.). 

Another  practical  application  of  base-metal  couples  has  been  found 
in  determining  temperatures  of  furnaces  used  for  bending  and  annealing 
large  searchlight  mirrors.  The  potentiometer  is  used  for  determining 
temperatures  while  the  furnace  is  being  heated  to  maximum  temperature. 
After  the  gas  has  been  turned  off,  the  cooling  curve  is  determined  by 
means  of  a  recording  pyrometer. 


494  PYROMETRY   IN   THE   MANUFACTURE    OF   OPTICAL   GLASS 

The  heat  control  of  pot  arches,  used  for  heating  glass  pots  before 
being  placed  in  the  melting  furnaces,  does  not  require  a  high  enough 
degree  of  accuracy  to  justify  the  installation  of  a  pyrometer  system. 
However,  since  the  maximum  temperature  used  for  this  operation  is 
approximately  1800°  F.,  base-metal  couples,  properly  protected,  could 
be  used  in  case  a  higher  degree  of  accuracy  than  is  used  at  present  should 
be  desired. 

Acknowledgment  is  due  Mr.  R.  J.  Montgomery,  of  the  Bausch  & 
Lomb  Optical  Co.'s  glass  plant,  for  furnishing  information  concerned 
with  the  use  of  the  platinum  element  and  the  optical  pyrometer  in 
connection  with  the  melting  operation. 


USE    OF    OPTICAL    PYROMETERS  495 


Use  of  Optical  Pyrometers  for  Control  of  Optical-glass  Furnaces 

BT   CLARENCE    N.    PENNEB,*   WASHINGTON,    D.    C. 
(Chicago  Meeting,  September,  1919) 

THE  manufacture  of  optical  glass  is  a  process  that  demands  careful 
regulation  and  control  at  all  stages  in  order  that  satisfactory  results  may 
be  obtained.  The  product,  to  serve  its  purpose,  must  meet  stringent 
requirements,  which  can  be  assured  only  by  careful  procedure  in  manu- 
facture. During  the  greater  part  of  the  time  that  a  pot  of  glass  is  in 
the  furnace,  temperatures  should  be  kept  within  certain  rather  narrow 
limits;  a  departure  on  either  side  is  likely  to  be  detrimental  to  the  glass 
and  may  result  in  total  loss.  If  the  temperature  during  melting  and 
fining  be  low,  the  melt  is  likely  not  to  fine  properly  and  the  glass  will 
contain  quantities  of  bubbles,  or  "seed,"  or  it  may  become  milky  and 
unfit  for  use;  too  high  a  temperature,  on  the  other  hand,  is  severe  on  the 
pot  and  may  cause  it  to  leak  or  cast  stones,  or  the  increased  solution  of 
pot  material  in  the  melt  may  add  to  the  color  of  the  glass  and  decrease 
its  transmission.  Moreover,  variations  in  the  working  temperature  will 
mean  variations  in  the  amount  of  selective  volatilization  and,  conse- 
quently, variations  in  optical  properties  from  pot  to  pot.  Further- 
more, in  the  closing  stages  of  furnace  treatment,  the  melt  is  cooled  until 
a  certain  degree  of  stiffness  is  attained,  when  the  pot  is  withdrawn. 
The  quality  of  the  glass — its  freedom  from  striae  and  bubbles — will  depend 
in  large  measure  on  whether  the  temperature  to  which  cooling  has  been 
carried  is  suitable.  For  each  type  of  glass  there  is  a  narrowly  restricted 
range  of  temperature  to  which  each  pot  of  the  given  type  should  be  cooled 
before  withdrawal.  A  quick  and  reliable  method  of  measuring  tempera- 
tures is,  therefore,  of  the  first  importance.  In  the  optical-glass  work 
conducted  by  the  staff  of  the  Geophysical  Laboratory  during  the  period 
of  the  war,  much  attention  was  given  to  the  matter  of  determining  what 
methods  would  meet  the  requirements.  This  article  will  deal  with  these 
investigations  and  with  the  application  of  the  results  to  actual  practice 
at  various  plants. 

At  the  time  that  the  work  of  the  Geophysical  Laboratory  was  begun 
at  the  Bausch  &  Lomb  plant,  each  melting  furnace  was  equipped  with 
a  thermocouple,  of  which  the  elements  were  platinum  and  an  alloy  of 
platinum  and  rhodium.  The  elements  were  inserted  in  tubes  set  in  the 
back  walls  of  the  furnaces  and  so  placed  that  the  position  of  the  ther- 
moj unctions  was  nearly  flush  with  the  inner,  or  heated,  side  of  the  rear 

*  Petrologist,  Geophysical  Laboratory,  Carnegie  Institution  of  Washington. 


496  USE   OP   OPTICAL  PYROMETERS 

walls  and  a  little  above  the  level  of  the  top  of  the  pot.  Leads  were 
brought  to  a  direct-reading  instrument  conveniently  placed  for  observa- 
tion by  the  furnace  men,  and  regulation  of  temperatures  was  dependent 
on  these  readings.  Although  the  thermocouples  were  protected  from 
contamination  by  tubes  of  dense  and  apparently  impervious  porcelain, 
it  was  recognized  that  their  readings  were  not  entirely  reliable.  Later 
work  showed  that  the  unreliability  was  greater  than  had  been  supposed. 
It  seemed  that,  for  constant  use,  a  pyrometer  of  the  Morse  or  Hol- 
born-Kurlbaum  type  would  meet  the  requirements  much  better  than 
thermocouples,  and  an  instrument  of  this  design  was  obtained  from  the 
Leeds  &  Northrup  Co. 

This  instrument  consists  essentially  of  a  telescope  with  a  small  in- 
candescent-lamp filament  placed  in  the  front  fodal  plane  of  the  eyepiece. 
The  telescope  is  directed  at  the  object  of  which  the  temperature  is  re- 
quired and  an  electric  current  is  sent  through  the  lamp  filament.  The 
strength  of  the  current  is  regulated  by  a  rheostat  until  the  brightness 
or  intensity  of  illumination  of  the  filament  matches  that  of  the  object 
sighted  upon.  The  strength  of  current  is  then  read  upon  a  milliam- 
meter,  and  the  corresponding  temperature  is  obtained  from  an  empirical 
calibration  table,  which  is  supplied  with  the  instrument.  For  measur- 
ing very  high  temperatures,  the  degree  to  which  the  heating  of  the  fila- 
ment would  have  to  be  carried  to  match  the  luminosity  of  the  furnace 
or  other  object  would  be  likely  to  produce  such  changes  in  the  filament 
as  would  affect  its  calibration  and  shorten  its  life.  Therefore,  the  instru- 
ment is  provided  with  an  absorption  screen,  which  may  be  readily  inserted 
between  the  lamp  filament  and  the  object  of  which  the  temperature  is  to  be 
measured,  and  which  cuts  down  the  intensity  of  radiation  from  the  latter. 
Naturally,  a  separate  calibration  table  must  be  used  when  the  screen  is 
inserted  and  the  sensitiveness  of  the  instrument  is  greatly  diminished. 
The  limit  at  which  it  is  considered  safe  to  make  frequent  use  of  the 
instrument  without  the  absorption  screen  is  about  1400°;  and  as  the 
accurate  determination  of  temperatures  much  higher  than  this  is  seldom 
necessary  in  optical-glass  furnaces,  the  use  of  the  screen  may  generally 
be  dispensed  with.  ; 

According  to  the  Stefan-Boltzmann  law,  the  complete  emission  of 
what  is  known  as  a  "black  body"  is  proportional  to  the  fourth  power 
of  the  absolute  temperature.1  From  this,  it  follows  that  the  brightness  of 
a  luminous  object  changes  very  rapidly  with  the  temperature  and  that 
comparatively  small  differences  of  temperature  (as  small  as  two  or  three 
degrees)  may  readily  be  perceived.  This  fact  is  of  great  importance  in 
the  practical  application  of  the  instrument. 

In  order  to  obtain  a  trustworthy  foundation  for  the  use  of  this  instru- 
ment a  rather  thorough  exploration  and  study  of  furnace  temperatures 

1  R.  W.  Wood:  "Physical  Optics."    614.    1911. 


CLARENCE   N.    FENNER  497 

was  carried  out  by  means  of  devices  to  be  described.  The  first  informa- 
tion sought  was  as  to  the  reliability  of  the  calibration  table;  to  obtain 
this  it  was  necessary  to  use  a  device  that  would  give  "black-body  radia- 
tion." A  "perfectly  black"  body  is  a  perfectly  absorbing  body  and 
emits  radiation  whose  intensity  is  a  function  of  the  temperature  alone. 
Most  substances  are  of  such  a  nature  that  when  they  are  heated  until 
they  become  luminous  the  intensity  of  their  radiation  is  dependent  upon 
an  emissivity  factor,  the  effect  of  which  is  such  that  two  bodies  at  the 
same  temperature  may  appear  unequally  bright.  Naturally  a  substance 
with  a  surface  possessing  the  power  of  reflection  will  send  to  the  eye 
reflected  rays  proceeding  originally  from  colder  or  hotter  objects  adjacent 
to  it,  and  to  a  proportional  degree  will  fail  to  send  forth  the  luminous 
rays  corresponding  to  its  own  temperature.  A  good  example  of  an 
almost  perfectly  reflecting  body  is  a  highly  polished  metallic  mirror. 
Though  cold  itself,  it  may  reflect  an  image  of  the  sun,  for  instance,  repre- 
senting a  temperature  of  many  thousand  degrees;  or,  it  may  be  heated 
to  a  temperature  of,  say,  1 000°  and  (if  the  surface  does  not  become  tar- 
nished) may  reflect  the  image  of  a  cold  and  black  substance  nearby  and 
fail  to  send  out  its  own  proper  radiation.  On  the  other  hand  "  black-body 
radiation"  is  given  out  by  a  substance  with  a  surface  that  possesses 
no  reflecting  power  and  therefore  absorbs  all  radiation  falling  upon  it.2 
No  substance  fulfills  this  condition  perfectly,  but  the  requirements  may  be 
met  satisfactorily  by  heating  uniformly  throughout  its  length  a  long  tube 
of  poorly  reflecting  material,  closed  at  one  end,  so  that  all  rays  proceed- 
ing from  the  bottom,  whether  they  originate  there  or  are  reflected  from 
another  part  of  the  tube,  correspond  to  one  and  the  same  temperature. 
To  meet  the  required  conditions  we  used  a  porcelain  tube  about  1  m. 
long  that  had  an  orifice  of  18  mm.,  and  was  stoppered  at  one  end  with  clay. 
This  tube  was  inserted  into  the  furnace  and  the  far  end  was  allowed  to 
rest  upon  the  rim  of  the  glass  pot  until  it  became  well  heated.  The 
optical  pyrometer  was  then  sighted  through  the  orifice  upon  the  clay 
plug  at  the  far  end,  and  a  reading  taken.  A  new  and  reliable  thermo- 
couple of  platinum  and  platinum-rhodium  was  then  inserted  into  the  tube 
and  readings  of  electromotive  force  were  taken  with  a  direct  reading 
milli voltmeter.  These  readings,  with  the  necessary  cold-junction  cor- 
rection, were  believed  to  represent  true  temperatures,  and  were  taken 
as  the  standard  of  reference.  Thus  tested,  the  temperatures  given 
by  the  calibration  tables  for  this  instrument  were  found  to  agree  surpris- 
ingly well  with  true  temperatures.  Some  of  the  results  are  given  in 
Table  1.  Later  tests  with  other  instruments  indicated  that  this  instru- 

1  For  a  more  complete  discussion  of  the  laws  of  radiation  and  absorption  see,  for 
example,  "Physical  Optics,"  by  R.  W.  Wood  (1911),  especially  591  and  following; 
or  "A  Text-book  of  Physics,"  edited  by  A.  W.  Duff  (1913),  279  and  following. 

32 


498 


USE    OF    OPTICAL    PYROMETERS 


ment  was  somewhat  exceptional  and  that  not  equal  care  in  calibration 
had  been  used  in  all  cases. 

TABLE  1. — Test  of  Optical  Pyrometer  by  Comparison  with  Thermocouple 
Readings  for  Black-body  Radiation 


Optical 
Pyrometer, 
Degrees  C. 

Thermocouple, 
Degrees  C. 

Difference, 
Degrees  C. 

Optical 
Pyrometer, 
Degrees  C. 

Thermocouple, 
Degrees  C. 

Difference, 
Degrees  C. 

1122                      1126                -4 

13921 

1076                      1080                -4 

1392  J 

1390 

+2 

1022 

1022                    0 

1389 

1389 

0 

1246 

1371  \ 

1248 

1246 

+2 

1366  j 

1365 

+3.5 

1250 

The  next  matter  on  which  information  was  desirable  was  that  of  the 
degree  of  agreement  of  temperatures  as  indicated  by  readings  on  the 


Fio.  1. — CONSTRUCTION  OP  TERMINAL  PORTION  OF  WATER-COOLED  DEVICE  FOR  EX- 
PLORING FURNACE  TEMPERATURES. 

furnace  walls  with  true  temperatures;  that  is,  to  ascertain  whether  the 
radiation  given  out  by  the  heated  walls  corresponded  with  black-body 
radiation,  or  whether  reflections  from  the  somewhat  glazed  surfaces 
of  the  refractory  lining  would  cause  significant  departures  from  correct 
results.  To  obtain  this  information  the  furnace  temperatures  were 
explored  with  a  specially  constructed  device,  by  which  a  thermocouple 
could  be  brought  to  any  desired  spot  and  readings  obtained,  and  to  com- 
pare these  readings  with  the  results  obtained  by  sighting  the  pyrometer 
on  the  adjacent  wall.  The  device,  shown  in  Fig.  1,  consisted  of  a  water- 
cooled  iron  tube  C  (or  assemblage  of  tubes)  9  or  10  ft.  (2.74  or  3.04  m.) 
in  length,  through  the  inner  orifice  of  which  the  compensating  leads  of 
a  thermocouple  were  carried.  The  thermocouple  proper  (consisting  of 
platinum  and  platinum-rhodium)  extended  into  a  porcelain  tube  B 
35  cm.  long  that  projects  beyond  the  water-cooled  part.  Fig.  1  shows  the 
terminal  portion  of  this  device.  The  thermoj unction  is  shown  at  A;  in 


CLARENCE    N.    FENNER 


499 


the  water-cooled  part  the  wires  are  connected  with  compensating  leads  of 
alloys  of  which  the  thermoelectric  force  over  the  range  of  temperature 
here  used  is  very  closely  equivalent  to  that  of  the  platinum  and  platinum- 
rhodium  wires  of  the  thermocouple,  so  that  no  appreciable  error  is  intro- 
duced by  subsidiary  currents  set  up  at  the  junctions.  Their  use  obviates 
the  need  of  carrying  the  platinum  and  platinum-rhodium  wires  back  10 
or  15  ft.  (3.04  or  4.57  m.)  to  the  meter.  The  thermocouple  wires  are  in- 


0       6"    1'     1'C"  2     2' 6"   3     3'6"  4'    4' 6"    5'     5'c"  6' 

FIG.  2. — CROSS-SECTION  OF  FURNACE,  SHOWING  POINTS  AT  WHICH  TEMPERATURE 
MEASUREMENTS  WERE  MADE,  AND  THE  CORRESPONDING  DISTANCE-TEMPERATURE  CURVE. 

sulated  by  porcelain  capillaries  (not  shown)  and  the  compensating  leads 
by  glass  tubes.  Asbestos  wool  D  is  packed  around  the  porcelain  tube 
to  hold  it  in  place  and  also  to  serve  as  a  heat  insulator  and  prevent  a 
too  severe  temperature  gradient  where  the  porcelain  tube  B  enters  the 
iron  tube  C.  The  iron  tube  C,  supply  pipe  E  for  the  water-circulating 
system,-  and  the  return  pipe  F  extend  back  to  the  outside  of  the  furnace, 
where  hose  connections  are  made  to  pipes  E  and  F  and  a  bushing  is  fitted 
to  the  .tube  C,  through  which  the  leads  are  carried.  The  fitting  G  by 


500 


USE    OF   OPTICAL   PYROMETERS 


which  the  various  parts  are  held  together  and  kept  in  position  was  the 
only  special  construction  required.  This  device  had  to  be  carefully 
handled  and  was  not  adapted  for  constant  use,  but  it  served  the  purpose 
for  which  it  was  devised  and  gave  the  required  information.  One  of 
the  first  purposes  for  which  it  was  used  was  to  ascertain  the  distribution 
of  temperatures  from  the  front  to  the  rear  of  a  furnace  in  which  a  pot 
of  glass  was  being  held  at  fining  temperature.  The  results  are  shown  in 
Table  2  and  Fig.  2. 

TABLE  2. — Distribution   of   Temperatures   in   a  Furnace   under    Fining 

Conditions  .   ••  • 


Distance    from     Front 
Face  of  Tuille, 
Feet 

Temperature, 
Degrees  C. 

Distance      from      Front 
Face  of  Tuille, 
Feet 

Temperature, 
Degrees  C. 

0.5 

1222 

3.5 

1396 

1.0 

1273 

4.0 

1403 

1.5 

1287 

4.5 

1403 

2.0 

1365 

5.0 

1403 

2.5 

1373 

5.5 

1398 

3.0 

1386 

6.0 

1395 

A  comparison  of  temperatures  as  determined  by  the  exploratory 
thermocouple  and  by  the  optical  pyrometer  showed  that  the  interior 
walls  of  the  furnace  gave  radiations  that  agreed  to  a  very  satisfactory 
degree  with  black-body  radiations  at  the  temperatures  at  which  most 
of  the  furnace  operations  are  conducted.  The  results  of  the  determina- 
tions are  given  in  Table  3. 

TABLE  3. — Comparison  of  Furnace  Temperatures  as  Read  by  Exploratory 
Thermocouple  and  by  Optical  Pyrometer 


Thermo- 
couple 
Reading, 
Degrees  C. 

Pyrometer 
Reading, 
Degrees  C. 

Difference 
in  Readings, 
Degrees  C. 

Thermo- 
couple 
Reading, 
Degrees  C. 

Pyrometer 
Reading, 
Degrees  C. 

Difference 
in  Readings, 
Degrees  C. 

1320 

1322 

+2 

1273 

1274 

+  1 

1315 

1320 

+5 

1398 

1397 

-1 

1311 

1320 

+9 

1398 

1397 

_  j 

1309 

1312 

+3 

1411 

1413 

+2 

1278 

1282                    +4 

1412 

1413                    +1 

The  very  close  agreement  shown  is  doubtless  due,  in  some  measure, 
to  a  counterbalancing  of  errors,  as  the  methods  used  were  not  of  the  degree 
of  precision  indicated  by  these  figures.  We  believed,  however,  that  the 
necessary  dependability  of  readings  at  these  temperatures  by  the  optical 
pyrometer  was  established.  At  lower  temperatures  the  agreement 


CLARENCE   N.    FENNER  501 

was  less  perfect.  At  a  temperature  of  1000°  or  1050°,  the  readings  taken 
by  the  optical  pyrometer  on  the  rear  wall  were  likely  to  be  as  much  as  40° 
or  50°  too  high,  and  might  be  even  more  in  error.  This  wa,s  probably 
to  be  ascribed  to  departure  from  theoretical  black-body  conditions. 
The  rear  wall  of  the  furnace  probably  receives  and  reflects  radiations 
from  hotter  regions  adjacent,  such  as  the  cap  on  which  the  flames  play 
or  from  the  flames  themselves,  and  a  source  of  error  is  thus  introduced. 
It  was  noticeable  that  when  the  temperature  was  not  very  high  the  parts 
of  the  rear  and  end  walls  that  are  immediately  adjacent  to  each  other 
were  likely  to  appear  of  unequal  brightness,  though  there  could  hardly 
be  much  actual  difference  in  temperature. 

These  investigations  supplied  sufficient  data  to  enable  us  to  use  the 
instrument  with  confidence  during  the  operations  of  melting  and  fining. 
It  was  therefore  used  daily  as  a  substitute  for  the  thermocouples,  though 
the  latter  were  still  considered  useful  to  a  certain  extent  when  properly 
controlled  by  the  pyrometer.  It  was  necessary,  however,  in  order 
to  use  the  thermocouples  at  all,  to  keep  a  constant  check  upon  their 
readings.  Not  only  were  their  indications  always  too  low  by  100°  to 
150°,  but  the  error  was  not  constant;  there  were  likely  to  be  gradual 
fluctuations  each  day.  No  investigation  was  made  into  the  reason  for 
this,  but  the  supposition  was  that  the  fluctuations  were  to  be  attributed 
to  contamination  by  the  combustible  gases  or  by  material  volatilized 
from  the  melts. 

The  furnace  men  were  instructed  in  the  use  of  the  optical  pyrometer, 
and  a  number  of  them  showed  considerable  skill,  so  that  we  felt  confident 
that  during  the  night  shifts  the  pyrometer  would  be  properly  used  for 
regulating  or  maintaining  temperatures.  Immediately  following  its 
introduction,  two  gratifying  results  ensued:  The  occasional  pots  of 
milky  glass  ceased  almost  entirely  and  the  losses  from  leakage  of  pots 
were  almost  eliminated.  We  think  these  results  were  due  to  the  better 
knowledge  of  temperatures  than  was  possible  before  and  to  the  ability 
to  keep  the  temperatures  where  wanted. 

Our  experience  with  the  optical  pyrometer  at  the  Bausch  &  Lomb 
plant  caused  us  to  install  similar  instruments  at  the  plants  of  the  Spencer 
Lens  Co.  at  Hamburg,  New  York,  and  of  the  Pittsburgh  Plate  Glass  Co. 
at  Charleroi,  Pa.,  when  the  work  of  the  Geophysical  Laboratory  was 
extended  to  these  plants.  We  felt  it  advisable  to  check  the  calibration 
of  the  new  instruments,  as  so  much  depended  on  their  correctness.  The 
results  were  not  as  favorable,  upon  the  whole,  as  with  the  first  instrument.3 
The  data  obtained  in  one  case  are  given  in  Table  4.  In  this  instance 
we  did  not  use  an  exploratory  thermocouple  and  therefore  were  not  able 

3  From  a  mechanical  standpoint  also  these  instruments  were  defective  in  a  number 
of  respects,  and  this  cause'd  us  a  good  deal  of  annoyance  and  trouble.  Later  in- 
struments have  been  considerably  improved. 


502  USE    OF    OPTICAL   PYROMETERS 

to  obtain  as  full  information  regarding  the  distribution  of  temperatures, 
but  the  method  employed  was  adequate  for  the  main  purpose  in  view. 
It  consisted  of  inserting  into  the  furnace  a  long  porcelain  tube,  stoppered 
with  clay  (similar  to  that  already  described),  so  that  the  closed  end  was 
over  the  center  of  the  pot;  of  reading  temperatures  with  the  optical  pyrom- 
eter sighted  through  the  tube  upon  the  stopper  (which  corresponded  to 
a  black  body)  and  also  upon  the  furnace  wall  back  of  the  pot;  and  of  com- 
paring these  with  readings  obtained  by  a  new  and  tested  thermocouple 
introduced  into  the  tube.  These  readings  were  taken  under  conditions 
that  were  probably  rather  less  favorable  than  those  ordinarily  met  in 
similar  testing  work  in  a  commercial  establishment;  that  is,  they  were 
made  during  the  pressure  of  other  duties  and  at  a  time  when  the  demands 
upon  furnace  capacity  forbade  much  delay  and  made  it  undesirable  to 
attempt  to  hold  the  temperatures  exactly  constant  during  each  set  of 
readings.  The  degree  of  precision  attained  was,  therefore,  considerably 
less  than  would  be  possible  under  laboratory  conditions,  but  the  results 
may  be  taken  as  fairly  representative  of  what  may  be  easily  realized 
in  practical  work  and  they  are  therefore  given  in  some  detail.  Two 
observers  A  and  B  worked  in  conjunction. 

A  porcelain  tube  39  in.  (1  m.)  long  was  inserted  through  the  small 
opening  in  the  tuille  and  rested  on  the  empty  pot.  The  distance  to  the 
end  of  the  tube  from  the  outside  of  the  tuille  was  37  in.  (93.9  cm.)  and 
the  distance  of  the  thermoj  unction  from  the  outside  of  the  tuille  was 
33  in.  (83.8  cm.).  The  cold  junction  of  the  thermocouple  was  in  ice 
water  and  the  readings  were  taken  by  a  direct-reading  millivoltmeter. 
Table  4  shows  that  the  readings  of  the  optical  pyrometer  were  in  general 
20°  to  30°  low;  25°  was  adopted  as  representing  the  error  with  a  reason- 
able degree  of  closeness. 

A  calibration  of  this  kind  is  not  very  difficult  and  should  always  be 
carried  out  when  a  new  instrument  is  put  in  service.  Without  this  cali- 
bration, it  may  be  possible  to  use  the  instrument  empirically  and  get 
reproducible  results,  when  all  the  conditions  are  kept  constant,  but  the 
object  should  be  to  determine  the  true  temperature  values.  It  is  only 
by  means  of  such  information  that  comparisons  may  be  made  with  other 
establishments  or  that  reproducibility  may  be  secured  when  a  change  of 
furnace  construction  is  made  or  a  new  working  procedure  is  put  into  effect. 
Such  a  calibration  is  necessary  in  order  not  only  to  check  the  manufac- 
turer's calibration,  but  also  to  determine  whether  the  furnace  walls 
indicate  a  temperature  that  corresponds  to  that  of  the  pot. 

There  is  probably  some  change  in  the  latter  respect  with  length  of 
service  of  the  lining.  A  new  lining  is  likely  to  have  less  reflecting  power 
and  to  give  truer  readings  than  one  that  has  been  in  service  for  several 
months  and  has  become  somewhat  glazed.  Moreover,  the  calibration 
of  the  pyrometer  itself  may  change.  With  several  instruments  that  we 


CLARENCE    N.    FENNER  503 

TABLE  4. — Data  Obtained  in  Calibration  of  Optical  Pyrometer 


No. 

Thermo- 
couple 
Reading, 
Degrees  C. 

Optical 
Pyrometer 
Reading, 
Degrees  C. 

Observe^ 

Average  of 
Part  Sighted   On                Optical 
With  Optical                  Pyrometer 
Pyrometer                   Readings, 
Degrees  C. 

Error  of 
Optical 
Pyrometer, 
Degrees  C. 

1 

1046 

1028 

B 

Inside  of  porcelain  tube 

1028 

B 

Inside  of  porcelain  tube 

1025 

A 

Inside  of  porcelain  tube 

1028 

A 

Inside  of  porcelain  tube 

1028 

A 

Rear  wall   of  furnace 

1022 

A 

Rear  wall  of  furnace 

1028 

B 

Rear  wall  of  furnace 

1028 

B 

Rear  wall  of  furnace            1027 

19  low 

2a 

1103 

1076 

A 

Inside  of  porcelain  tube 

1076 

A 

Inside  of  porcelain  tube 

1066 

B 

Inside  of  porcelain  tube 

1076 

B 

Inside  of  porcelain  tube          1074 

28  low 

26 

1110 

1082 

B 

Rear  wall  of  furnace 

1082 

B 

Rear  wall  of  furnace 

1087 

A 

Rear  wall  of  furnace 

1085 

A 

Rear  wall  of  furnace            1084 

26  low 

2c 

1110 

1087 

B 

Inside  of  porcelain  tube 

1089 

B 

Inside  of  porcelain  tube 

1092 

A 

Inside  of  porcelain  tube 

1089 

A 

Inside  of  porcelain  tube          1089 

21  low 

3a 

1189 

1155 

B 

Inside  of  porcelain  tube 

1157 

B 

Inside  of  porcelain  tube 

1154 

A 

Inside  of  porcelain  tube 

1161 

A 

Inside  of  porcelain  tube 

1161 

A 

Inside  of  porcelain  tube 

1152 

A 

Inside  of  porcelain  tube          1157 

32  low 

35 

1189 

.     1161 

B 

Rear  wall  of  furnace 

1161 

B 

Rear  wall  of  furnace 

« 

1156 

A 

Rear  wall  of  furnace 

1156 

A 

Rear  wall  of  furnace            1159 

30  low 

4 

1225 

1200 

B 

Inside  of  porcelain  tube 

1208 

B 

Inside  of  porcelain  tube 

1216 

B 

Inside  of  porcelain  tube 

1219 

A 

Inside  of  porcelain  tube 

1219 

A 

Inside  of  porcelain  tube 

1223 

A 

Inside  of  porcelain  tube 

1226 

B 

Inside  of  porcelain  tube 

1228 

B 

Inside  of  porcelain  tube 

1219 

B 

Inside  of  porcelain  tube 

1221 

B 

Rear  wall  of  furnace 

1221 

B 

Rear  wall  of  furnace 

1219 

A 

Rear   wall  of  furnace 

1216 

A 

Rear  wall  of  furnace 

1219 

A 

Rear  wall  of  furnace 

1237 

1218 

13  low 

Av.,      1231 

5 

1248 

1212 

B 

Inside  of  porcelain  tube 

1219 

B 

Inside  of  porcelain  tube 

1216 

B 

Inside  of  porcelain  tube 

1223 

A 

Inside  of  porcelain  tube 

1226 

A 

Inside  of  porcelain  tube 

1228 

A 

Inside  of  porcelain  tube 

1225 

B 

Inside  of  porcelain  tube  • 

1221 

B 

Inside  of  porcelain  tube 

1224 

B 

Rear  wall  of  furnace 

1224 

B 

Rear  wall  of  furnace 

1226 

A 

Rear   wall   of   furnace     . 

1223 

A 

Rear  wall  of  furnace 

1223 

A 

Inside  of  porcelain  tube          1222 

26  low 

6 

1316  \ 
1323  / 

1298 

B 

Inside  of  porcelain  tube 

1292 

B 

Inside  of  porcelain  tube 

1295 

B 

Inside  of  porcelain  tube 

1285 

A 

Inside  of  porcelain  tube 

1300 

A 

Inside  of  porcelain  tube 

1302 

A 

Inside  of  porcelain  tube 

1297 

A 

Rear   wall  of  furnace 

1300 

A 

Rear  wall  of  furnace 

1306 

B 

Rear  wall  of  furnace 

1304 

B 

Rear  wall  of  furnace 

1316 

1298 

20  low 

Av.,      1318 

"The  inconsistency  of  this  result  with  the  others  of  the  series  is  evidently  due  to  variations  in  the 
temperature  of  the  furnace  while  the  readings  were  in  progress. 


504  USE    OF   OPTICAL   PYROMETERS 

were  using,  the  current  required  to  produce  a  given  intensity  of  filament 
luminosity  changed  gradually  with  use.  For  these  reasons  it  is  essential 
to  make  occasional  recalibrations  of  the  instruments  in  order  to  be  able 
to  place  reliance  upon  their  indications.  The  most  troublesome  part  of 
such  a  calibration  is  sighting  through  the  long  porcelain  tube  upon  a 
rather  small  area  at  the  end.  Naturally  there  is  likely  to  be  some  varia- 
tion in  the  results,  and  therefore  sufficient  readings  should  be  taken  to 
lessen  the  error.  To  match  the  luminosity  of  the  filament  with  that  of  a 
large  object,  such  as  the  furnace  wall,  is  comparatively  easy,  and  in  a 
series  of  such  readings  the  difference  between  the  highest  and  lowest 
should  seldom  exceed  5°  or  6°.  Different  observers  also  should  agree 
closely  in  their  readings.  Unless  extraordinary  demands  are  being  made 
upon  furnace  capacity,  which  will  forbid  keeping  the  furnace  out  of 
operation  for  any  considerable  period,  it  will  usually  be  possible  to  hold 
the  temperature  steady  during  a  set  of  readings,  which  will  aid  greatly 
in  giving  consistent  results.  If  a  laboratory  is  available,  in  which  an 
electric  resistance  furnace  forms  part  of  the  equipment,  the  work  of 
calibrating  the  lamp  may  be  considerably  facilitated,  as  the  necessary 
apparatus  may  be  set  up  in  more  convenient  form  and  more  constancy 
of  temperatures  may  be  maintained.  In  order  to  obtain  black-body 
conditions  under  such  circumstances  we  used,  in  some  of  our  calibration 
work,  baffles  so  disposed  around  the  orifice  of  the  furnace  as  to  cut  off  from 
the  chamber  into  which  the  instrument  was  sighted  practically  all  radia- 
tion from  regions  of  lower  temperature.4  The  true  value  of  the  tempera- 
ture within  the  chamber  was  given  by  a  thermocouple.  It  should  be 
noted  that  such  a  method  of  calibration  gives  results  that  apply  to  black- 
body  conditions  only,  and  that  for  practical  use  it  is  still  necessary  to 
determine  to  what  degree  the  walls  of  a  given  furnace  fulfill  these 
conditions. 

The  use  of  the  instrument  in  daily  work  hardly  requires  extended  de- 
scription. During  a  large  part  of  the  time  that  a  pot  of  glass  is  in  the 
furnace  the  temperature  should  be  maintained  at  a  constant  high  level, 
usually  about  1400°  C.,  and  all  that  is  required  is  that  the  pyrometer  is 
used  often  enough  to  maintain  the  constancy  of  temperature.  Later,  in 
the  final  stages  of  stirring,  the  fire  is  turned  off  and  the  temperature  is 
allowed  to  drop.  When  a  certain  temperature  is  reached  (which  will 
vary  with  the  type  of  glass)  stirring  is  stopped  and  the  pot  is  removed. 
It  is  of  great  importance  that  this  be  done  at  the  proper  moment,  and  the 
optical  pyrometer  is  well  adapted  for  following  the  temperature  of  the 
cooling  glass.  A  few  minutes  before  the  end  of  operations,  an  extra- 
polation is  made  on  a  plot  of  the  cooling  curve  to  the  required  tempera- 
ture and  the  corresponding  time  for  removal  is  thus  determined.  A  pre- 

4  See  "Methods  of  Measuring  Temperature,"  by  Ezer  Griffiths  (1918),  116,  or 
"Measurement  of  High  Temperatures,"  by  G.  K.  Burgess  and  H.  Le  Chatelier 
(1912),  240. 


CLARENCE   N.    FENNER  505 

caution  that  should  be  kept  in  mind  is  to  observe  some  definite  rule  as  to 
the  part  of  the  surface  of  the  glass  upon  which  readings  are  taken,  as 
there  are  noticeable  differences  in"  brightness  over  the  surface  during  the 
final  stages  of  stirring.  A  good  plan  is  to  sight  upon  the  area  immediately 
behind  the  stirring-rod,  as  the  movement  of  the  rod  through  the  glass 
brings  hotter  material  to  the  surface  and  the  area  mentioned  is  likely 
to  give  the  brightest  glow. 

Although  the  paper  has  referred  more  especially  to  the  procedure 
adopted  when  open  pots  are  used,  the  same  principles  apply  to  covered 
pots.  With  these  the  arrangement  is  such  that  in  sighting  at  the  interior 
wall  of  the  pot  the  reading  is  even  more  likely  to  correspond-to  that  of  a 
black  body  and  to  represent  the  temperature  of  the  melt  than  when  the 
wall  back  of  the  pot  is  sighted  upon,  as  is  done  when  open  pots  are  used. 
There  is  a  little  uncertainty  as  to  the  effect  upon  the  readings  produced 
by  the  volatilized  sublimates,  but  it  does  not  seem  that  they  should 
be  a  factor  of  significance.  It  is  only  as  these  fumes  escape  into  the 
open  air  and  are  condensed  that  sighting  through  them  should  have  a 
noticeable  effect  upon  the  readings,  and  even  there  they  are  so  tenuous 
that  it  is  doubtful  whether  the  effect  is  appreciable.  Moreover,  they 
often  come  out  in  such  a  manner  that  it  is  possible  to  sight  under  or  over 
them  rather  than  through  them. 

Some  workmen  may  be  found  to  whom  the  regulation  of  temperatures 
by  means  of  an  instrument  of  this  kind  may  be  entrusted  with  confidence. 
Naturally,  such  operations  as  plotting  a  temperature-time  curve  of  a 
pot  of  cooling  glass  require  some  special  ability,  but  the  procedure  may 
be  simplified  by  having  sheets  of  coordinate  paper  prepared,  with  the 
hours  of  the  day  printed  along  the  bottom  and  the  degrees  of  tem- 
perature at  the  side.  The  use  of  these  is  quite  easily  understood.  Never- 
theless, the  superintendent  or  foreman  must  exercise  considerable  super- 
vision in  order  to  get  the  best  results.  This,  however,  is  true  of  all 
steps  in  the  making  of  optical  glass. 

DISCUSSION 

CLARENCE  N.  FENNER. — Mr.  Gelstharp,  chief  chemist  of  the  Pitts- 
burgh Plate  Glass  Co.,  has  directed  my  attention  to  the  fact  that  that 
company  was  using  optical  pyrometers  obtained  from  the  Leeds 
&  Northrup  Co.  several  months  before  the  members  of  the  Geophysical 
Laboratory  arrived  at  Charleroi.  I  am  glad  to  make  this  correction. 
The  chief  object  of  my  paper  was  to  emphasize  the  necessity  of  careful 
temperature  control  in  the  making  of  optical  glass  and  to  describe  the 
kind  of  tests  to  which  an  optical  pyrometer  should  be  subjected  at  a 
glass  plant  in  order  to  obtain  information  regarding  the  closeness  with 
which  its  readings  correspond  with  true  temperatures  under  the  condi- 
tions which  prevail  in  each  particular  case. 


506     PYROMETRY  AS  APPLIED  TO  MANUFACTURE  OF  OPTICAL  GLASS 


Pyrometry  as  Applied  to  Manufacture  of  Optical  Glass 

BY   CARL   W.    KEUFFEL,*   M.  E.,  NEW   YORK,    N.   Y. 
(Chicago  Meeting,  September,  1919) 

THE  manufacture  of  optical  glass  is  a  new  industry  in  this  country. 
In  1914,  after  the  war  started,  the  supply  of  optical  glass  from  Europe 
was  cut  off,  but  as  there  was  a  fairly  large  stock  of  glass  on  hand,  it  was 
not  until  the  end  of  1915  that  the  optical  instrument  makers  made  serious 
efforts  to  produce  their  own  glass.  These  early  attempts  were  generally 
made  by  practical  glass  men  and,  due  primarily  to  the  fact  that  very  little 
scientific  help  was  used,  the  first  attempts  were  unsuccessful.  It  was  not 
until  the  whole  problem  was  attacked  by  technically  trained  men  who 
used  advanced  methods  of  research  that  good  results  were  obtained. 
These  researches  proved  that  the  accurate  control  of  the  temperature  was 
a  most  important  factor  in  the  manufacture  of  optical  glass. 

The  early  glass  maker  judged  temperatures  with  his  eyes;  later,  Seger 
cones  were  introduced  into  the  furnace  at  various  stages  of  the  melt. 
Then  radiation,  indicating  and  recording  pyrometers  with  rare-metal 
thermocouples  were  introduced.  Now,  in  addition  to  these,  an  optical 
pyrometer  every  hour  or  so  is  sighted  right  into  the  pot  and  on  the  molten 
glass. 

Figs.  1  and  2  show  how  pyrometers  are  used  to  control  the  melt.  In  Fig. 
2,  Y  is  the  clay  pot  in  which  the  batch  is  melted  to  glass  within  the  fur- 
nace X.  The  rare-metal  thermocouple  S,  protected  by  a  clay  tube, 
extends  through  the  wall  and  about  4  in.  into  the  furnace.  This  couple 
is  connected  to  an  indicator  T  and  a  recorder  U  so  that  a  complete 
record  of  the  furnace  temperatures  is  obtained.  The  section  of  the  curve 
ABCD,  Fig.  1,  shows  how  gradually  and  carefully  the  clay  pot  must  be 
heated  to  get  rid  of  moisture  AB  and  during  the  calcining  at  CD.  The 
stem  of  an  expansion  pyrometer  R  is  extended  through  a  hole  in  the  door 
Z  of  the  furnace  and  into  the  pot  in  order  to  more  closely  follow  the  tem- 
peratures of  the  clay  pot  itself  during  the  burning  process,  that  is  from 
A  to  D.  After  the  pot  is  calcined,  the  temperature  can  be  rapidly  raised 
to  E  (about  1430°  C.);  from  E  to  F  this  temperature  is  much  higher 
than  at  any  other  stage  in  order  to  slightly  overburn  the  pot,  which  will 
become  very  dense  and  shrink  to  its  final  size.  The  temperature  is  then 
lowered  to  about  1400°  C.  and  at  G  to  H  the  batch  is  introduced  in  about 
four  to  five  charges.  From  H  to  M  the  glass  is  thoroughly  mixed  by 

*  Supervisor,  Optical  Department,  Keuffel  &  Esser  Co. 


CARL    W.    KEUFFEL 


507 


means  of  a  stirring  machine  and  a  clay  stirring  rod  until  it  is  homogeneous. 
This  stirring  is  kept  up  until  a  certain  point  N  is  reached,  where  the  glass 
becomes  quite  viscous,  when  the  whole  operation  is  stopped.  The  tem- 
perature is  then  reduced  to  room  temperature  at  such  a  rate  that  the 


Temp. 
Deg.C 


1500^ 


1000" 


600  °_ 


E  F 


Mon.  Tues.  Wed.  Thur.  F'ri. 

FIG.  1. — TEMPERATURE  CYCLE. 


Sat. 


glass  in  the  pot  will  solidify  and,  due  to  strain  formed  during  this  cooling, 
break  up  into  small  chunks.  The  size  of  the  chunks  depends  on  the  rate 
of  cooling.  During  the  part  GHMN  of  the  cycle,  accurate  temperatures 
are  taken  on  the  surface  of  the  glass  Y,  Fig.  2,  in  the  pot  by  means  of  an 


FIG.  2. — SECTION  OP  MELTING  FURNACE. 

optical  pyrometer  V.  It  is  by  watching  this  part  of  the  cycle  closely, 
and  controlling  temperatures  accurately,  that  it  has  been  possible  to  re- 
produce successfully  the  various  kinds  of  optical  glass.  In  fact,  the 
quality  of  the  glass  produced  depends  more  on  accurate  temperature 
control  than  on  any  other  one  factor. 

The  chunks  of  glass  produced  must  be  molded  or  pressed  into  various 


508         PYROMETRY    AS    APPLIED    TO    MANUFACTURE    OF    OPTICAL    GLASS 

forms  to  make  them  ready  for  use  in  optical  instruments.  This  is  done 
by  slowly  heating  the  chunks  to  a  bright  red,  when  they  will  be  soft  enough 
to  be  pressed  into  form.  These  formed  pieces  must  then  be  cooled  at  a 
certain  rate  and  in  such  a  way  that  no  internal  strains  are  set  up.  A 
lens  or  prism  made  from  glass  that  is  not  reasonably  free  from  strain  may 
very  seriously  affect  the  definition  of  the  instrument.  Prisms  and  large 
lenses  for  high-grade  instruments,  such  as  range  finders  and  periscopes, 
are,  therefore,  annealed  in  an  electric  furnace  that  is  controlled  by  an 
automatic  pyrometer  regulator.  This  regulator  is  so  constructed  that 
between  600°  and  200°  C.  the  furnace  temperature  can  be  made  to 
drop  anywhere  from  1°  C.  every  2  or  3  hr.  to  10°  C.  per  hr.  or  more.  For 
every  different  kind  of  glass  a  special  cooling  curve  must  be  followed; 
and  here  again  it  has  been  proved  that  perfect  temperature  control  is 
absolutely  essential  in  order  to  produce  a  good  product. 


PYROMETER    SHORTCOMINGS    IN    GLASS-HOUSE    PRACTICE  509 


Pyrometer  Shortcomings  in  Glass-house  Practice 

BY  W.    M.    CLARK,*    PH.   B.,    AND    CHARLES    D.    SPENCER,*    CLEVELAND,    OHIO 
(Chicago  Meeting,  September,  1919) 

OUR  interest  in  the  matter  of  pyrometers  and  pyrometry  is  primarily 
that  of  a  user  of  considerable  quantities  of  heat-measuring  equipment; 
and  while  we  may  be  somewhat  critical  on  the  subject  we  have  aimed  to 
make  the  nature  of  these  comments  constructive. 

The  glass  industry  in  America  dates  back  to  1609,  when  Capt.  John 
Smith  started  a  small  glass  plant  near  Jamestown,  Va.,  with  a  few  Italian 
workmen,  to  manufacture  glass  beads  for  trading  with  the  Indians. 
While  the  industry  has  expanded  until  it  has  reached  a  volume  of  over 
$200,000,000  per  annum,  the  means  of  heat  determination  in  Colonial 
days  was  the  human  eye,  and  in  much  of  the  industry  the  same  means  is 
used  today.  About  15  yr.  ago  the  largest  glass-house  supply  dealers 
began  to  sell  platinum  pyrometers,  but  as  nearly  all  of  these  proved  un- 
satisfactory, pyrometers  met  a  temporary  rebuff  from  the  old-time  glass 
manufacturers.  As  their  ruggedness  of  construction,  reliability,  and 
stability  have  been  improved,  however,  pyrometers  have  gradually  won 
their  way  into  more  and  more  extensive  use  throughout  the  glass  trade. 
But  there  are  still  many  applications  where  pyrometers  are  not  used, 
largely  because  of  cost  and  upkeep  considerations. 

There  are  two  distinct  fields  in  glass  works  for  pyrometry.  The  first 
calls  for  accurate  temperature  measurement  and  the  second  only  de- 
mands knowledge  of  relative  comparative  fluctuations  of  temperature, 
to  know  whether  the  work  is  becoming  hotter  or  colder.  For  example, 
in  a  lehr  for  annealing  glassware  and  removing  strain  we  want  accurate 
temperatures  to  know  that  the  ware  is  heated  to  the  critical  temperature 
for  rapid  removal  of  strain.  On  the  other  hand,  in  a  glass  pot  furnace 
nearly  120  ft.  (36  m.)  in  circumference,  where  the  pyrometer  couples 
only  project  inside  the  furnace  wall  for  less  than  2  in.  (50  mm.),  it  is 
inconceivable  that  these  few  inches  represent  the  accurate  temperature 
at  every  point  of  the  120-ft.  circumference  but  the  relative  readings  as 
to  whether  the  pyrometer  shows  schedule  temperature  fluctuations 
are  valuable  indications  for  control.  The  glass  melting  in  the  pots  is, 
however,  a  sensitive  pyrometer  because  the  duration  of  the  time  of  melt- 
ing shows  clearly  whether  the  furnace  is  too  hot  or  too  cold.  If  the  glass 


*  National  Lamp  Works  of  General  Electric  Co. 


510  PYROMETER   SHORTCOMINGS    IN    GLASS-HOUSE    PRACTICE 

melts  and  is  workable  in  22  hr.  when  it  should  be  ready  in  20  hr.,  the 
cause  must  be  insufficient  heat. 


LACK  OF  STANDARDIZATION  OP  COUPLES 

One  of  the  manufacturers  maintains  that  platinum-pi atinum-iridium 
couples  are  more  stable  under  glass-house  conditions.  Others  use  Her- 
aueus  platinum-platinum-rhodium  wire  of  two  different  diameters. 
Others  use  platinum-platinum-rhodium  wire  of  different  manufacture 
and  different  electromotive  force  from  the  preceding.  Couples  of  different 
manufacture  are  not  interchangeable.  The  porcelain  tubes  are  also 
from  diversified  sources  and  of  varying  degrees  of  excellence. 

COLD-JUNCTION  COMPENSATION 

There  are  a  variety  of  schemes  proposed  to  meet  this  condition,  such 
as  burying  the  cold  junction,  introducing  it  into  a  thermos  bottle,  using 
compensating  lead  wires  and  returning  to  a  thermostat  box,  water-jacket- 
ing the  cold  junction,  etc.  The  average  glass  manufacturer  does  not 
understand  the  subject  and  generally  omits  cold-junction  compensation 
altogether,  because  it  is  usually  presented  in  a  bewildering  indefinite 
manner  and  an  impractical  arrangement  proposed  at  considerable  expense. 
Standardized  practical  cold-junction  compensation  methods  would  be 
desirable. 


Some  of  the  most  successful  developments  along  the  line  of  machinery 
and  equipment  have  resulted  from  producing  standard  lines  to  meet 
good  manufacturing  practice  so  that  the  reasonable  requirements  of  the 
trade  are  met.  The  resulting  concentration  of  production  on  a  few  types 
has  led  to  volume  production  and  warranted  an  investment  in  molds, 
machinery,  jigs,  punches,  and  dies,  etc.,  enabling  the  manufacturer  to 
turn  out  more  perfect  fittings,  interchangeable  parts  and  a  much  better 
piece  of  equipment  mechanically  and  at  a  greatly  reduced  cost,  over 
hand-fitted  parts  and  small-scale  production. 

A  survey  of  the  various  types  of  indicators  and  recorders  on  the  mar- 
ket will  show  a  wide  variety  of  instruments  built  to  meet  similar  require- 
ments with  different  meritorious  and  bad  features  combined,  because 
they  are  turned  out  by  small-scale  manufacturing,  combining  various 
patent  features,  or  attempts  to  evade  competitors'  patents.  Patent- 
license  arrangements  between  the  manufacturers  and  centralized  produc- 
tion of  the  different  types  would  tend  to  bring  about  greater  stability, 
accuracy,  and  lessened  cost. 


W.    M.    CLARK    AND    CHARLES   D.    SPENCER  511 

Where  setscrews  and  binding  posts  are  involved  some  of  the  manu- 
facturers upset  the  heads  of  the  screws  so  that  the  turning  knobs  cannot 
be  screwed  off,  dropped,  and  lost.  This  should  be  universally  adopted 
in  the  interest  of  avoiding  loose  connections  and  the  substitution  of  make- 
shift nuts  and  binding  posts  to  replace  special  parts  belonging  to  instru- 
ments and  couples. 

Brief  directions  for  the  setting  up,  wiring,  and  adjustment  of  every 
instrument  should  be  pasted  on  the  instrument  itself  or  on  the  case 
containing  the  instrument.  Or  a  label  should  state  that  information 
regarding  the  instrument  would  be  sent  by  the  manufacturer,  on  request. 

In  some  cases  the  temperature  readings  desired  lie  within  a  range  of  a 
few  hundred  degrees  but  the  scale  on  the  instrument  may  be  divided 
equally  over  1600°  C.  In  the  case  of  furnace  temperatures,  for  instance, 
the  part  of  the  scale  that  is  read  will  lie  only  between  800°  C.  and  1400°  C. 
Greater  accuracy  in  reading  temperatures  results  from  suppressing  the 
zero  point  and  opening  up  the  scale  over  the  part  where  readings  are 
desired. 

After  platinum  has  been  exposed  to  reducing  gases  for  some  time,  it 
becomes  "sick,"  or  brittle  and  crystallized.  By  boiling  in  nitric  acid 
and  heating  to  redness  for  a  prolonged  period  it  is  possible  to  revivify 
the  platinum  and  prolong  its  life  materially. 

Standardization  and  calibration  of  the  couples  is  the  next  problem. 
The  average  plant  is  not  equipped  for  this  maintenance  or  calibration 
and  couples  are  generally  used  until  they  break.  It  would  seem  as  if 
this  was  properly  a  function  of  the  service  department  of  the  pyrometer 
manufacturer. 

With  optical  pyrometers  our  experience  has  been  somewhat  limited, 
but  we  have  used  one  of  the  latest  model  recommended  by  the  Geophys- 
ical Laboratory.  It  is  quite  expensive,  heavy  to  carry  around,  and  the 
focusing  lens  on  the  end  is  not  fastened  on  by  a  catch  in  a  slide  or  by 
some  other  safety  arrangement  such  as  could  easily  be  applied,  so  that 
it  is  only  a  question  of  time  before  the  lens  is  dropped  off  and  broken. 

GLASS-HOUSE  NEEDS 

In  a  glass  works  there  are  a  multitude  of  places  where  it  would  be 
desirable  to  have  temperature  control  in  the  melting  and  heating  proc- 
esses and  equipment  relating  to  combustion.  To  properly  equip  a  plarit 
with  heat  indicators  or  recorders  at  every  point  where  this  knowledge 
would  be  of  value  to  the  operator  would  involve  prohibitive  expense  in 
the  present  stage  of  the  development  of  the  art  of  pyrometry.  Therefore 
only  the  most  important  locations  are  equipped.  If  an  instrument, 
like  the  Northrup  tin  pyrometer  for  example,  could  be  developed  at  a 
reasonable  cost  and  of  a  rugged  type  it  should  have  great  value  and  ex- 
tensive application  in  glass  works.  Large  numbers  of  the  graphite  ex- 


512  PYROMETER   SHORTCOMINGS    IN    GLASS-HOUSE   PRACTICE 

pansion  type  pyrometers  are  still  in  use,  although  they  are  inaccurate, 
because  they  are  cheap,  rugged,  and  convenient.  An  accurate  instru- 
ment combining  these  good  points  would  have  a  still  wider  field. 

Briefly,  the  need  of  glass  manufacturers,  as  we  see  it,  is  to  have  reli- 
able heat  gages  rather  than  refined  instruments.  It  is  customary  in 
machine  shop  practice  to  furnish  to  skilled  workmen  gages  and  fine  tools 
of  close  precision,  below  0.001  in.  Close-precision  instruments,  however, 
are  generally  confined  to  the  laboratories.  Our  plea  is,  therefore,  for 
pyrometer  development  along  similar  lines,  to  have  practical  apparatus 
for  the  skilled,  non-technical  craftsman  to  handle  and  use;  to  have 
pyrometer  production  for  the  trade  become  a  utility  industry  producing 
articles  that  are  bought  by  the  consumer  principally  on  the  basis  of 
quality  or  efficiency  for  the  price  and  without  thought  of  their  pleasing 
his  taste  or  fancy.  They  should  be  judged  solely  on  the  basis  of  per- 
formance in  proportion  to  price.  What  mechanical  principles  are  em- 
ployed and  whether  the  designs  are  pleasing  to  the  eye  should  not  be  the 
consideration  if  efficiency  is  predominant. 

In  brief,  we  feel  that  if  pyrometer  equipment  resulted  from  the  same 
intensive  organized  industrial  methods  that  produce  typewriters,  adding 
machines,  time  clocks,  and  other  modern  office  and  factory  appliances, 
there  would  be  a  widely  increased  usage  of  applied  pyrometry,  improve- 
ment in  design,  and  much  reduced  cost  due  to  improved  standardized 
production  and  distribution  methods.  The  quality,  accuracy,  and  pre- 
cision of  special,  more  refined  pyrometers  for  scientific  work  would  also 
benefit  from  standardization  of  the  utilitarian  types. 


PYROMETRY   IN    THE   MANUFACTURE    OF    CLAY    WARES  513 


Pyrometiy  in  the  Manufacture  of  Clay  Wares 

BY   F.    K.    PENCE,*  B.    A.,    CER.    E.,    ZANESVILLE,    OHIO 
(Chicago  Meeting,  September,  1919) 

THE  reduction  of  the  firing  of  clay  wares  to  a  science  has  been  one  of 
the  most  difficult  problems  of  modern  ceramic  engineering.  The  number 
of  factors  involved  in  the  treatment  of  these  wares  has  been  such  a  com- 
plicated composite  of  the  chemical  and  the  physical  properties  that, 
according  to  the  old  practice,  results  depended  rather  on  the  art  of  the 
operator  than  on  any  scientific  data  available. 

The  combined  influence  of  the  chemical  and  physical  properties  of 
clay  minerals  renders  it  difficult  to  apply  the  data  obtained  in  one  opera- 
tion to  the  forecasting  of  details  to  be  applied  in  another. 

In  maturing  clay- ware  bodies  by  the  application  of  heat,  the  chemical 
reactions  involved  are,  in  general,  incomplete.  The  object  frequently 
is  to  secure  a  certain  physical  quality,  as  density,  color,  or  strength, 
rather  than  any  particular  chemical  composition  or  development.  This 
is  more  generally  true  of  the  so-called  crude  clay  products.  In  the  case  of 
certain  more  high-grade  wares,  as  the  porcelains,  it  is  necessary  to  reach 
the  temperature  required  to  bring  about  a  certain  molecular  develop- 
ment or  crystalline  structure;  but  even  here,  time  and  temperature  are 
at  work  and  must  be  considered  jointly. 

The  conditions  attending  the  firing  of  clay  wares  has  led  to  the  use  of 
certain  devices  in  the  measurement  of  the  progress  of  the  firing  process 
whereby  the  influence  of  the  various  burning  factors  on  the  clay  composi- 
tion under  fire  is  indicated.  In  the  simplest  application  of  this  practice, 
samples  of  the  ware  itself  are  so  placed  as  to  be  drawn  for  observation  at 
various  intervals  during  the  burn.  Although  the  appearance  is  affected 
by  the  quick  cooling  treatment,  by  experience,  this  practice  is  successfully 
used,  particularly  in  the  brick  industry.  The  settle  of  the  setting  of  brick, 
as  measured  from  the  top  of  the  kiln  may  also  serve  as  a  guide  in  the 
firing  operation.  Another  example  is  found  in  the  stoneware  industry, 
where  the  practice  of  arranging  test  pieces  made  of  the  same  composition 
as  the  ware,  including  the  glaze  coating,  is  of  assistance,  particularly  as 
the  appearance  of  the  slip  glaze  in  fusion  gives  a  fairly  definite  end  point. 

*  Production  Superintendent  and  Ceramic  Engineer,  American  Encaustic  Til- 
ing Co. 


514  PYROMETRY   IN   THE    MANUFACTURE    OF    CLAY   WARES 

This  pyrometric  principle,  in  which  there  is  indicated  the  influence  of 
temperature  treatment  rather  than  absolute  temperature,  has  its  most 
general  application  in  the  use  of  shrinkage  disks  and  pyrometric  cones. 
The  shrinkage  •  disks  are  so  composed  as  to  parallel  in  their  action  the 
progress  of  the  maturing  processes  in  the  ware;  in  this  way  their  field  is 
more  limited  than  that  of  the  pyrometric  cones.  In  the  latter  case,  the 
slender,  pointed,  triangular-shaped  pieces  are  composed,  in  general, 
of  mineral  composition  similar  to  that  of  clay  bodies  or  glazes  maturing 
at  the  same  temperature  at  which  the  cone  fuses.  The  cones  are  made 
in  a  series  of  succeeding  numbers  with  an  approximate  temperature 
difference  of  20°  C.  between  the  fusion  point  of  succeeding  cones.  By 
using  numbers  of  the  series  covering  the  maturing  temperatures  of  a 
given  ware,  the  end  point  in  the  firing  of  the  ware  can  be  established. 
A  convenient  practice  is  to  use  both  shrinkage  disks  and  pyrometric 
cones,  in  which  the  former  serve  as  a  guide  during  the  earlier  stages 
of  the  firing  and  the  latter  afford  a  more  delicate  indication  of  the 
conditions  near  or  at  the  temperature  of  maturity.  In  all  these  opera- 
tions, there  must  be  a  preliminary  study  in  which  the  relation  between 
the  action  of  the  clay  ware  and  the  action  of  the  pyrometric  indicator  has 
been  established.  This  relation  then  serves  as  a  guide  to  succeeding 
operations. 

This  principle  is  fundamental  in  the  application  of  devices  measuring 
absolute  temperatures  in  the  manufacture  of  clay  wares.  The  electrical 
pyrometer,  with  accessory  recorders,  affords  a  means  of  obtaining  time- 
temperature  curves,  showing  heat  development  at  all  stages  of  the  burning 
process.  By  determining  through  preliminary  research,  the  time- 
temperature  curve  suited  to  a  given  ware,  a  convenient  guide  is  afforded. 
Some  difficulty  is  encountered  in  locating  the  metal  couple  at  desired 
points  in  the  kiln,  particularly  in  the  case  of  large  kiln  installations. 
The  expense  of  first  cost  and  upkeep  in  equipping  the  ordinary  periodic 
kiln  with  couples  at  all  points  where  heat  observations  are  desired  is  also 
a  deterring  factor.  In  the  case  of  the  continuous  tunnel  kiln,  a  series  of 
metal  couples  at  proper  intervals  is  of  distinct  service  in  controlling  a 
temperature  level  under  conditions  where  slight  influences  produce  rapid 
changes. 

Modern  developments  in  the  optical  pyrometer  have  brought  it  to  a 
state  of  accuracy  which  makes  it  a  valuable  addition  to  pyrometric 
equipment.  It  is  particularly  serviceable  in  the  wide  distribution  of 
temperature  readings  that  may  be  obtained.  Its  application  in  the  use 
of  research  furnaces  and  of  high-temperature  installations  is  especially 
convenient. 

In  practically  all  cases,  however,  in  which  absolute  or  approximate 
temperatures  are  measured  by  means  of  metal-couple  (electrical)  or 


F.    K.    PENCE  515 

optical  pyrometers,  the  use  of  shrinkage  disks,  pyrometric  cones,  or 
similar  devices  is  also  recommended.  In  this  way,  influences  other  than 
absolute  temperature  may  be  noted  and,  in  general,  a  more  convenient 
means  of  obtaining  a  record  from  all  parts  of  the  kiln  contents  afforded. 
It  must  be  remembered,  however,  that  for  each  type  of  clay  ware,  or 
wares  of  the  same  type  where  size,  shape,  or  physical  property  is  markedly 
changed)  a  preliminary  study  must  be  made  to  determine  the  relation 
between  the  action  of  the  ware  and  the  record  of  the  pyrometer  or  com- 
bination of  pyrometers  used. 


516         APPLICATION    OF    PYROMETRY   TO    THE    CERAMIC    INDUSTRIES 


Application  of  Pyrometry  to  the  Ceramic  Industries 

BY   C.   B.    THWING,*    PH.    D.,    PHILADELPHIA,    PA. 

(Chicago  Meeting,  September,  1919) 

IT  is  likely  that  among  most  races,  owing  to  the  ease  of  finding  and 
working  clay,  the  making  of  clay  utensils  was  learned  earlier  than  the 
molding  of  metal  implements.  The  ancients  made  good  pottery  and 
durable  brick  in  kilns  that  wasted  fully  one-half  the  heat  and  spoiled 
fully  one-fourth  of  the  ware;  and,  taking  the  clay  working  industry 
as  a  whole,  we  are  doing  the  same  today. 

The  chemical  and  physical  changes  involved  in  the  burning  of  clay 
are  so  many  and  vary  so  much  with  the  composition  of  the  clay  that  it  is 
not  surprising  that  the  rule-of-thumb  methods  so  commonly  employed 
fail  to  give  uniform  results.  In  its  passage  from  plastic  clay  to  the 
vitreous  or  stonelike  character  the  mingled  minerals  and  accidental 
organic  impurities  of  which  the  clay  is  blended  must  pass  through  four 
stages,  the  second  and  third  usually  overlapping  and  the  fourth  often 
beginning  before  the  third  is  complete.  These  stages  in  their  order  with 
the  rising  temperature  are  drying,  dehydration,  oxidation,  vitrification. 

In  the  first  part  of  the  drying  stage,  usually  accomplished  in  part 
outside  the  kiln,  the  water  mixed  with  clay  is  dried  out  at  temperatures 
not  much  exceeding  the  boiling  point  of  water.  In  the  later  part  of  this 
stage,  within  the  kiln,  the  hygroscopic  water  absorbed  from  the  air  is 
expelled  at  somewhat  higher  temperatures,  this  stage  being  known  as 
watersmoking.  Some  clays  are  very  sensitive  to  temperature  during 
the  drying  stage  and  if  the  temperature  exceeds  100°,  the  ware  becomes 
checked.  Other  clays  may  be  dried  more  rapidly  without  danger. 

In  down-draft  kilns,  the  draft  is  slight  during  the  watersmoking 
stage  and  the  water  driven  off  from  the  top  rows  condenses  on  the  ware 
in  the  bottom  of  the  kiln,  with  the  result  that  high  temperatures  are 
likely  to  be  reached  at  the  top  of  the  kiln  before  the  bottom  is  fully  dry. 
The  lag  in  temperature  of  the  bottom  persists  to  the  second  stage  in 
spite  of  the  better  draft  then  prevailing. 

In  the  second  stage,  the  water  of  crystallization,  or  so-called  "  chemical 
water,"  is  set  free  at  a  temperature  of  about  600°  C.  The  liberation  and 
evaporation  of  this  chemical  water  requires  much  heat,  the  result  for- 
tunately being  that  the  temperature  advances  but  little  even  if  the  firing 
is  continued  uniformly. 

During  the  third  stage  the  organic  matter,  carbon  chiefly,  is  oxidized. 

*  President,  Thwing  Instrument  Co. 


C.   B.    THWING  517 

The  amount  of  carbon  may  vary  from  %  to  6  per  cent.  With  clays  high 
in  carbon,  considerable  time  is  required  to  complete  the  oxidation;  es- 
pecially if  much  iron  in  the  form  of  ferrous  oxide  is  present,  since  the 
latter  takes  oxygen  to  convert  it  to  the  ferric  state,  which  imparts  the  red 
color  to  red  brick  and  terra  cotta. 

If  care  is  not  taken  to  admit  enough  air  and  to  keep  the  temperature 
below  800°  C.  until  the  second  and  third  stages  are  complete,  weak  and 
porous  ware  will  result,  due  to  the  explosive  action  of  the  steam  and 
carbonic  gas  formed  within  the  ware.  More  ware  is  ruined  or  damaged 
at  this  stage  than  during  all  of  the  rest  of  the  burn  combined. 

During  the  fourth  and  last  stage  of  burning,  the  ware  shrinks  in 
volume.  Part  of  it  becomes  vitrified  and  binds  the  unvitrified  particles 
together  to  give  the  hardness  and  strength  desired.  At  this  stage  also 
occur  the  changes,  such  as  oxidation  of  iron,  which  determine  the  color 
of  the  finished  product. 

The  first  to  attempt  to  measure  the  temperatures  in  ceramic  kilns 
was  Josiah  Wedgewood,  the  famous  English  potter.  He  inserted  blocks 
of  standard  dimensions  in  the  kiln  and  withdrew  them  at  intervals,  after 
the  beginning  of  shrinkage,  and  measured  their  length.  The  method  has 
been  revived  in  recent  years,  the  specimens,  however,  being  in  the  form  of 
rings.  Seger's  pyrometric  cones  were  an  attempt  to  use  the  principle  of 
observing  the  effect  of  heat  on  material  similar  to  that  being  burned  and 
to  extend  the  range  of  temperature  measured.  The  cones  are  designed 
to  cover  intervals  of  30°  C.  but  cannot,  of  course,  be  said  to  indicate  tem- 
perature as  closely  as  30°  C.,  as  the  softening  point  is  a  function  not  only 
of  the  temperature  but  also  of  the  time  and,  in  the  case  of  the  lower 
numbers  in  particular,  of  the  nature  of  the  kiln  gases.  An  advantage 
of  the  method  is  that  it  permits  taking  comparative  indications  at 
several  place's  in  the  kiln  at  small  expense  and  that  some  of  the  locations 
may  be  near  the  center  of  the  kiln,  where  it  is  not  practicable  to  measure 
the  higher  temperatures  with  thermocouples. 

A  third  method  of  roughly  measuring  the  temperature  is  that  of 
withdrawing  samples. of  the  ware  itself  and  observing  its  appearance. 

A  fourth  method,  often  used  where  brick  of  standard  size  are  burned, 
is  to  judge  when  the  kiln  is  finished  by  the  "settle,"  or  measured  shrink- 
age, of  the  entire  mass  in  the  kiln. 

All  of  the  methods  named  are  useful  but  they  give  no  indications  dur- 
ing the  earlier  part  of  the  burn,  when  damage  is  most  likely  to  be  done; 
they  cover,  at  the  best,  only  a  few  temperatures;  and  they  take  no  ac- 
count of  falling  temperatures  and  give  no  clue  to  the  duration  of  time 
at  which  any  temperature  attained  was  carried.  What  is  required  is  a 
device  that  will  give  a  graphic  time-temperature  chart  of  the  tempera- 
ture of  two  or  more  points  in  the  kiln  over  the  entire  period  of  the  burn. 
Such  a  record  makes  it  possible  to  establish  a  standard  curve,  and  by 


518         APPLICATION    OF    PYROMETRY    TO   THE    CERAMIC    INDUSTRIES 

following  such  a  curve,  at  the  same  time  observing  proper  condition  as 
to  oxidizing  or  reducing  atmosphere,  etc.,  men  of  ordinary  intelligence 
can  burn  ware  of  uniform  quality  even  though  they  have  had  little  firing 
experience. 

It  is  now  possible  to  equip  any  ceramic  plant  with  pyrometers  to  give 
accurate  records  meeting  the  requirements  described  at  a  cost  that  is 
soon  repaid  in  improved  quality  of  ware  and  enormous  saving  in  fuel. 
Such  time-temperature  records  result  in  more  intelligent  as  well  as  more 
faithful  service  on  the  part  of  the  burner,  while  giving  at  the  same  time 
to  the  head  burner,  ceramist,  or  superintendent,  full  information  on  which 
to  base  changes  in  practice,  with  a  view  to  further  improvement  in 
operation. 

At  present  most  ware  is  burned  in  periodic  kilns  of  the  down-draft 
type,  in  which  the  hot  furnace  gases  enter  from  the  sides  of  the  kiln, 
rise  to  or  toward  the  top,  and  escape  by  openings  at  the  bottom  through 
ducts  to  the  stack.  This  method  of  burning  is  being  superseded  by 
continuous  tunnel  kilns  of  various  types,  differing  in  details,  but  alike  in 
general  method.  The  tunnel  kiln  provides  a  long  kiln  of  small  cross- 
section,  divided  into  a  series  of  zones,  in  which  the  temperature  increases 
to  a  maximum  and  then  diminishes  toward  the  exit  end,  through  which 
small  loads  of  ware  are  passed  at  a  suitable  rate.  The  circulation  of  hot 
gases  is  in  the  direction  opposite  to  the  direction  of  flow  of  the  material 
through  the  kiln.  The  gas  and  air  for  combustion  enter  over  the  cooling 
ware  and  so,  being  heated  to  the  combustion  point  on  reaching  the  hot 
zone,  the  products  of  combustion  serve  to  heat  the  entering  stream  of 
ware  at  any  desired  rate  of  increase,  determined  by  the  length  of  the  kiln. 

The  control  of  temperatures  by  pyrometers  is  much  easier  in  the 
continuous  kiln  than  in  the  intermittent  kiln,  since,  owing  to  the  dimen- 
sions and  shape  of  the  kiln,  the  ware  need  never  be  far  distant  from  a 
pyrometer  during  its  passage  through  the  kiln.  In  continuous  kilns,  it 
is  advisable  to  record  the  temperatures  at  six  points  in  the  kiln,  while  the 
same  number  of  additional  couples  are  connected  to  an  indicating 
pyrometer.  In  a  round  down-draft  kiln,  at  least  one  point  in  the  crown 
and  one  point  near  the  bottom  should  be  recorded;  while  in  a  rectangular 
kiln,  top  and  bottom  temperatures  should  be  recorded  near  each  end  of 
the  kiln.  It  is  desirable  to  have  the  top  and  bottom  temperatures  re- 
corded on  the  same  galvanometer  for  easy  comparison,  while  on  the 
rectangular  kiln  a  chart  having  two  sections,  each  showing  the  two 
records  at  one  end  of  the  kiln,  is  a  convenient  arrangement. 

For  temperatures  not  exceeding  1200°  C.  (2200°  F).  good  base-metal 
couples,  properly  protected  in  high-grade  porcelain  tubes,  have  proved 
entirely  adequate;  for  higher  temperatures,  platinum  couples  must  be 
used.  For  the  ranges  where  they  are  available,  base-metal  couples  are 
preferable  because  of  their  greater  ruggedness  and  low  cost  and  because 


DISCUSSION  519 

they  generate  three  times  as  much  current  as  platinum  at  the  same  tem- 
perature, thus  making  possible  the  use  of  more  rugged  galvanometers 
in  the  recorders. 

While  the  use  of  pyrometers  for  controlling  the  burning  of  clay  ware 
ought  to  become  universal,  pyrometers  plus  fire  will  not  burn  good  ware; 
fire  suitably  controlled  by  means  of  the  information  furnished  by  pyrome- 
ters will  surely  do  the  work.  Pyrometers  will  unquestionably  furnish 
most  important  information,  which  will  be  valuable  just  in  proportion 
as  it  is  studied  and  applied  to  the- problem  in  hand.  I  am  a  firm  believer 
in  putting  all  of  the  information  given  by  the  pyrometer  at  the  disposal 
of  the  burner.  It  is  usually  easy  to  locate  the  recorder  where  the  burner 
can  follow  the  record.  A  glance  at  the  chart  tells  him  not  only  the  tem- 
perature at  the  moment  but  also  the  trend  of  temperature  change  in  a 
way  that  no  number  of  observations  read  from  an  indicator  can  do. 
When  the  records  are  made  accessible  to  the  burners,  they  will  inevitably 
vie  with  one  another  in  making  the  best  possible  chart. 

Where  waste  heat  from  the  kilns  is  used  for  drying  the  ware  it  is 
very  desirable  to  record  the  temperature  in  the  drying  tunnels,  especially 
if  the  clay  is  sensitive  to  drying  conditions.  Multiple-record  recorders 
of  the  base-metal  thermocouple  type  make  it  possible  to  obtain  such 
records  in  compact  form  at  a  moderate  outlay. 

DISCUSSION 

FRANCIS  T.  OWENS,*  Watsontown,  Pa.  (written  discussionf). — In  the 
second  paragraph,  Dr.  Thwing  mentions  the  various  stages  through  which 
clay  ware  must  pass  but  he  does  not  analyze  the  third  stage,  oxidation, 
sufficiently.  There  are  two  elements  to  dispose  of  in  a  great  many  of  the 
shales  that  must  be  burned;  these  elements  are  carbon  and  sulfur.  Dr. 
Thwing  speaks  of  the  carbon,  but  intimates  that  it  is  safe  to  have  a 
temperature  of  800°  C.,  which  we  find  to  be  rather  dangerous  on  a  great 
many  shales.  With  one  shale  that  we  handle,  we  cannot  go  above 
535°  C.  until  we  have  passed  this  particular  stage.  At  800°  C.  the  sulfur 
will  begin  to  pass  off  and  it  is  very  necessary  that  plenty  of  air  be  ad- 
mitted at  this  time  and  a  very  even  temperature  maintained.  I  realize 
fully  that  this  paper  is  not  meant  to  be  much  more  than  a  suggestion  in 
the  use  of  pyrometers  in  burning  of  clay  wares,  but  lest  some  one  be  led 
astray,  I  call  attention  to  the  above  results  of  our  practice. 

Our  experience  with  cones  is  that  if  a  cone  is  brought  to  a  temperature 
near  its  fusion  point  and  is  then  allowed  to  cool  off  80°  to  100°  F.,  the  cone 
will  show  an  error  of  from  30°  to  100°  in  its  fusion  point.  We  have  not 
discovered  just  why  this  is,  but  have  had  this  experience  in  two  or  three 


Factory  Manager,  Fiske  &  Co.,  Inc.  f  Received  Sept.  18, 1919. 


520         APPLICATION    OF    PYROMETRY   TO   THE    CERAMIC    INDUSTRIES 

instances.  For  that  reason,  cones  are  not  a  sure  guide  and,  while  we 
use  them,  we  would  not  think  of  attempting  to  burn  a  kiln  without 
pyrometers. 

Dr.  .Thwing  speaks  of  the  necessity  for  study  of  the  information  ob- 
tained when  using  pyrometers;  this  is  one  of  the  points  that  should  be 
emphasized  greatly.  It  is  not  enough  to  have  records  to  look  at,  the 
heat  records  of  each  kiln  should  be  traced  on  cross-section  paper.  A 
study  of  the  records  will  soon  show  that  no  two  kilns  act  in  exactly  the 
same  way.  Charts  should  be  drawn  of  those  burns  showing  the  best 
results,  then  a  composite  chart  should  be  made  from  these  various  charts; 
in  this  way  a  burning  guide  that  will  insure  good  results  throughout  the 
entire  burning  plant  may  be  obtained. 

The  writer  strongly  advises  the  use  of  a  recording  pyrometer  both  at 
the  hot  end  and  the  cold  end  of  the  dryer.  We  have  found  that  where 
wares  are  difficult  to  dry,  the  trouble  is  due  chiefly  to  conditions  in  the 
dryer  that  we  were  not  aware  of  until  we  began  to  make  records  for  the 
entire  24  hr.  A  recording  mercury  thermometer  will  do  the  work  nicely 
in  a  dryer,  but,  as  every  one  understands,  is  not  adapted  for  kilns. 

R.  C.  PURDY,  Worcester,  Mass. — The  Norton  Co.  has  86  periodic 
kilns,  hence  considerable  experience  with  pyrometry  and  other  methods 
of  control.  Better  regulation  has  been  obtained  since  pyrometers  have 
been  installed  for  the  control  of  every  kiln.'  There  are  three  central 
stations  in  which  twelve-point  and  six-point  recorders  are  placed.  Be- 
fore these  pyrometers  were  installed,  Seger  pyrometric  cones  were  the 
sole  means  of  judging  the  rate  of  increase  in  temperature.  There  was 
no  way  of  telling  how  the  burning  had  progressed  in  the  initial  stages; 
there  was  nothing  on  which  to  base  changes  in  procedure;  and  there  was 
no  control  over  the  men  who  fired  the-  kilns.  The  men  were  easily 
trained  to  follow  a  desired  time-and-temperature  curve.  The  firemen 
now  plot  on  a  small  coordinate^chart,  independent  of  the  automatic 
record  in  the  recorder,  on  which  chart  is  recorded  all  other  data  of  the 
burn  so  that  when  the  kiln  is  finished  the  entice  record  is  on  one  sheet 
or  chart.  Besides  this  permanent  complete  record  of  ea^h  kiln  firing, 
the  pyrometers  have  proved  very'  valuable  in  transferring  the  kiln  con- 
trol from  the  shop  man,  without  records,  to  the  laboratories,  where  com- 
plete records  are  used  as  a  basis  for  instructions  and  rules. 

It  is  important  to  control  the  time-temperature  treatment  of  a  kiln 
throughout  the  entire  burn.  Some  of  the  kilns  are  8  ft.  in  diameter, 
others  15,  18,  and  20  ft.,  and  each  has  its  own  rate  of  heat  treatment  and 
maximum  temperature.  All  kilns  are  finished  to  the  same  Seger  cone, 
but  with  cones  exactly  alike  the  temperature  recorded  will  vary  widely, 
dependent  on  size  of  the  kiln.  Fundamentally,  the  cause  of  differences 
in  temperatures  with  the  same  cone  indication  is  the  difference  in  rate 
of  heat  treatment  necessitated  by  difference  in  size  of  kiln. 


DISCUSSION  521 

A  grinding  wheel,  to  the  uninitiated,  appears  as  a  rough  looking  thing 
in  the  burning  of  which  almost  any  degree  of  heat  treatment  would 
suffice.  As  a  matter  of  fact,  our  heat-treatment  specifications  are  close. 
We  must  produce  an  exact  grade  of  toughness  or  hardness,  denoted  by 
the  penetration  of  a  tool.  We  have  learned  by  practice  the  time-temper- 
ature treatment  that  is  best  for  each  size  of  kiln,  a  knowledge  we  did  not 
have  before  the -installation  of  recording  pyrometers. 

The  burning  of  ceramic  ware  is  a  heat-treatment  proposition  and  is 
affected  by  both  time  and  temperature.  No  clay  ware  is  absolutely 
homogeneous.  It  is  composed  of  sizable  particles  of  different  minerals. 
The  size  of  particles,  or  the  surface  exposed  by  the  fluxing  mediums  in 
the  ceramic  ware,  determines  the  rate  at  which  fusion  will  progress. 
Fusion  of  clay  ware  is  a  progressive  reaction  and  generally  that  progress 
increases  in  rate  as  the  temperature  increases.  It  is  the  rate  of  fusion 
that  determines  the  character  of  the  ware.  Pyrometry  people  often 
forget  that.  They  seem  to  think  it  is  sufficient  if  we  have  a  measured 
temperature.  We  must  have  a  time-temperature  treatment  for  which 
the  cones  are  absolutely  necessary,  as  they  are  the  only  available  means 
of  control  that,  positively  measures  the  total  effect  of  heat  treatment. 
We  finish  the  kilns  by  the  cones,  bringing  the  cones  to  the  specified 
stage  of  deformation  by  holding  the  kiln  at  a  specified  maximum  temp- 
erature. The  burning  is  carried  on  at  a  specified  time-temperature  rate 
until  a  specified  temperature  is  obtained.  The  kiln  is  then  held  at  tfyat 
temperature  until  the  cones  show  that  the  kiln  has  had  the  desired  heat 
treatment.  Cones  show  by  their  deformation  a  given  intensity  of  heat 
treatment  which  cannot  be  measured,  even  approximately,  by  a  tem- 
perature recorder.  The  pyrometers  are  valuable  tools  when  the  data 
obtained  from  them  are  used  with  an  understanding  of  their  limitations 
in  a  given  case.  For  ceramic  ware,  the  Seger  pyrometric  cones  are  the 
best  tools  we  have  for  determining  when  sufficient  heat  treatment  has 
been  given. 


522  PYROMETRY   IN    ROTARY    PORTLAND    CEMENT    KILNS 


Pyrometry  in  Rotary  Portland  Cement  Kilns 

BY  LEO   I.   DANA,*  B.    S.,   AND  C.   O.   FAIRCHILD.f  B.   S.,    WASHINGTON,    D.    C. 
(Chicago  Meeting,  September,  1919) 

As. a  part  of  an  investigation  conducted  by  the  Cement  Section  of  the 
Bureau  of  Standards,  at  the  plant  of  the  Security  Cement  &  Lime  Co., 
Security,  Md.,  the  High-temperature  Measurements  Section  was  called 
upon  to  measure  temperatures  in  a  dry-process,  coal-burning  kiln.  Al- 
though the  measurements  were  made  only  under  the  conditions  existing 
in  one  plant,  the  methods  employed  and  the  conclusions  drawn  apply, 
in  a  general  way,  to  those  existing  in-most  plants.  As  far  as  we  have  been 
able  to  learn,  no  sufficiently  thorough  methods  of  making  accurate 
pyrometric  measurements  in  the  sintering  zone  and  in  the  rear  end  of 
rotary  cement  kilns  have  been  described.  It  is  believed  that  in  this 
investigation,  data  were  obtained  that  can  serve  as  a  foundation  for 
further  study  of  the  problems  of  pyrometry  in  cement  kilns. 

IMPORTANCE  OF  TEMPERATURE  MEASUREMENTS  IN     .   . 
ROTARY  CEMENT  KILNS 

The  manufacture  of  Portland  cement  is  today  fairly  well  standardized. 
The  various  steps  in  the  manufacture  have  been  developed  to  such  an 
extent  that  the  process  is  almost  automatic.  In  spite  of  these  strides  the 
efficiency  of  utilization  of  heat  in  the  kiln  is  very  low  and  an  undue  pro- 
portion is  wasted;  for  example,  about  50  per  cent,  ordinarily  goes  up  the 
stack.  In  order  to  prevent  these  losses,  not  only  should  means  be  pro- 
vided to  utilize  the  waste  heat  but  instruments,  such  as  pyrometers,  should 
be  installed  to  study  arid  control  the  operation  with  the  view  of  minimiz- 
ing the  losses.  At  the  same  time  such  control  will  produce  a  more 
uniform  product.  The  saving  in  fuel  that  may  be  effected  by  the  proper 
use  of  pyrometers  would  be  much  more  than  their  cost.  Where  these 
pyrometers  should  be  installed  and  what  they  can  show  are  very  briefly 
indicated  in  the  following  paragraphs  and  discussed  in  greater  detail 
later. 

The  problem  of  temperature  measurements  in  rotary  cement  kilns 
may  be  divided  into  two  parts :  Temperatures  in  the  sintering  zone  and 
temperatures  at  the  rear  end  of  the  kiln  and  in  the  stack.  A  knowledge 

*  Assistant  Physicist,  U.  S.  Bureau  of  Standards, 
t  Associate  Physicist,.  U.  S.  Bureau  of  Standards. 


LEO   I.     DANA    AND    C.    O.    FAIRCHILD  523 

of  the  temperature  in  the  clinkering  zone  can  serve  to  indicate  the  uni- 
formity with  which  the  clinker  is  being  burned.  Of  greater  importance, 
however,  is  the  determination  of  the  burning  temperature,  required  by 
mixes  of  various  chemical  compositions. 

Temperature  measurements  in  the  rear  end  of  the  kiln  are  useful 
as  a  general  pyrometric  control  of  kiln  operation.  Records  of  these 
temperatures  can  show  whether  the  fuel  is  burning  properly  and  record 
certain  abnormalities  of  kiln  operation  such  as  the  stopping  of  the  kiln  and 
the  fuel,  the  occurrence  of  "rings,"  and  the  "flooding"  of  fuel.  A 
pyrometer  in  the  stack,  in  addition  to  following,  in  a  general  way,  the 
temperature  variations  indicated  by  a  couple  in  the  rear  end,  can  show 
the  regularity  of  the  draft  and  whether  the  temperature  of  the  gases  is 
proper  for  treater  dust  precipitators  or  waste-heat  boilers. 

The  measurement  of  temperatures  in  the  clinkering  zone  is  the  more 
difficult  problem  and  has  hitherto  been  seldom  attempted;  in  order  to 
measure  these  temperatures  some  form  of  optical  or  radiation  pyrometer  is 
necessary.  To  measure  the  temperatures  in  the  rear  end,  however,  is 
simpler  and  can  be  accomplished  with  base-metal  thermocouples. 

TEMPERATURES  IN  THE  CLINKERING  ZONE 

General  Considerations. — Investigations  conducted  by  the  Geophys- 
ical Laboratory,  of  Washington,  have  shown  that  with  the  proper  pro- 
portions of  lime,  alumina,  and  silica  of  the  highest  purity,  a  temperature 
of  1650°  C.  for  a  sufficient  length  of  time  is  required  for  the  complete 
burning  of  a  perfectly  burned  Portland  cement.  The  resulting  clinker 
consists  of  three  compounds,  dicalcium  silicate,  tricalcium  silicate,  and 
tricalcium  aluminate,  all  of  which  are  cementing  constituents.  Of  the 
three,  tricalcium  silicate  is  the  most  active  and  important.  At  1335°  C., 
however,  a  flux,  which  is  a  molten  eutectic,  begins  to  form;  this  flux 
promotes  the  formation  of  the  above-mentioned  cementing  constituents. 
As  the  temperature  is  raised  above  1335°  C.,  the  amount  of  the  three 
compounds  increases,  particularly  the  tricalcium  silicate,  until  they  are 
completely  formed  at  1650°  C. 

•  These  statements  hold  for  materials  of  the  highest  purity.  In  the 
case  of  commercial  Portland  cement,  which,  in  addition  to  the  lime, 
alumina,  and  silica,  contains  impurities  such  as  the  oxides  of  iron,  mag- 
nesium, sodium,  potassium,  and  sulfur,  the  temperature  for  complete 
burning  has  been  found  to  be  considerably  lower  than  1650°  C.1 

Temperature  measurements  for  two  days  in  the  kiln  investigated 
by  this  Bureau  gave  an  average  burning  temperature  of  1380°  C.  for  a 
clinker  of  the  following  average  composition.2 

1  G.  A.  Rankin:  Portland  Cement.     Jnl.  Frank.  Inst.  (1916)  181,  776. 

2  We  are  indebted  to  Mr.  H.  A.  Bright  of  the  Chemistry  Division  of  this  Bureau 
for  the  analysis. 


524  PYROMETRY   IN   ROTARY   PORTLAND    CEMENT   KILNS 

PER  CENT. 

CaO 61.80 

A12O3 6.67 

SiO2 23.64 

MgO 3.00 

Fe2O3 3.01 

Alkalies 1 . 33 

S03 0.50 

Ignition  loss ..."..! 0 . 05 

Thus  the  total  impurities  amounted  to  about  7.9  per  cent.  Rankin 
states  that  a  cement  containing  6.7  per  cent,  impurities  required  a  burning 
temperature  of  1425°  C.  This  means,  in  the  case  of  the  first-mentioned 
clinker,  that  the  temperature  at  which  the  flux  begins  to  form  must  be 
considerably  lower  than  1380°  C.,  probably  around  1100°  C.;  thus  because 
of  the  presence  of  impurities,  the  burning  temperatures  of  commercial  raw 
mixes  are  considerably  lower  than  those  of  materials  of  the  highest  purity. 
In  addition,  it  has  been  shown  that  the  same  cementing  compounds  are 
formed  in  the  commercial  clinker  in  approximately  the  same  proportions 
as  occur  in  pure  clinker,  even  though  the  commercial  clinker  has  been 
burned  at  a  considerably  lower  temperature.  The  burning  temperature 
of  a  commercial  Portland  cement  is  widely  varied  by  its  composition;  to 
what  extent  each  impurity  affects  the  burning  temperature  has  not  yet 
been  determined. 

Possible  Methods. — In  rotary  cement  kilns,  there  is  a  zone  of  high 
temperature  beginning  a  few  feet  from  the  end  at  which  the  fuel  is  burned 
and  extending  for  10  to  20  ft.  (3  to  6  m.)  (the  dimensions  of  a  kiln  being 
125  by  8  ft.).  The  average  temperature  in  this  zone  is  supposed  to  be 
that  necessary  to  sinter  or  burn  the  clinker.  As  a  rule  there  undoubtedly 
is  a  small  temperature  gradient  in  this  zone.  The  temperatures  one 
may  encounter  in  commercial  practice  range  from  about  1200°  to  1500°  C. 

Two  types  of  pyrometers  can  be  used,  thermoelectric  and  radiation 
or  optical  pyrometers.  In  the  case  of  thermoelectric  pyrometers,  a  long 
couple  may  be  pushed  in  through  a  hole  in  the  end  of  the  kiln  to  the  center 
of  the  burning  zone.  Such  a  couple  would  have  to  be  made  of  platinum 
platinum-rhodium.  To  procure  the  required  refractory  protection  tube 
about  15  ft.  (4.5  m.)  in  length  would  be  very  difficult;  besides,  such  a 
couple  would  measure  the  temperatures  of  the  hot  gases  and  not  of  the 
clinker.  This  method,  therefore,  is  hardly  feasible. 

A  method  that  has  been  suggested  is  the  installation  of  a  couple  in  a 
hole  bored  in  the  side  of  the  kiln  approximately  at  the  center  of  the  hot 
zone  and  flush  with  the  surface  of  the  lining.  In  addition  to  the  problem 
of  a  proper  protection  tube  for  a  rare-metal  thermocouple  and  the  mechan- 
ical difficulties  of  taking  measurements  on  a  rotating  kiln,  there  is  great 
uncertainty  as  to  what  temperature  this  couple  would  measure.  It  is 
well  known  that  a  coating  of  varying  thickness  builds  up  on  the  refractory 


c 

LEO    I.    DANA   AND    C.    O.    FAIRCHILD  525 

lining  of  the  kiln  and  consequently  there  would  be  a  variable  temperature 
gradient  from  the  surface  of  the  coating  to  the  hot  junction  of  the 
thermocouple. 

The  only  means  that  at  present  give  promise  of  reasonable  accuracy 
are  the  optical  or  radiation  pyrometers.  Of  these  the  types  most  suitable 
are  the  Wanner  and  Morse  optical  pyrometers  and  the  Fery  and  Thwing 
radiation  pyrometers.  The  indication  of  the  latter  departs  from  the 
true  temperatures  more  widely  than  does  the  indication  of  the  former 
due  to  the  high  absorption  of  infra-red  radiation  by  carbon  dioxide  and 
water  vapor.  The  Holborn  and  Kurlbaum  modification  of  the  Morse 
optical  pyrometer,  in  addition  to  allowing  the  observer  to  distinctly  see 
the  object  sighted  upon  while  measuring  its  temperature,  is  more  reliable, 
more  precise,  and  more  rapid  than  the  other  types.3 

The  work  of  this  Bureau  has  shown  that  accurate  temperature  meas- 
urements cannot  be  made  with  an  optical  pyrometer  in  a  rotary  kiln 
using  coal  as  fuel,  while  the  fuel  is  burning  or  while  the  kiln  is  rotating, 
for  the  following  reasons:  Powdered-coal  (as  well  as  oil)  flames  possess 
very  great  intrinsic  brilliancy  and  by  sighting  through  the  flame  on  the 
lining  of  the  kiln  or  on  the  clinker  the  apparent  temperature  of  the  flame 
is  being  measured.  This  apparent  temperature  is  usually  considerably 
higher  than  the  true  temperature  of  the  clinker  and  is  much  the  same 
under  constant  coal  and  air  conditions  irrespective  of  the  temperature 
of  the  clinker.  If  one  sights  on  the  lining  or  on  the  clinker,  but  not 
through  the  flame,  an  error  is  introduced  by  the  reflection  of  light  from 
the  brilliant  flame.  That  is,  in  addition  to  light  received  from  the  hot 
lining  or  clinker,  reflection  from  the  latter  of  light  from  the  brilliant 
flame  occurs.  Another  source  of  error  is  caused  by  the  cement  dust 
resulting  from  the  rotation  of  the  kiln,  the  unburned  coal  dust,  and  the 
ash  dust.  This  dust,  in  the  line  of  sight,  acts  as  an  absorption  screen 
and  reduces  the  intensity  of  the  light  reaching  the  pyrometer.  Errors 
arising  from  dust  and  flame  are  indeterminate  and  consequently  make 
accurate  temperature  measurements  impossible  while  the  fuel  is  burning 
and  the  kiln  is  rotating. 

Procedure  Recommended. — We  believe  it  to  be  necessary  to  stop  the 
flame  and  the  kiln  in  order  to  measure  the  temperature  in  the  hot  zone. 
It  may  also  be  found  advisable  to  cut  off  the  air  supply  to  prevent  the 
dust  blown  up  by  the  air  from  interfering  with  the  measurements. 
Because  of  the  air  currents  blowing  through  the  kiln  and  because  of 
radiation,  the  kiln  cools  rapidly  after  cutting  off  the  fuel  supply.  Since 
it  may  take  from  5  to  15  sec.  for  the  smoke  to  clear  sufficiently  to  allow 
measurements  to  be  taken  and  about  10  sec.  after  this  to  get  the  measure- 
ment, the  temperature  of  the  spot  sighted  upon  will  fall  appreciably. 

3  This  pyrometer  is  manufactured  in  this  country  by  the  Leeds  &  Northrup  Co., 
Philadelphia,  Pa. 


526  PYROMETRY    IN    ROTARY    PORTLAND    CEMENT   KILNS 

Therefore,  the  temperature  of  the  kiln  cannot  be  estimated  by  a  single 
measurement.  By  taking  a  series  of  temperature-time  measurements, 
as  rapidly  as  possible,  the  temperature  of  the  kiln  at  the  stopping  of  the 
flame  can  be  estimated  with  a  fair  degree  of  accuracy  by  extrapolation  of 
the  temperature-time  curve,  as  described  in  more  detail  below. 

The  proper  spot  to  sight  upon  should  be  the  hottest  in  the  kiln,  which, 
as  a  rule,  will  be  found  in  the  center  of  the  sintering  zone;  this  is  about 
8  to  10  ft.  (2.4  to  3  m.)  from  the  end  of  the  kiln.  The  proper  position  of 
the  sighting  spot  around  the  circumference  may  be  determined  by  the 
following  observations.  After  stopping  kiln,  coal,  and  air,  the 
dust  begins  to  clear  and  the  clinker  that  had  built  up  on  the  lower  right- 
hand  quadrant  of  the  kiln  (the  direction  of  rotation  being  anti-clockwise 
facing  the  kiln)  falls  in  slides  for  about  2  or  3  sec.  About  10  sec.  (an 
average  value)  after  stopping  the  kiln  the  uppermost  layer  of  clinker  in 
the  quadrant  falls  over,  uncovering  the  lining.  The  thickness  of  the 
layer  that  falls  over  may  vary  from  about  6  to  12  in.  (15  to  30  cm.).  In 
all  the  measurements,  a  spot  on  the  lining  that  is  uncovered  by  the  falling 
of  the  clinker  should  be  sighted  upon  with  the  optical  pyrometer. 

The  alternative  of  sighting  on  the  surface  of  the  clinker  should  not  be 
employed,  for  the  following  reasons.  First,  it  has  been  found  by  experi- 
ment that  the  surface  of  the  lining  cools  more  slowly  than  the  surface  of 
the  clinker,  probably  because  the  former  is  smoother  and  slightly  less 
exposed  than  the  latter.  Second,  the  surface  of  the  cooling  clinker 
represents  a  mottled  appearance,  which  makes  matching  of  the  pyrome- 
ter filament  against  this  surface  difficult.  Third,  the  lining  under  the 
thick  layer  of  clinker  (the  latter  falling  on  the  average  10  sec.  after  stop- 
ping the  kiln)  does  not  cool  appreciably  until  the  clinker  does  fall,  for 
the  only  way  it  can  cool  is  by  conduction  through  the  hot  lining  and 
bed  of  hot  clinker — and  this  cooling  is  relatively  slow.  The  surface  of 
the  clinker,  on  the  other  hand,  starts  to  cool  almost  immediately  after 
stopping  the  coal.  Thus,  as  far  as  the  cooling  of  the  uncovered  spot  of 
the  lining  is  concerned,  the  interval  from  the  stopping  of  the  kiln  and  the 
falling  of  the  clinker  layer  is  time  gained  in  measuring  the  temperature; 
that  is,  during  the  interval  most  of  the  dust  disappears  and  for  this 
interval  no  extrapolation  would  be  necessary.  Extrapolated  curves  of 
readings  taken  on  the  surface  of  the  clinker  and  on  the  surface  of  the 
lining  show  that  the  temperatures  obtained  by  the  two  methods  are  in 
substantial  agreement;  but  those  readings  taken  on  the  lining  are  more 
reliable  because  the  slower  cooling  of  the  lining  and  the  interval  between 
stopping  the  kiln  and  the  falling  of  the  clinker  results  in  a  smaller  correc- 
tion being  necessary  from  the  extrapolation  of  the  temperature-time 
curve. 

The  pyrometer  should  be  kept  fixed  in  position.  The  direction  in 
which  it  is  sighted  will  depend  on  the  position  of  the  hole  in  the  front- 


LEO    I.    DANA    AND    C.    O.    FAIRCHILD  527 

end  housing  of  the  kiln  and  the  distance  of  the  center  of  the  sintering 
zone  from  the  end.  The  procedure  in  taking  the  measurements  may  be  as 
follows:  On  signal,  the  burner  throws  off  the  belt  feeding  the  coal  and 
stops  the  kiln  and  air  blast,  practically  simultaneously.  At  the  instant 
of  shutting  off  the  coal,  a  stop  watch  is  started.  The  observer  notes  the 
instant  at  which  the  heavy  layer  of  clinker  falls  and  the  assistant  records 
the  time.  As  soon  as  possible  after  the  clinker  falls,  the  observer  matches 
the  pyrometer  filament  against  the  uncovered  spot  on  the  lining,  sighting 
near  the  layer  of  clinker;  the  assistant  records  the  time  and  the  current 
through  the  lamp.  Four  or  five  such  readings  are  made  as  rapidly  as 
possible,  the  usual  interval  between  each  reading  being  about  10  sec.  A 
signal  is  then  given  to  the  burner  to  start  the  kiln  and  the  time  of  starting 
may  be  noted. 

In  the  Bureau  investigation,  of  sixty-three  observations  of  the  time 
the  layer  of  the  clinker  fell  after  stopping  the  kiln,  the  mean  value  was  10 
sec.  with  an  average  deviation  of  3  sec.  The  average  time  of  stopping 
the  kiln  for  a  reading,  computed  from  104  measurements  was  1  min.  and 
20  sec.  The  kiln  was  stopped  for  a  measurement  every  %  hr.,  which  it 
is  believed  is  a  satisfactory  interval. 

In  the  case  of  plants  burning  oil  as  fuel  we  believe  the  same  considera- 
tions as  outlined  above  should  hold.  In  those  burning  gas,  the  flame 
is  not  so  bright  and  does  not  emit  so  much  light;  at  any  rate,  the  kiln 
should  be  stopped  because  of  the  dust.  Thus,  probably  the  same  pro- 
cedure as  in  coal  plants  should  be  followed. 

Errors  in  Measurements. — Even  though  the  lining  in  the  burning 
zone  is  heated  fairly  uniformly  while  the  kiln  rotates,  there  probably  is  a 
slight  departure  from  black-body  conditions  due  to  rapid  cooling  after 
stopping  the  fuel  supply.  Nevertheless  the  emissivity  of  the  surface  of  the 
lining  and  also  of  the  clinker,  is  high;  the  error  due  to  departure  from 
black-body  conditions  perhaps  does  not  amount  to  more  than  10°  C. 
Thus,  while  the  absolute  values  of  the  temperature  readings  may  all  be 
10°  C.  too  low,  this  departure  is  very  probably  the  same  for  all  readings, 
and  the  comparison  of  the  readings,  which  is  the  most  important  object, 
is  not  affected  by  this  source  of  error. 

The  smallest  increment  of  temperature  that  can  easily  be  measured 
with  the  Leeds  &  Northrup  optical  pyrometer  in  its  industrial  form  is 
about  4°  C.  The  calibration  of  the  pyrometer,,  with  reference  to  a  stand- 
ard optical  pyrometer,  may  generally  be  relied  upon  to  remain  good  to 
±  10°  C.  Including  the  possible  error  due  to  departure  from  black-body 
conditions,  it  is  believed  that  the  temperature  measurements,  consider- 
ing only  pyrometry  errors,  may  be  estimated  good  to  ±20°  C. 

The  fact  that  the  temperature-versus-time  curve  is  extrapolated  to 
estimate  the  temperature  of  the  kiln  is  not  regarded  as  introducing  any 
uncertainty  into  the  measurement,  but  rather  to  make  it  more  reliable 


528 


PYROMETRY    IN    ROTARY    PORTLAND    CEMENT    KILNS 


than  any  single  measurement.  On  account  of  the  rapidity  with  which 
the  matching  must  be  made,  a  single  measurement  might  be  in  error 
because  of  incorrect  matching.  These  errors  are  partly  eliminated  by 
drawing  the  temperature-time  curve  through  the  points.  Nevertheless, 
occasionally  it  is  impossible  to  see  the  clinker  fall  because  of  dust;  in 
such  cases  the  average  value  for  the  time  of  fall  of  the  clinker  counted 
from  the  instant  of  stopping  the  coal  and  kiln,  may  be  assumed  and  the 
curve  extrapolated  for  this  average  time.  In  these  instances,  as  well  as 
in  others  where  the  points  do  not  fall  very  well  on  a  straight  line,  the 
error  is  greater.  As  a  rule  the  temperature-time  points  follow  very 
closely  a  straight  line,  thus  making  the  extrapolation  easy  and  precise, 
see  Fig.  1. 


nt 

1400 
90 
80 
70 
60 
50 
|      40 
I      30 

£      20 
* 

»     10 

ittoo 

g     90 
ft 
30 

70 
60 
1260 

.< 

Time  from  Stopping     Extra- 
Curve  No.      of  Coal  and  Kiln       polated 
to  Falling  of  Clinker     Temp. 
°C 
I                     10  seconds                1400 
II                      8                               1350 

^^T 

s 

Extrapolated^*^ 

NT 

I 

Temperature 

A 

*«> 

(Clinker  Fell) 

^ 

»V_ 

r 

Firit 

V 

"••w 

®v 

^V. 

Be 

adin 

1 

^x, 

^v. 

^s 

vy 

3* 

\ 

^x, 

\ 

\ 

^v 

-V, 

*»^o 

O^N 

^s 

^ 

Sj 

"X 

\ 

^ 

S 

\H 

)                    10                   20 

30                  40                  50                  60                   70 

Time  in  Seconds  from  Instant  of  Stopping  Coal  and  Kiln 

FIG.  1. — TYPICAL  EXTRAPOLATION  PLOTS  OF  TEMPERATURE  READINGS. 

Since  in  the  investigation  made  by  the  Bureau  the  average  time  from 
the  fall  of  the  clinker  from  the  lining  to  the  first  reading  was  13  sec.  and 
the  average  rate  of  cooling  of  the  lining  was  2°  C.  per  second,  the  average 
value  of  the  correction  resulting  from  extrapolation  was  26°  C. 

It  is  a  peculiarity  of  kiln  operation  that  the  sintering  zone  fluctuates 
back  and  forth  for  several  feet.  One  of  the  functions  of  the  burner  is  to 
keep  this  zone  the  proper  length  and  in  the  proper  position  by  control 
of  the  kiln  speed,  the  rate  of  addition  of  the  raw  feed — these  two  factors 
are  coupled  and  controlled  together — and  the  coal  and  air  supply. 
Since  the  pyrometer  is  kept  fixed  in  alignment,  it  may  be  that  the  maxi- 
mum temperature  is  not  always  being  measured.  It  is  not  ordinarily 
possible  to  observe  the  extent  of  these  small  fluctuations.  At  times, 
however,  the  sintering  zone  can  be  observed  to  be  flagrantly  out  of 
position,  in  which  case  the  temperature  readings  should  indicate  that 
such  a  condition  exists. 


LEO   I.  DANA   AND    C.    O.    FAIRCHILD  529 

Significance  of  Measurements. — The  extent  to  which  the  cementing 
constituents  in  the  burning  of  a  Portland  cement  clinker  are  formed 
depends  principally  on  two  factors :  the  temperature  of  burning  and  the 
time  of  burning.  In  actual  operation  of  the  kiln,  the  burner  judges  the 
proper  degree  of  burning  by  the  appearance  of  the  clinker  in  the  kiln  and 
as  it  leaves  the  kiln.  To  say  the  least,  this  is  a  very  imperfect  method. 
He  controls  the.  degree  of  burning  by  varying,  first,  the  speed  of  the  kiln 
together  with  the  amount  of  raw  feed  and,  second,  the  amount  of  coal. 
By  the  former  operation,  the  time  of  burning  is  controlled;  and  by  the 
latter,  the  temperature  of  burning  is  controlled.  Within  certain  limits, 
it  is  possible  to  burn  a  clinker,  on  the  one  hand,  for  a  short  time  at  a  high 
temperature  and,  on  the  other,  for  a  longer  time  at  a  lower  temperature 
and  still  produce  the  same  degree  of  burning.  Since,  in  practice,  both 
the  time  and  temperature  are  varied  to  some  extent  and  since  ordinarily 
only  the  burning  temperature  can  be  measured,  it  is  evident  that  the 
temperature  measurements  cannot  properly  be  compared  and  correlated 
unless  the  time  of  burning  is  approximately  the  same.  Although  the 
speed  of  the  kiln  can  be  measured,  other  factors  that  enter  into  the  time  of 
burning  cannot  be  estimated;  these  are  the  length  of  the  sintering  zone 
and  the  inside  diameter  of  the  kiln,  both  of  which  vary  from  time  to  time. 
With  normal  operation  of  the  kiln,  it  is  believed  that  the  times  of  burning 
are  not  sufficiently  divergent  to  vitiate  comparison  of  the  burning 
temperatures. 

Several  conditions  occasionally  arise,  however,  which  produce  ab- 
normal operation  of  the  kiln  and,  accordingly,  the  temperature  readings 
taken  during  these  intervals  should  not  be  considered  as  taken  with  the 
normal  time  of  burning.  Some  of  these  conditions  are : 

1.  When  the  partly  calcined  raw  material  builds  up  around  the  cir- 
cumference just  before  the  sintering  zone  and  restricts  the  opening  of 
the  kiln  at  this  point,  the  distribution  of  temperature  along  the  kiln 
changes  considerably.     The  ring  formed  in  such  a  case  tends  to  cut  down 
the  draft  making  the  front  part  of  the  kiln  hotter  than  normal.     In 
addition,  the  sintering  zone  appears  to  be  shorter  than  normal  and  closer 
to  the  front  of  the  kiln.     Thus,  whenever  a  ring  forms  in  the  kiln,  the 
clinker  is  probably  burned  at  a  higher  temperature  for  a  shorter  time  or 
may  even  be  overburned. 

2.  Sometimes  an  excess  of  raw  feed  enters  the  kiln.     This  large  amount 
of  the  material  abstracts  considerable  heat  from  the  sintering  zone,  tending 
to  lower  its  temperature.     When  this  condition  goes  too  far  it  is  necessary 
to  slow  down  the  kiln,  or  even  to  stop  it,  in  order  that  the  large  mass  of 
material  may  be  burned  to  the  proper  degree. 

3.  Excess  of  coal,  or  "flooding"  of  coal,  causes  a  lowering  in  tempera- 
ture of  the  sintering  zone.     Such  a  large  amount  of  powdered  coal  may 
come  through  that  very  little  will  burn,  owing  to  the  limited  amount  of  air. 

34 


530  PYROMETRY   IN   ROTARY   PORTLAND    CEMENT   KILNS 

The  excess  of  coal  absorbs  a  large  quantity  of  heat  and  cools  the  hot  zone. 
At  the  same  time,  some  of  the  coal  burns  all  along  the  kiln,  and  the  rear  end 
becomes  hotter. 

Conclusions  from  Measurements. — Emphasis  should  be  placed  on  the 
fact  that  a  single  measurement  of  the  burning  temperature  has  little 
meaning;  for  a  gross  accidental  error  may  have  occurred  in  the  measure- 
ment ;  or,  for  some  reason,  the  kiln  may  not  have  been  operating  normally. 
Consequently  conclusions  from  the  data  can  only  be  drawn  when  a 
succession  of  temperatures  over  a  sufficient  length  of  time  have  been  taken. 
For  this  purpose,  readings  should  be  taken  through  8  or  10  hr.  of  each  day 
and  for  several  days.  A  very  vital  correlation  is  one  between  the  chemical 
compositions  of  the  raw  mix  and  clinker  and  the  burning  temperatures. 
It  may  not  be  amiss  to  point  out  the  factors  and  conditions  that  may 
exist  to  make  this  correlation  uncertain. 

1.  In  the  first  place,  there  are  the  abnormal  conditions  of  kiln  opera- 
tion, mentioned  before,  under  which  temperatures  are  not  being  measured 
properly. 

2.  The  factors  of  time  and  temperature  of  burning,  as  well  as  the 
chemical  composition  of  mix  and  clinker,  control  the  extent  of  formation 
of  the  cementing  constituents  in  the  clinker.     The  question  of  variation 
in  time  of  burning  has  been  discussed  previously.     A  cement  of  the  same 
composition  may  be  burned  over  a  considerable  range  of  temperature  and 
for  the  same  time  and  still  produce  a  cement  passing  specifications.     That 
is,  the  part  burned  at  the  higher  temperatures  will  contain  a  greater 
proportion  of  cementing  constituents.     Thus  a  petrographic  examination 
of  the  clinker  would  be  necessary  in  order  to  judge  this  factor.     To 
properly  compare  burning  temperatures  with  chemical  composition,  it 
is  necessary  to  assume  that  equilibrium  has  been  attained;  that  is,  that 
the  maximum  amount  of  cementing  constituents  possible  has  been  formed 
or  that  about  the  same  proportion  of  the  theoretical  quantity  has  been 
formed  in  each  burn. 

3.  Several  other  external  factors  of  more  or  less  indeterminate  effect 
enter  into  making  the  chemical  composition  uncertain,     (a)  It  is  known 
that  part  of  the  fine  coal  ash  blowing  through  the  kiln  enters  into  combi- 
nation with  the  raw  mix  or  partly  burned  mix.     Since  the  quantity  and 
composition  of  the  coal  may  vary  from  time  to  time,  this  effect  may  not 
be  constant.     (6)  Part  of  the  coating  on  the  kiln  lining  falls  off  occasion- 
ally into  the  partly  burned  mix  or  the  clinker.     This  coating  is  composed 
of  material  from  previous  mixes  and  its  admixture  possibly  can  change 
the  composition  of  the  mix  very  appreciably,     (c)  Two  raw  mixes  may 
have  identical  chemical  compositions  but  require  different  burning  tem- 
peratures for  the  reason  that  the  raw  mix  materials  are  in  different  phys- 
ical or  chemical  conditions.     For  example,  a  mix  in  which  treater  dust  is 
added  may  have  the  same  chemical  composition  as  one  in  which  none  is 


LEO    I.    DANA    AND    C.    O.    FAIRCHILD  531 

added;  but  the  treater  dust  mix  would  no  doubt  require  a  different  burn- 
ing temperature  because  that  part  of  the  mix  represented  by  the  treater 
dust  had  been  burned  previously. 

In  case  the  temperature  readings  are  fairly  constant,  say  within 
50°  C.,  we  believe  it  is  proper  to  average  the  readings.  Such  a.  mean, 
however,  is  not  considered  as  having  much  significance  when  the  tem- 
perature rises  or  falls  considerably  during  the  day.  In  the  latter  cases, 
the  range  of  temperature  covered  by  the  best  representative  line  drawn 
through  the  separate  readings  may  be  found.  This  process  is  justified 
and  is  probably  of  value  when  only  a  few  chemical  analyses  of  the  raw 
mix  or  clinker,  composed  of  a  number  of  samples  taken  during  a  period 
of  several  hours,  are  made. 

When  correlating  the  burning  temperature  with  the  chemical  com- 
position of  the  raw  mix,  one  should  not  forget  to  take  account  of  the  time 
of  travel  of  the  mix  from  the  feed  end  to  the  clinkering  zone  at  the  point 
where  the  temperature  is  measured. 

The  measurement  of  the  temperature  in  the  clinkering  zone  is  not 
an  easy  and  convenient  matter;  to  do  it  properly  requires  considerable 
practice,  two  persons,  and  the  assistance  of  the  burner.  In  addition,  the 
kiln  must  be  stopped  for  about  1^  min.  for  each  temperature  measure- 
ment. However,  the  loss  of  production  entailed  is  very  slight.  The 
measurement  of  temperature  in  the  clinkering  zone  is  not  to  be  recom- 
mended for  a  continuous  pyrometric  control  of  a  kiln,  but  rather  as  a 
special  aid  to  be  used  at  intervals  in  the  study  of  the  relation  between 
the  chemical  composition  of  the  raw  mix  and  clinker  with  the  burning 
temperature,  and  in  the  introduction  and  investigation  of  any  new  factors 
that  may  have  an  appreciable  effect  on  the  burning  temperature. 

TEMPERATURES  AT  REAR  END  OF  KILN  AND  IN  STACK 

There  probably  are  considerably  greater  differences  in  the  various 
cement  plants  in  the  temperatures  of  the  rear  end  and  stack  gases  than 
there  are  in  the  temperatures  in  the  sintering  zone.  The  reason  for  this 
is  that  the  rear  end  and  stack  temperatures  depend,  to  a  large  extent,  on 
the  construction  of  the  rear-end  housing  and  stack  and  on  the  process 
while  the  temperature  in  the  burning  zone  is  governed  principally  by  the 
mix.  Kiln  construction  and  the  process  differ  widely  in  various  plants, 
while  the  composition  of  the  mix  must  lie  within  definite  limits.  Con- 
sequently, in  the  following  discussion  we  shall  have  to  refer  in  more 
detail  to  the  conditions  that  existed  in  the  kiln  investigated  by  this 
Bureau,  and  the  statements  cannot  be  as  general  as  in  the  discussion  of 
the  temperatures  of  the  clinkering  zone. 

Rear-end  Temperatures. — The  average  temperature  of  the  gases 
leaving  the  end  of  the  kiln  experimented  on  was  about  650°  C.  Occasion- 


532  PYROMETRY   IN    ROTARY    PORTLAND    CEMENT   KILNS 

ally  the  temperature  rose  as  high  as  800°  C.  and  fell  as  low  as  500°  C.  The 
temperature  of  the  couple  fluctuated  as  much  as  10°  or  15°  C.  in  5  min. 
The  atmosphere  at  the  rear  end  was  extremely  dusty,  containing  the 
alkaline  cement  dust,  and  was  sometimes  slightly  reducing  or  slightly 
oxidizing. 

The  hot  junction  of  the  thermocouple  was  placed  near  the  axis  of  the 
kiln  about  1  ft.  in  from  the  plane  of  the  rear  end.  As  far  as  the  position 
of  introduction  and  the  length  of  the  couple  are  concerned,  these  depend 
on  the  construction  of  the  rear-end  housing  of  the  kiln.  It  was  found 
possible  to  place  a  7-ft.  couple  through  a  hole  in  the  top  of  the  stack  base 
so  that  the  hot  junction  would  be  in  the  correct  position. 

One  of  the  difficulties  that  occurs  with  the  rear-end  couple  is  the 
building  up  of  cement  dust  on  the  pyrometer  protection  tube.  This 
dust  cakes  to  such  an  extent  that  a  large  mass  of  dust  may  form  around 
the  hot  junction,  introducing  a  large  time  lag  in  the  indications  and  even 
swamping  out  short-period  variations  in  temperature  of  the  surrounding 
gases.  If  the  couple  is  placed  too  near  the  feed  pipe,  a  bridge  of  caked 
dust  may  form  between  the  pipe  and  couple  making  the  effect  worse. 
To  prevent  this,  every  part  of  the  couple  should  be  placed  at  least  2  ft. 
from  any  part  of  the  caked  feed  pipe.  It  may  be  necessary  to  bend  the 
couple  in  order  to  do  this.  At  present  the  only  way  that  appears  feasible 
to  prevent  the  caked  dust  from  affecting  the  temperature  readings  materi- 
ally is  to  remove  this  dust  periodically;  at  least  once  every  12  hr.  Per- 
haps as  simple  a  way  as  any  is  to  have  the  couple  protection  tube  rest  in  a 
sleeve  which  extends  from  the  head  of  the  couple  to  about  half  way  down 
the  tube  and  which  fits  the  tube  rather  closely.  Each  time  the  tube  is 
to  be  cleaned,  it  is  pulled  up  through  this  sleeve  and  the  accumulated 
dust  is  shaved  off. 

The  choice  of  the  proper  material  for  the  protecting  tube  of  the  thermo- 
couple is  another  source  of  difficulty.  It  has  been  demonstrated  that 
iron  tubes  will  not  last  long  in  the  atmosphere  existing  in  the  end  of  a 
dry-process  kiln  because  they  are  oxidized  and  rapidly  attacked  by  the 
dust.  A  porcelain,  clay,  or  other  such  refractory  tube  would  also  be 
attacked.  Probably  the  best  tube  that  can  be  obtained  at  present  is  one 
of  nichrome  or  chromel  of  fairly  thin  walls.  In  wet-process  plants  where 
the  temperature  at  the  rear  is  considerably  lower  and  the  atmosphere 
less  dusty  than  in  dry  process  plants,  it  is  probable  that  little  trouble 
would  be  experienced  from  the  corrosion  of  protecting  tubes  and  the 
caking  of  dust.  A  base-metal  thermocouple  of  iron-constantan  or 
chromel-alumel  is  satisfactory.  The  chromel-alumel  couple  will  have  a 
longer  life  and  can  withstand  a  higher  temperature  but  is  slightly  more 
costly. 

To  record  the  temperature  of  the  couple  any  one  of  the  galvanometric 
thermocouple  recorders  on  the  market  may  be  used.  Since  most  of  these 


LEO   I.    DANA   AND    C.    O.    FAIRCHILD  533 

recorders  are  not  as  dustproof  as  they  should  be  for  use  in  a  cement  plant, 
they  should  be  mounted  ia  a  dust  proof  cabinet.  Information  con- 
cerning thermocouple  and  recorder  installations  is  given  elsewhere.4 

The  installation  of  pyrometers  in  the  rear  end  of  rotary  kilns  has  not 
been  common  in  cement-mill  practice.  Because  they  can  serve  a  useful 
purpose,  we  believe  that  they  should  be  employed  more  frequently.  In 
plants  that  do  not  attempt  to  save  some  of  the  heat  carried  away  in  the 
gases  leaving  the  kiln,  records  of  temperatures  at  the  rear  end  should  in- 
dicate how  to  reduce  this  loss  to  a  minimum  by  correlating  the  changes 
in  temperatures  with  the  length  of  flame,  the  draft  through  the  kiln, 
and  other  factors.  In  plants  where  the  waste  gases  are  utilized  to  heat 
boilers,  a  pyrometer  at  the  rear  end,  as  well  as  one  at  the  waste-gas 
entrance  and  exit  of  the  boiler,  will  show,  in  addition  to  the  above-men- 
tioned correlations,  the  optimum  temperature  of  the  gases,  and  will  give 
a  record  of  the  attainment  of  this  condition.  Although  there  are  other 
means  of  recording  the  time  of  stopping  of  a  kiln  (the  supply  of  fuel  is 
usually  stopped  when  the  kiln  stops),  a  pyrometer  in  the  rear  end  by 
recording  the  temperature  of  the  gas  will  more  nearly  approximate  the 
true  time  the  kiln  was  ineffective  in  production.  As  stated  before,  the 
temperature  at  the  rear  end  is  abnormally  low  when  a  ring  occurs  in  the 
kiln;  as  a  rule  one  should  be  able  to  tell  from  the  records  the  duration  of 
a  ring.  Again,  when  there  is  a  large  excess  of  fuel  the  temperature  at  the 
rear  is  abnormally  high.  It  appears,  from  data  obtained,  that  when  the 
kiln  is  operating  normally,  the  rise  and  fall  of  the  maximum  temperature 
in  the  clinkering  zone  is  followed  by  a  similar  change  in  temperature 
of  the  rear-end  couple  greatly  diminished  in  magnitude. 

Stack  Temperatures. — The  stack  temperatures  encountered  in  the  kiln 
investigated  averaged  about  250°  C.;  they  ranged  from  about  200°  to 
300°  C.  The  gases  after  passing  out  the  rear  end  of  the  kiln  were  pur- 
posely cooled  by  the  admixture  of  air  entering  the  stack  base  from  the 
outside.  The  atmosphere  was  considerably  less  dusty  than  at  the  rear 
end;  and  because  the  dust  was  at  so  low  a  temperature  it  was  not  corrosive 
and  did  not  cake  to  the  extent  it  did  in  the  end  of  the  kiln. 

The  couple  was  placed  in  the  stack  about  15  ft.  from  the  floor  of  the 
stack  base.  This  couple  should  be  a  sufficient  distance  up  the  stack  to 
measure  the  temperature  of  the  well-mixed  gases  in  case  air  is  admitted 
below. 

If  the  temperatures  are  as  low  as  indicated,  the  installation  may  be 
similar  to  the  rear-end  couple  but  much  simplified  because  there  should 
be  no  trouble  from  errors  in  measurements  due  to  caking  of  dust  on  the 
pyrometer  tube  and  no  corrosion  of  an  iron  protecting  tube.  An  iron- 
constantan  couple  should  be  very  satisfactory  for  these  low  temperatures. 

4  Thermoelectric  Pyrometry,  by  Foote,  Harrison,  and  Fairchild.  Recording  Py- 
rometry,  by  Fairchild  and  Foote.  This  volume. 


534  PYROMETRY    IN    ROTARY    PORTLAND    CEMENT   KILNS 

On  the  other  hand,  if  the  temperatures  are  considerably  higher,  for  ex- 
ample above  500°  C.,  the  same  precautions  will  probably  have  to  be  taken 
as  with  rear-end  couples. 

If  air  is  mixed  with  the  gases  leaving  the  end  of  the  kiln  and  the  tem- 
perature, humidity,  and  quantity  of  this  air  are  constant,  the  indica- 
tions of  the  stack  couple  should  correspond  in  direction  of  variation  to 
those  of  the  rear-end  couple.  Usually,  however,  the  air  conditions  are 
not  constant,  and  the  couples  will  not  correspond. 

With  less  exactness  than  the  rear-end  couple,  the  records  of  the  stack 
pyrometer  will  also  indicate  the  stopping  of  the  kiln,  the  formation  of 
a  ring  in  the  kiln,  and  flooding  of  fuel.  In  case  the  waste  gases  pass 
through  dust  precipitators,  the  stack  couple  should  show  whether  the 
temperature  of  the  gases  is  proper  for  the  precipitators. 

This  discussion  of  the  application  of  pyrometric  methods  to  the  opera- 
tion and  control  of  cement  kilns,  while  far  from  complete,  will  possibly 
serve  to  indicate  some  of  the  advantages  and  possibilities  of  such  methods. 
We  are  indebted  to  Mr.  J.  C.  Pearson,  of  the  Cement  Section  of  this 
Bureau,  for  affording  us  the  opportunity  of  undertaking  this  investigation. 


APPLICATION    OF    PYROMETERS    TO    CERAMIC    INDUSTRY  535 


Application  of  Pyrometers  to  the  Ceramic  Industry 

BY    JOHN    P.    GOHEEN,*   PHILADELPHIA,    PA. 
(Chicago  Meeting,  September,  1919.) 

RECENTLY  the  head  burner  at  a  brick  plant  with  over  40  years'  experi- 
ence said  that  he  had  burned  brick  by  guess  for  over  half  his  lifetime  and 
had  used  pyrometers  for  2%  years  but  hoped  that  he  would  never  have 
to  burn  kilns  without  pyrometers  again.  The  ceramic  engineer  in  making 
a  special  study  of  the  burning  of  clay  ware  and  the  effect  the  heat  of  the 
kiln  has  on  color  and  vitrification-  is  also  rapidly  placing  the  burning  of 
clay  on  a  twentieth-century  basis. 

Clay  ware  may  be  divided  into  various  groups,  brick  and  drain  tile; 
sewer  pipe;  firebrick;  and  pottery,  which,  in  turn  may  be  subdivided 
into  special  classes.  As  nearly  as  the  analysis  of  the  clay  type  of  kiln, 
etc.,  will  allow,  there  are  practically  the  same  problems  to  solve 
in  burning  in  all  of  the  processes  or  operations.  The  underlying 
"objective  is  the  same,  namely,  the  production  of  the  best  grade  of  ware 
under  the  most  efficient  and  economical  manufacturing  system. 

The  principal  problem  in  the  production  of  clay  ware  is  the  burning  off 
of  the  kilns.  The  most  modern  steam  shovel  may  dig  the  clay  and  the 
most  advanced  types  of  pug  mills  and  wire  cut  machines  or  molds  of 
various  kinds  may  be  installed,  but  unless  the  drying  and  burning  of  the 
ware  is  successful,  all  of  the  previous  operations  will  be  useless.  On 
this  final  operation  depends  the  entire  result  of  the  plant. 

In  reality,  burning  of  all  clay  ware  is  purely  a  heating  proposition. 
The  kilns,  or  furnaces,  should  be  so  designed  as  to  give  equal  distribution 
of  heat.  Consequently,  the  problems  of  design  and  construction  of 
kilns  to  produce  equal  distribution  of  heat  require  serious  consideration,  as 
do  also  the  questions  of  heat  control  and  draft  control.  The  three  most 
important  factors  in  the  burning  of  practically  all  types  of  kilns  may  be 
summarized  as  follows:  The  installation  of  dependable  pyrometers  by 
which  the  temperature  can  be  accurately  measured;  the  draft  gage  to 
show  just  how  much  air  or  draft  should  be  allowed -to  come  in  over  the 
grate  bars  to  effect  good  combustion  and  to  allow  the  rate  of  heating  to  be 
controlled  by  adjustment  of  the  dampers;  the  construction  of  the  kiln; 
since  it  is  the  container  of  heat,  it  must  be  built  according  to  modern  prin- 
ciples. These  three  primary  factors  are  here  considered  with  reference 
to  the  burning  of  the  ware  and  the  resultant  Economical  advantages. 


*  Secretary,  The  Brown  Instrument  Co. 


536  APPLICATION  OF  PYROMETERS  TO  CERAMIC  INDUSTRY 

PYROMETER  INSTALLATION 

The  first  requisite  for  efficient  burning  is  a  pyrometer.  The  equip- 
ment to  best  meet  the  condition  of  each  particular  plant  should  be 
carefully  selected  in  order  to  secure  the  greatest  returns  from  the 
investment.  The  purchaser  who  buys  on  a  price  basis  alone  is  not  doing 
justice  to  his  business  experience.  The  equipment  that  will  be  most  suit- 
able for  all  plants  should  consist  of  a  thermocouple  in  each  kiln,  a  high- 
resistance  indicating  pyrometer  for  the  use  of  the  burner,  and  a  record- 
ing pyrometer  to  be  used  as  a  check  to  prove  whether  or  not  the  burning 
has  been  carried  out  as  directed;  also  rotary-type  and  plug-type  switches 
to  permit  any  thermocouple  to  be  connected  to  indicating  and  recording 
instruments. 

Thermocouples. — While,  in  general,  the  equipment  outlined  is  suitable 
for  all  plants,  particular  attention  should  be  given  to  the  selection  of  the 
thermocouples.  There  have  been  developed  within  the  past  2  to  3  years 
a  number  of  types  of  thermocouples  composed  of  base-metal  alloys. 
These  alloys  are  somewhat  limited  in  their  application  on  account  of  the 
rapid  oxidation  of  the  elements  under  high  temperature,  and,  therefore, 
should  only  be  used  under  favorable  circumstances.  The  most  satis- 
factory thermocouple  is  composed  of  pure  platinum  wire  on  one  side,, 
and  on  the  other  side  a  wire  composed  of  90  per  cent,  platinum  and  10  per 
cent,  rhodium.  The  melting  point  of  the  wire  in  the  couple  is  over  31 50°  F. 
(1732°  C.)  so  that  it  may  be  used  in  nearly  all  kilns.  Protecting  tubes 
composed  of  a  very  dense  and  highly  glazed  porcelain  are  used  to  protect 
the  platinum  wire  from  attack  and  contamination  of  the  gases  of 
combustion.  These  tubes  are  in  turn  protected  by  outside  tubes  of 
"corundite"  fireclay.  This  thermocouple  should  be  used  in  nearly  all 
applications,  particularly  when  the  temperature  will  exceed  2000°  F. 
(1093°  C.).  Some  plants  use  base-metal  thermocouples  for  kilns  burning 
sewer  pipe  and  firebrick,  which  require  temperatures  frequently  in  excess 
of  the  melting  point  of  the  base-metal  elements. 

Should  the  conditions  warrant,  a  base-metal  thermocouple  composed  of 
a  nickel-chromium  alloy,  with  a  melting  point  of  2600°  F.  (1427°  C.),  may 
be  used.  In  order  to  protect  this  couple  from  oxidation,  porcelain  tubes 
must  be  used  and  preferably  another  outer  tube  of  fireclay  to  protect  the 
porcelain  tube  from  injury. 

Indicating  and  Recording  Instruments. — The  indicating  and  recording 
instruments  should  be  built  up  with  an  internal  resistance  of  at  least  600 
ohms,  in  order  to  overcome  the  change  in  lead  wire  resistance  either  due 
to  the  use  of  different  length  of  leads  or  temperature  changes  along  the 
leads.  The  possible  source  of  error  is  proportionate  to  the  resistance  of 
the  meter  to  the  ratio  of  increased  resistance  of  lead  wire  or  thermocouple 
over  the  normal  external  resistance  of  the  circuit.  Accordingly  instru- 


JOHN   P.    GOHEEN 


537 


FIG.  1. — a,  RECORDING  INSTRUMENT  IN  OFFICE;  6,   INDICATING  INSTRUMENT  FOR 
BURNER;  c,  GROUP  OF  KILNS. 


538  APPLICATION    OF   PYROMETERS   TO    CERAMIC    INDUSTRY 

ments  of  a  lower  resistance  than  600  ohms  only  minimize  the  error.  This 
can  readily  be  shown  by  the  following  example :  Assume  that  a  pyrome- 
ter reads  accurately  when  using  50  ft.  (15  m.)  of  No.  12  gage  single-con- 
ductor copper  wire.  If  500  ft.  of  this  wire  is  added,  as  the  resistance  per 
foot  is  0.00159  ohm,  for  500  ft.  this  resistance  will  be  0.795  ohm.  If  a 
5-ohm  instrument  is  used,  the  error  produced  by  this  wire  is  its  resistance 
0.795  ohms  divided  by  the  resistance  of  the  instrument,  or  5  ohms,  which 
equals  15.9  per  cent.;  or  at  1500°  F.  (816°  C.),  an  error  of  238.5°  F. 
(132.5°  C.).  If  a  500-ohm  instrument  is  used,  the  error  will  be  the  resis- 
tance of  the  wiring,  0.795  ohm,  divided  by  the  resistance  of  the  instru- 
ment, 600  ohms,  which  equals  0.13  per  cent.,  or  at  1500°  F.  an  error  of 
2°  F.  This  error  is  so  small  that  it  would  be  impossible  to  read  on  the 
scale  of  a  pyrometer  graduated  to  2000°  F.  or  3000°  F. 

It  can  be  readily  understood  that  only  the  most  improved  form  of 
construction  will  embody  these  advantages.  The  recording  pyrometer 
should  also  be  of  similar  high-resistance  construction  and  of  a  simplified 
design.  It  is  best  to  use  an  individual-chart  instrument  for  each  kiln 
under  fire  so  that  when  filed  the  charts  can  easily  be  referred  to.  A 
recorder  producing  more  than  one  record  is  more  complicated ;  and  as  the 
various  kilns  are  brought  up  to  heat  and  burned  off,  it  is  difficult  to  decide 
just  when  to  tear  off  the  chart.  In  Fig.  1  is  shown  the  indicating  pyro- 
meter and  switchboard  for  use  of  the  burner  and  also  a  circular-chart 
high-resistance  recording  pyrometer  with  rotary  type  switch  so  that  any 
kiln  may  be  connected  with  the  recorder  as  placed  under  fire.  Addi- 
tional recorders  may  be  used  if  desired.  The  chart  is  produced  by  means 
of  a  circular  carbon-paper  disk  which  revolves  with  the  chart.  This 
eliminates  the  use  of  ink  in  any  form  and  the  consequent  cleaning  and 
filling  of  the  pen.  As  the  chart  is  rotated  by  an  8-day  clock,  a  tempera- 
ture record  of  an  entire  run  may  be  made  on  a  single  chart. 

Another  type  of  recording  pyrometer  is  the  continuous-chart  recorder, 
which  is  supplied  with  a  60-day  roll  of  chart  paper.  This  may  be  clipped 
off  at  the  end  of  each  burn  and  then  plugged  on  to  another  kiln.  This 
type  of  recorder  also  produces  a  permanent  record  without  the  use  of  ink 
by  means  of  a  carbon  ribbon  passing  between  the  chart  and  the  recording 
arm. 

Installation  of  Pyrometers. — In  the  installation  of  pyrometer  equip- 
ment on  kilns,  extreme  care  should  be  taken  to  follow  the  best  practice. 
The  thermocouples  should  extend  about  3  in.  (7  cm.)  inside  the  kiln 
crown  so  that  the  temperature  recorded  should  be  as  close  to  the  true 
temperature  as  possible  and  not  seriously  affected  by  heat  radiation 
from  the  kiln  walls.  At  some  plants,  an  effort  was  made  to  protect 
the  thermocouple  by  placing  it  so  that  it  did  not  extend  through  the 
crown  inside  the  kiln.  Naturally  the  temperature  was  seriously  affected 
by  the  heat  radiation  and  heat  absorption  of  the  brick  composing  the 


JOHN    P.    GOHEEN  539 

crown,  and  dependable  results  could  hardly  be  expected.  Moisture- 
proof  caps  should  be  provided  for  the  couples,  so  that  rain  or  dust  can- 
not seep  down  the  porcelain  or  firebrick  tubes.  The  wire  should  be 
supported  by  staybolts  at  proper  intervals  so  that  undue  stresses  will 
not  be  placed  on  the  thermocouple. 

In  general,  the  results  that  may  be  expected  by  the  installation  of 
pyrometers  are  as  follows:  (1)  Securing  proper  temperature  of  kiln 
from  which  the  best  temperature  curve  may  be  plotted  and  used  for 
burning  all  similar  clays.  (2)  Efficient  operation  of  the  kilns  by  the 
burner,  who  has  definite  temperature  curves  to  designate  the  condition 
of  the  kiln  at  various  periods  of  the  burn.  (3)  Elimination  of  uneven 
firing,  thus  enabling  the  burner  to  bring  the  kilns  up  to  heat  without 
any  set-backs.  (4)  Saving  of  fuel,  by  shortening  the  time  of  burn  due 
to  more  even  rate  of  firing.  (5)  Making  more  rapid  turnover  of  plant 
capacity  due  to  shorter  time  of  burning.  (6)  Producing  actual  records 
of  the  proper  burning  conditions  so  that  the  management  will  have 
access  to  this  information  at  any  time.  (7)  Providing  valuable  data 
for  the  ceramic  engineer,  in  analyzing  the  clay  and  the  relation  that  cer- 
tain temperatures  have  on  shades  and  quality.  (8)  Eliminating  careless 
firing  of  the  kilns,  especially  during  the  night  shift  when  the  firemen  are 
inclined  to  shirk  their  work,  as  the  records  would  show  their  negligence. 
Statistics  show  that  approximately  75  per  cent,  more  fuel  is  used  in 
bringing  up  a  kiln  with  30,000  or  50,000  brick  that  has  been  allowed 
to  drop  back  150  °  F.  than  if  the  set-back  had  not  occurred. 

DRAFT  EFFECTS  IN  KILN  BURNING 

The  second  requisite  for  good  kiln  control  is  the  use  of  proper  draft 
conditions.  It  is  almost  impossible  to  secure  good  temperature  control 
of  the  kiln  without  knowing  the  amount  of  draft  and  the  change  in 
draft  conditions.  A  very  simple  form  of  draft  gage  is  shown  in  Fig.  2. 
This  consists  of  an  inclined  glass  tube  with  oil  and  spirit  line  level.  The 
gage  is  graduated  in  hundredths  of  inches  and  may  be  attached  per- 
manently to  each  kiln  stack  or  used  portably  by  mounting  on  a  tripod 
or  stand.  The  effects  of  changes  in  draft  in  the  control  of  the  kiln  are 
very  important,  but  apparently  receive  little  attention.  When  there 
is  a  strong  draft,  the  kiln  will  burn  faster  and  the  dampers  in  the  stacks 
should  be  adjusted  accordingly. 

KILN  CONSTRUCTION 

Apparently  every  burner  and  superintendent  has  original  ideas 
concerning  kiln  construction,  which  they  have  carried  out  at  the  various 
plants  so  that  there  are  almost  as  many  different  types  of  kiln  construe- 


540 


APPLICATION   OF   PYROMETERS   TO   CERAMIC   INDUSTRY 


tion  as  there  are  plants.  One  of  the  most  common  mistakes  is  to  build  a 
kiln  without  giving  proper  attention  to  the  flue  capacity.  As  a  result 
the  burner  is  unable  to  bring  his  heat  down  to  the  bottom  and  only  the 
upper  courses  will  be  burnt  to  color,  proper  degree  of  hardness,  or  com- 
plete vitrification,  depending  on  which  of  these  is  desired.  Kilns  with 
proper  flue  regulation  to  support  the  pyrometric  control  should  have 
a  uniform  burn  from  the  third  course  upward,  if  used  to  advantage. 
Flues  should  be  arranged  so  that  the  draft  can  be  easily  regulated  by 
the  burner;  hot  spots  or  cold  spots  are  entirely  unnecessary.  By  closing 
or  opening  the  dampers,  quick  changes  in  draft  should  be  secured  which 
in  turn  is  shown  by  the  information  given  by  the  pyrometer  readings. 
While  there  are  innumerable  types  of  kilns  a  few  seem  to  have  proved 
their  worth  and,  accordingly,  have  become  practically  the  standard  of 
their  type.  The  installation  of  the  pyrometer  equipment  depends  con- 
siderably on  the  general  scheme  of  burning. 


FIG.  2. — DRAFT  GAGE  0  TO  2"  OF  WATER. 

The  old-fashioned  scove  kiln  can  be  eliminated  entirely,  as  this  has 
no  practical  manner  of  control  and  depends  entirely  on  natural  draft. 
The  round  down-draft  kiln  is  very  largely  used  for  burning  brick,  drain 
tile,  and  sewer  pipe,  and  is  known  as  a  periodic  kiln.  The  thermocouple 
is  placed  in  the  top  of  the  kiln  near  the  apex.  The  heat  from  the  fire 
boxes  passes  to  the  top  and  is  then  drawn  down  through  the  brick, 
which  are  piled  in  courses  with  about  1  in.  (2.5  cm.)  space  between  each 
brick.  The  necessity  for  good  draft  conditions  can  readily  be  understood 
as  the  heat  must  be  drawn  down  through  the  brick  and  out  through  the 
flues  in  the  bottom  of  the  kiln  to  the  stack.  In  Fig.  1  is  shown  a  plant 
with  its  pyrometer  equipment  controlling  the  burning  of  the  round 
down-draft  kilns.  The  indicating  and  recording  instruments  are  very 
similar  for  all  kilns  but  the  application  of  the  thermocouple  to  the  kilns 
differs  with  the  construction  of  the  kilns. 


JOHN    P.    GOHEEN  541 

In  installing  a  pyrometer  equipment  on  the  periodic  kiln,  the  thermo- 
couple is  usually  installed  about  3  in.  inside  the  kiln  and  about  3  ft. 
from  the  apex.  The  couple  is  then  in  the  hottest  part  and  registers 
the  highest  temperature.  This  is  a  very  important  point  in  connection 
with  the  use  of  pyrometers  for  kiln  burning.  The  temperature  readings 
give  the  burner  a  working  temperature  with  the  highest  temperature  as 
the  guide.  Thermocouples  installed  through  the  side  of  the  kiln  may 
be  more  affected  by  local  changes  in  draft  conditions.  More  than  one 
thermocouple  for  the  average  size  of  round  kiln  has  proved  unnecessary, 
though  some  plants  have  attempted  to  use  them  for  determining  bottom 
temperatures.  As  it  requires  an  extremely  long  thermocouple  to  pene- 
trate sufficiently  far  toward  the  center  of  the  kiln,  this  extra  thermocouple 
has  not  proved  very  popular.  Also,  with  good  conditions  for  controlling 
the  draft,  the  burner  can  soon  learn  how  to  bring  the  heat  at  the  top  of 
the  kiln  down  to  the  lower  courses  while  he  holds  the  top  steady. 
Such  good  control  has  been  secured  at  some  plants  that  the  temperature 
records  show  where  kilns  have  been  held  for  periods  as  long  as  48  hr. 
without  showing  a  rise  or  drop  in  temperature  exceeding  25°  F.  while  the 
heat  was  being  pulled  down  and  allowed  to  soak  through  the  lower  part 
of  the  kiln.  It  is  in  this  distribution  of  the  heat  uniformly  all  through 
the  kiln  that  pyrometers  have  proved  so  useful. 

Pottery  kilns  are  .somewhat  more  difficult  to  handle  from  the  view- 
point of  an  installation  of  pyrometers,  due  to  their  construction,  which 
requires  the  thermocouple  to  be  frequently  installed  first  through  the 
outside  wall  and  then  through  the  bag  wall  into  the  kiln  chamber.  The 
length  of  couple  necessary  and  the  peculiar  draft  conditions  in  the  bottle- 
shaped  kilns  have  not  encouraged  many  pyrometer  installations  at  these 
plants,  but  the  urgent  need  of  pyrometric  control  is  recognized.  An 
elaborate  system  of  shrinkage  scales  has  been  devised  at  some  plants, 
where  tests  are  taken  from  the  kiln  and  measured  for  their  shrinkage  on 
a  shrinkage  scale. 

In  addition  to  the  periodic  kilns  there  are  continuous  kilns,  which  by 
a  process  of  rotation  keep  certain  parts  or  chambers  under  fire  at  all 
times.  The  surplus  heat  from  the  chamber  under  fire  is  used  to  heat  up 
other  chambers  and  accordingly  save  a  great  quantity  of  fuel.  Some  of 
these  kilns  have  as  many  as  60  to  90  chambers  of  40,000  brick  capacity. 
A  number  of  ingenious  methods  have  been  adopted  to  carry  the  heat 
from  one  chamber  to  another.  Some  types  of  kilns  carry  the  heat  through 
flues  while  one  kiln  of  the  Haigh  continuous  type  uses  a  paper  curtain, 
which  burns  away  when  the  kiln  has  reached  a  certain  temperature 
and  automatically  connects  the  next  chamber  to  the  one  being  burned. 
The  fuel  used  is  generally  producer  gas;  it  is  then  only  necessary  to  light 
the  burner  of  the  new  chamber  at  the  proper  time  and  advance  the 


542  APPLICATION    OF   PYROMETERS   TO    CERAMIC    INDUSTRY 

burning  of  the  various  chambers  in  rapid  succession.  The  remaining 
chambers  are  meanwhile  cooling  off  or  are  being  loaded  or  unloaded. 
If  possible  to  stress  the  importance  of  pyrometer  control  of  continuous 
kilns  over  periodic  kilns,  it  would  seem  that  when  more  than  one  chamber 
is  intimately  connected  with  the  hot  zone  the  advantage  of  a  pyrometer 
is  greatly  enhanced.  By  proper  regulation  of  the  temperature  in  the  hot 
chamber,  the  rate  of  heating  of  the  two  or  three  chambers  in  advance  is 
affected  proportionately.  The  thermocouples,  whether  of  rare  metal  or 
a  nickel-chromium  alloy,  are  installed  through  the  crown.  Usually  the 
crown  of  a  continuous  kiln  is  very  thick  to  prevent  heat  waste  so  the 
thermocouples  are  about  30  to  36  in.  (76  to  91  cm.)  long  and  penetrate 
about  3  in.  inside  the  chamber.  As  the  kilns  often  have  as  many  as  90 
chambers,  the  pyrometer  installation  should  be  carefully  installed  with 
the  wiring  out  of  the  way.  Usually  the  kilns  are  covered  with  a  shed  so 
that  the  wires  may  be  carried  overhead. 


FIG.  3. — AMERICAN  DRESSLEE  TUNNEL  KILN. 


Another  kiln  of  interest,  namely  the  American  Dressier  tunnel  kiln , 
is  a  continuous  kiln  in  which  the  floor  of  the  kiln  moves  along  at  a  certain 
rate  and  the  material  is  passed  through  a  hot  zone.  Platinum-rho- 
dium thermocouples  are  located  in  the  hot  zone  while  nickel-chromium 
thermocouples  are  placed  at  intervals  of  about  20  ft.  down  the  kiln. 
After  the  ware  is  loaded  on  the  movable  floor  it  is  not  touched  until  ready 
to  be  unloaded  at  the  other  end.  Again  the  degree  of  success  is  dependent 
on  good  kiln  operation  and  the  pyrometer  is  of  very  great  importance  in 
establishing  proper  temperatures.  Fig.  3  shows  this  type  of  continuous 
kiln  with  the  thermocouples  located  along  the  top.  Indicating  and  record- 
ing instruments  are  placed  in  the  office  of  the  head  burner  where  complete 
data  of  the  temperatures  are  kept.  It  might  be  of  interest  to  note  that 
this  kiln  has  also  been  placed  in  service  at  steel  plants  for  the  annealing 
and  heat  treatment  of  various  grades  of  steels. 


JOHN  P.  GOHEEN  543 

DRYING  CLAY  WARE 

Another  important  temperature  problem  in  clay-working  plants  is 
the  proper  drying  of  the  ware.  Much  improvement  has  been  brought 
about  in  equipment  for  this  work.  The  waste  heat  from  the  kilns  is 
largely  used  to  heat  the  dryers,  so  it  is  necessary  to  control  the 
amount  of  heat  allowed  to  reach  the  dryer.  A  recording  thermometer 
has  become  instrumental  in  aiding  in  proper  dryer  regulation.  In  con- 
struction, the  equipment  consists  of  a  bulb  filled  with  a  nitrogen  gas  under 
pressure.  This  pressure  is  carried,  by  means  of  capillary  tubing  protected 
with  bronze  flexible  armor,  to  a  helix  to  which  the  recording  arm  is  attached. 
A  24-hr,  record  chart  is  supplied  so  that  an  actual  temperature  record  is 
produced  of  the  temperature  in  the  dryer.  By  properly  drying  the  clay 
ware,  the  very  start  of  trouble  in  burning  can  be  greatly  eliminated. 

CONCLUSIONS 

Almost  any  plant  can  show  the  results  of  poor  burning  either  from 
carelessness  or  "hard  luck"  with  that  particular  kiln.  Almost  all  plants 
will  show  the  result  of  good  burning  and  it  is  to  increase  the  percentage  of 
good  burns  that  pyrometers  have  been  so  widely  recognized  as  a  real 
essential  for  the  up-to-date  plant  management.  Other  checks  on  the  kiln 
must  not  be  overlooked,  such  as  "tests"  and  cones.  Cones  show  the 
relative  kiln  condition  but  have  little  to  do  with  the  temperature  of  the 
kiln.  It  is  their  function  to  tell  whether  a  certain  condition  of  the  kiln 
has  been  reached,  but  their  limit  of  usefulness  practically  stops  there. 
Pyrometers,  however,  serve  as  a  continuous  guide  for  the  burning  of 
the  kilns  from  the  start  until  the  kiln  is  finally  burned  off. 

•  It  is  due  to  the  persistent  demand  by  the  ceramic  engineer  and  up-to- 
date  manager  that  this  somewhat  severe  condition  for  the  application  of 
pyrometers  has  been  solved.  It  is  for  the  future  to  decide  whether 
pyrometric  control  will  be  carried  to  the  extent  where  the  burner  sets 
the  pyrometer  and  by  means  of  automatic  control  the  pyrometer  will 
control  the  heating  of  the  kiln  by  a  certain  gradual  increase.  The 
automatic  control  of  temperature  has  already  been  solved  for  the  control 
of  metallurgical  furnaces,  and  its  applications  for  gas-fired  kilns  is  not 
beyond  the  impossible.  Comparatively  much  bigger  things  may  be 
expected  through  the  future  development  of  pyrometers  for  the 
ceramic  industry. 


544  PYROMETRY    IN   BLAST-FURNACE   WORK 


Pyrometry    in  Blast-furnace  Work* 

BY    P.    H.    ROYSTERf    AND    T.    L.    JOSEPH,  $    WASHINGTON,    D.    C 
(Chicago  Meeting,  September,  1919) 

FOR  a  number  of  years  the  Bureau  of  Mines  has  been  investigating 
certain  problems  relating  to  the  blast  furnace.  In  the  course  of  these 
investigations  it  was  desirable  to  measure,  with  the  optical  pyrometer, 
the  temperatures  occurring  in  the  hearth  of  the  furnace.  Since  no  sys- 
tematic measurements  of  these  temperatures  have  been  published  a  state- 
ment of  the  results  may  be  of  interest.  The  data  given  in  this  paper 
were  obtained  at  thirty-two  blast  furnaces  operated  by  seventeen  com- 
panies; twenty  of  these  furnaces  were  making  iron,  eight  ferromanganese, 
and  five  spiegeleisen.  The  collection  of  this  information  was  made 
possible  by  the  cooperation  of  the  American  Manganese  Co.,  Bethlehem 
Steel  Corpn.,  Brier  Hill  Steel  Co.,  Buffalo  Union  Furnace  Co.,  Clinton 
Iron  &  Steel  Co.,  Donner  Steel  Co.,  John  B.  Guernsey  &  Co.,  Jones  & 
Laughlin  Steel  Co.,  E.  E.  Marshall,  the  McKinney  Steel  Co.,  National 
Tube  Co.,  New  Jersey  Zinc  Co.,  Republic  Iron  &  Steel  Co.,  Seaboard 
Steel  &  Manganese  Corpn.,  Southeastern  Iron  Corpn.,  the  Youngstown 
Sheet  &  Tube  Co.,  and  the  Wharton  Steel  Co. 

TEMPERATURE  OF  BLAST-FURNACE  HEARTH 

The  blast  furnace  was,  by  long  odds,  the  earliest  piece  of  industrial 
apparatus  operating  at  what  it  is  now  the  fashion  to  call  high  tempera- 
tures, i.e.,  above  1400°  C.  (2552°  F.).  It  is  a  furnace,  moreover,  whose 
operation  is  believed  to  be  extremely  susceptible  to  small  changes  in 
hearth  temperature.  Since  these  temperatures  have  not  been  measured 
by  the  majority  of  the  men  who  have  originated  or  accepted  this  theory 
of  the  importance  of  hearth  temperature,  it  is  perhaps  permissible  to 
wonder  just  how  this  theory  came  to  receive  such  universal  acceptance. 
The  men  operating  blast  furnaces  did  not — in  fact  could  not,  if  the  theory 

*  Published  by  permission  of  the  Director  of  the  U.  S.  Bureau  of  Mines.  Report 
of  research  under  the  joint  auspices  of  the  U.  S.  Bureau  of  Mines  and  the  National 
Research  Council. 

t  Assistant  Physicist,  U.  S.  Bureau  of  Mines. 

I  Assistant  Metallurgical  Chemist,  U.  S.  Bureau  of  Mines. 


P.    H.    ROYSTER    AND   T.    L.    JOSEPH  545 

is  correct — wait  for  the  invention  of  pyrometric  instruments  to  supply  the 
means  of  temperature  control.  Just  as  they  invented  the  theory  that 
hearth  temperature  should  play  an  important  part  in  furnace  control, 
they  also  invented  a  method  of  measuring  that  temperature.  It  would  be 
difficult  to  assign  a  date  to  the  origin  of  this  pyrometric  theory.  Blast- 
furnace legend,  in  fact,  may  claim  for  itself  the  invention  of  the  first  high- 
temperature  pyrometer;  that  is,  if  other  conditions  are  the  same,  high 
silicon  and  low  sulfur  in  the  metal  indicate  a  high  hearth  temperature  and 
a  drop  in  silicon  while  a  rise  in  sulfur,  not  attributable  to  other  causes, 
indicates  a  fall  in  hearth  temperature. 

This  has  never  been  shown  to  be  true,  of  course.  Moreover,  it  is 
difficult  to  get  such  a  statement  from  many  furnacemen  who  believe  in 
and  act  on  it.  Nevertheless,  the  most  readily  apparent  phenomenon  in 
blast-furnace  practice  is  the  implicit  faith  of  the  furnace  operator  in  the 
theory  and  the  confidence  with  which  he  changes  his  burden  or  his  blast 
temperature,  or  puts  on  extra  coke  in  answer  to  the  chemist's  report  on 
the  last  cast.  A  chemist's  report  on  the  last  cast  means  silicon  and 
sulfur.  A  cloud  of  uncertainty  often  hangs  over  much  that  goes  into  and 
comes  out  of  a  blast  furnace.  Complete  analyses  of  slag  are  not  often 
made  and  gas  analyses  are  almost  rare,  but  the  writers  are  not  aware 
of  any  furnace  now  operating  at  which  silicon  and  sulfur  is  n  ot  run  on 
each  cast,  reported  immediately,  scrutinized  carefully,  and  discussed 
earnestly.  The  furnace  operator  has  found  this  theorem  so  satisfactory 
a  guide  to  furnace  operation  that  he  has  had  no  cause  to  worry  over  his 
inability  to  set  down  his  knowledge  of  the  temperature  of  his  hearth  in 
terms  of  such  and  so  many  degrees  centigrade  on  the  gas-thermometer 
scale.  It  is  easy  to  understand,  therefore,  why  no  attempt  at  pyro- 
metric control  has  been  made,  although  instruments  able  to  measure  the 
temperatures  encountered  have  been  available  for  two  decades. 

It  was  the  writer's  purpose  in  making  the  temperature  measurements 
reported  in  this  paper  to  establish,  on  as  definite  a  quantitative  basis  as 
possible,  the  relation  between  the  temperature  in  the  furnace  hearth  and 
the  analysis  of  the  metal  made.  The  'silicon-sulfur  theorem  is  unfortu- 
nately rather  indefinite.  The  silicon,  for  example,  is  alleged  to  depend 
not  only  on  the  temperature  but  also  on  the  slag  composition  and  .volume, 
the  size  of  the  furnace,  the  tonnage  made  per  day,  and  the  character  of  the 
charge.  These  factors  are  in  doubt  in  that  each  can  mean  one  or  more  of 
several  things.  The  temperature  of  the  hearth  may  mean  the  tempera- 
ture of  the  metal  or  of  the  slag  in  the  hearth,  or  it  may  mean  the  tem- 
perature of  the  solid  stock  in  the  combustion  zone  or  of  the  products  of 
combustion  arising  from  that  zone.  By  the  composition  of  the  slag  one 
usually  means  its  "basicity."  This  again  doesn't  mean  anything  in 
particular;  it  may  refer  to  the  percentage  of  lime  or  of  bases,  or  to  the 
ratio  of  lime  to  silica,  of  bases  to  silica,  of  lime  to  acids,  or  of  bases  to 

35 


546  PYROMETRY    IN  BLAST-FURNACE    WORK 

acids.  The  size  of  the  furnace  may  mean  its  hearth  diameter,  its  bosh 
diameter,  or  its  volume;  the  "  character  of  the  stock"  may  mean  anything. 
In  order  successfully  to  correlate  the  temperature  measurements  obtained 
at  the  several  furnaces,  therefore,  it  was  necessary  at  the  same  time  to 
secure  all  the  information  available  concerning  both  the  furnaces  and  the 

furnace  practice. 
i 

TEMPERATURE    MEASUREMENTS    AND    OPERATING    DATA    FROM    IRON 
BLAST-FURNACE  OPERATION 

A  summary  of  the  temperature  measurements  made  by  the  writers 
and  some  of  the  operating  data  obtained  from  the  furnace  records  for 
twenty  iron  furnaces  are  given  in  Table  1.  The  furnaces  are  numbered 
arbitrarily  from  1  to  20,  as  shown  in  the  first  column.  Temperatures 
measured  with  a  set  of  Morse  type  optical  pyrometers  are  given  in  the 
second,  third,  and  fourth  columns.  Under  the  caption  Tuyere  Tem- 
peratures are  given  the  temperature  readings  observed  when  the  pyro- 
meter was  sighted  through  the  tuyere  stock  and  along  the  axis  of  the  blow- 
pipe, through  the  tuyere  and  into  the  furnace.  These  temperatures  are 
corrected,  as  well  as  was  practicable,  for  the  absorption  of  the  glass 
screen  in  the  tuyere  sight.  The  slag  and  metal  temperatures  are  those 
observed  when  the  pyrometer  was  sighted  on  the  surface  of  the  slag  and 
metal  streams  in  their  respective  runners  at  flush  and  at  cast.  To  the 
observed  readings  has  been  applied  an  appropriate  emissivity  correc- 
tion. The  details  of  the  measurements  and  their  probable  accuracy 
will  be  discussed  later.  The  metal  and  slag  analyses  were  made  by  the 
companies'  chemists.  The  figures  in  the  last  four  columns  were  taken 
from  the  furnace  records  and  have  their  usual  significance.  The  fuel 
consumption  is  given  in  terms  of  pounds  of  carbon  in  place  of  the  usual 
pounds  of  coke  per  ton  of  metal. 

COMPARISON    OF   SILICON-SULFUR    PYROMETER   WITH   OPTICAL 

PYROMETER 

A  casual  examination  of  the  figures  in  Table  1  reveals  little  to  support 
the  theorem  that  the  temperature  of  the  furnace  can  be  determined  from 
the  silicon  or  sulfur  in  the  metal  or  from  any  simple  relation  based  on 
metal  and  slag  analyses.  The  silicon  varies  from  0.92  to  2.40  per  cent., 
the  tuyere  temperature  from  1595°  to  1862°  C.  (2903°  to  3384°  F.),  the 
slag  temperature  from  1437°  to  1543°  C.  (2619°  to  2809°  F.),  and  the 
metal  temperature  from  1426°  to  1473°  C.  (2599°  to  2683°  F.).  The 
range  of  variation  found  in  this  limited  number  of  furnaces  is:  silicon, 
1.48  per  cent.,  tuyere  temperature  267°  C.,  slag  temperature  106°,  and 
metal  temperature  47°.  The  silicon  in  the  hottest  metal  is  1.14  per  cent. 


P.    H.    ROYSTER    AND    T.    L.    JOSEPH 


547 


CO     I      cocococotopococoeoMcococoiocoeoeocoeoco 


8  8  8    8    S    S  8  8  8  8  S     S  8 


548 


PYROMETRY   IN  BLAST-FURNACE   WORK 


and  in  the  coldest  metal  1.05  per  cent.;  the  silicon  in  the. metal  corre- 
sponding to  the  hottest  slag  is  1.70  per  cent.,  and  in  the  metal  accompany- 
ing the  coldest  slag  it  is  1.05.  The  metal  made  by  the  furnace  having 
the  highest  tuyere  temperature  carries  1.27  per  cent,  silicon.  The  furnace 
showing  the  coldest  tuyeres,  however,  made  metal  with  the  lowest  sili- 
con; that  is,  furnace  9,  tuyere  temperature  1595°  and  silicon  0.92  per 
cent. 

.  The  twenty  furnaces  were  arranged  in  the  order  of  the  increasing 
silicon  content  of  the  metal  and  were  divided  into  four  groups.  The 
operating  quantities  for  these  furnaces  were  averaged  by  groups  and  the 
results  are  shown  in  Table  2.  It  is  rather  difficult  to  establish  any  basis 
for  the  silicon-sulfur  theorem  from  these  data.  The  three  temperatures 
that  might  possibly  indicate  hearth  temperature  are  included  in  this 
table  as  well  as  the  three  expressions  that  might  indicate  the  basicity  of 
the  slag;  for  the  most  part  none  of  these  six  quantities  vary  markedly 
with  the  silicon  in  the  metal.  Group  I  has  average  basicity  and  a  low 
hearth  temperature;  group  IV  has  average  hearth  temperature  and  a 
low  basicity;  groups  II  and  III  have  both  average  hearth  temperature 
and  average  basicity. 

TABLE  2. — Furnaces  Grouped  According  to  Silicon  Content 


Group 

I 

II 

•  III                       IV 

1 

Furnaces  included  . 

5,  6,  9,  14, 
20 
1.02 
0.038 
1661 
1485 
1438 
1.28 
1.39 
1.04 
480 

4,  8,  12,  13, 
15 
1.23 
0.034 
1712 
1495 
1460 
1.31 
1.41 
1.01 
814 

7,  10,   11, 
16,  19 
1.49 
0.033 
1728 
1483 
1458 
1.33 
1.47 
1.01 
1048 

1,  2,  3,  17, 
18 
1.96 
0.031 
1718 
1530 
1467 
1.12 
1.25 
0.86 
690 

Silicon  in  metal.  .  .  . 

Sulfur  in  metal  

Tuyere  temperature,  degrees  C. 
Slag  temperature,  degrees  C  .  .  .  . 
Metal  temperature,  degrees  C  .  . 
CaO/SiO2  - 

CaO  +  MgO/Si02. 

CaO  +  MgO/SiO2  - 
Viscosity  of  slag  .... 

r-A!203  

TABLE  3. — Furnaces  Grouped  According  to  Tuyere  Temperatures 


Group 

V 

VI 

VII 

'  VIII 

Furnaces  included  

11,  15,  16, 

12,  13,  14, 

1,  4,  6,  7, 

2,  3,  8,  9, 

Tuyere  temperature,  degrees  C. 
Slag  temperature,  degrees  C  .  .  .  . 
Metal  temperature,  degrees  C  .  . 
Silicon  in  metal  

17,  18 
1827 
1516 
1467 
1.55 

19,  20 
1704 
1497 
1455 
1.18 

10 
1649 
1497 
1452 
1.48 

1623 
1491 
1450 
1.59 

Slag  temperature  minus  metal 
temperature,  degrees  C  

49 

42 

45 

41 

P.    H.    ROYSTEB   AND    T.    L.    JOSEPH 


549 


In  order  to  look  at  these  data  in  the  other  direction,  the  furnaces  were 
rearranged  in  the  order  of  decreasing  tuyere  temperatures,  and  averaged 
in  four  groups.  The  results  of  this  arrangement,  as  given  in  Table  3, 
show  that  the  slag  is  quite  uniformly  45°  hotter  than  the  metal  and  that 
the  metal  temperature  is  quite  closely  proportional  to  the  tuyere  tempera- 
tures, the  metal  temperature  rising  8°  for  every  100°  increase  in  tuyere 
temperature.  In  this  case  group  IV,  with  the  coldest  hearth,  shows  the 
highest  silicon. 

CALCULATION  OF  SILICON  IN  METAL 

With  the  aid  of  more  or  less  complete  data  on  a  number  of  furnaces,  it 
is  usually  impossible  to  miss  a  relation  existing  between  the  various  quan- 
tities once  preconceived  theories  have  been  discarded.  In  the  present 
case  it  can  be  shown  that  the  silicon  in  the  metal  can  be  calculated  from 
the  following  equation  with  an  average  error  of  ten  points  of  silicon  and  a 
maximum  error  of  twenty  points. 

If        S  =  per  cent,  of  silicon  in  metal; 

t  =  temperature  of  slag,  degrees  C. ; 
T  =  tons  of  metal  made  in  24  hr.; 
D  —  hearth  diameter,  in  feet; 
F  =  pounds  of  coke  per  ton  of  metal; 
M  =  per  cent,  of  silica  in  the  coke, 
then 

FM  r  T  n 

S  =  *? ; "  [0.238  +  0.143  p  +  0.0012  (t  -  1500)  J  (1) 

There  are  insufficient  data  from  the  first  six  furnaces  to  enable  one  to 
compare  the  actual  results  with  the  equation.  For  the  remaining  four- 
teen furnaces  sufficient  data  of  sorts  are  available,  and  the  silicon  has  been 
calculated  from  this  equation;  the  results  are  shown  in  Table  4. 

TABLE  4. — Calculated  Silicon  Contents 


Furnace    i     Silicon. 

Nnmhor             Actual, 

Per  Cent. 

Silicon, 
Calculated, 
Per  Cent. 

Difference, 
Per  Cent.  X 
10* 

Furnace        5gon. 
Number      ,£§& 

Silicon, 
Calculated, 
Per  Cent. 

Difference, 
Per  Cent.  X 
10> 

7               1.51 

1.58 

-  8 

14 

1.08 

1.03 

5 

8 
9 

1.29 
0.96 

1.21 

1.07 

8 
-11 

15 
16 

1.27 
1.59 

1.36 
1.45 

-  9 
14 

10 

1.37 

1.26 

11 

17 

1.70          1.61 

9 

11 

1.42 

1.26 

16 

18            1.76          1.57 

19 

12 

1.14 

1.24 

-10 

19            1.57          1.46 

11 

13 

1.14 

1.19 

-   5 

20            0.97          1.01             -  4 

In  Fig.  1  the  calculated  and  the  actual  values  of  the  silicon  are  shown 
diagrammatically.     It  is  rather  difficult  to  attribute  the  agreement  solely 


550 


PYROMETRY   IN   BLAST-FURNACE   WORK 


to  coincidence;  in  fact,  it  is  definitely  evident  from  Table  4  and  Fig.  1 
that  the  equation  is  an  approximation  of  the  true  relation.  It  must  not 
be  concluded,  however,  that  the  equation  can  give  the  answer  to  the  prob- 
lem with  any  finality.  It  can  be  regarded  as  nothing  more  than  the  best 
quantitative  expression  that  has  so  far  been  offered  for  predicting  the 
silicon  in  the  metal. 

The  interpretation  of  the  equation  1  is  rather  simple.  FM  is  the 
pounds  of  silica  per  ton  of  metal  in  the  form  of  coke  ash.  This  quantity, 
divided  by  47.7,  indicates  the  silicon  the  metal  would  carry  if  all  the 
silica  in  the  coke  ash  were  reduced  to  silicon  and  entered  the  metal  and  if 
none  of  the  silica  from  the  ore  or  from  the  stone  were  reduced.  Actually, 
the  silica  required  to  produce  the  silicon  in  the  average  metal,  for  these 


/s 

I 

r 

s- 

1.0 
0.8 

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^ 

\ 

M: 

- 

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\ 

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^. 

§ 

V 

1 

\ 

i 

/ 

S. 

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( 

\\ 

i 

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^ 

p> 

t 

\ 

AC 

tu 

jl_. 

IS 

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V 

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s 

\ 

V« 

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. 

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1 

^ 

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1 

>~>o 

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f/a 

fee 

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^8           9          10         II          12         t3         14         IS        16         /7         /6         19        2t 

FURNACE  NUMBER 
FIG.  1. 

fourteen  furnaces,  is  about  54  per  cent,  of  the  silica  in  the  coke  ash. 
The  values  for  the  individual  furnaces  vary  from  47.4  to  66.5  per  cent. 
The  furnace  "recovers"  therefore  something  better  than  50  per  cent,  of  the 
silicon  in  the  coke  ash.  This  silicon  recovery  varies  largely,  and  almost 
exclusively,  with  the  tons  of  metal  made  per  square  foot  of  hearth  area. 
In  addition,  but  to  a  much  less  important  extent,  it  varies  with  the  tem- 
perature of  the  slag.  An  increase  of  100°  in  slag  temperature  increases 
the  silicon  about  0.06  per  cent.  Since  the  silicon  is  thus  so  slightly 
affected  by  the  slag  temperature,  it  is  obviously  impossible  practically 
to  determine  the  slag  temperature  from  the  silicon  in  the  metal.  It  is 
scarcely  within  the  scope  of  the  present  paper  to  discuss  this  problem  fur- 
ther; although  the  question  of  silicon  control  may  be  of  importance  to  the 
general  problem  of  furnace  operation  and  control,  the  sole  reason  for  con- 
sidering it  here  was  to  show  that  it  is  but  a  slight  indication  of  the  hearth 
temperature  theoretically  and  that  practically  it  is  no  indication  at  all. 


P.    H.    ROYSTER    AND   T.    L.    JOSEPH 


551 


Furnaces  have  been  operated  many  years  on  the  assumption  that  the 
silicon  in  the  metal  is  a  hearth  thermometer,  and  it  is  undeniable  that  a 
cold  furnace  (i.e.,  a  furnace  producing  low-silicon  metal)  can  be  cured  by 
increasing  the  coke  charge  per  ton  of  metal.  Equation  1  shows  that  the 
silicon  can  be  raised  by  increasing  the  coke  consumption,  by  virtue  of  the 
fact  that  such  an  increase  in  coke  consumption  increases  the  amount  of 
silicon  in  the  coke  ash,  quite  independent  of  any  change  it  might  produce 
in  the  temperature  of  the  hearth. 


SULFUR  IN  THE  METAL  AS  A  PYROMETER 


If 


s  =  per  cent,  sulfur  in  metal; 
S  =  per  cent,  sulfur  in  slag; 
t  =  temperature  of  metal,  in  degrees  centigrade; 
B  =  ratio  CaO  +  MgO  ^  A1203  +  Si02 
it  can  be  shown  that  the  empirical  equation 

s  =  0.074  +  0.0183  S  -  0.00061  (t  -  1400)  -  0.037  B 


(2) 


0,05 


0.03 


O.OZ 


aoi 


\f 


IZ      13       14-15     16      17      Id 


3        4-       J-       6        7        8        9        /O       // 

FURNACE  NUMBER 
FIG.  2. 

agrees  with  the  facts  shown  in  Table  1  within  an  average  error  of 
0.0045  per  cent,  sulfur.  Unlike  the  expression  found  for  the  silicon  in 
the  metal,  this  relation  between  sulfur  and  the  related  operating  quan- 
tities agrees  in  a  general  way  with  the  usual  ideas  held  by  furnacemen. 
The  values  of  the  actual  sulfur  and  of  the  calculated  sulfur  in  the  metal 
for  the  first  eighteen  furnaces  are  given  in  Table  5  and  are  shown  diagram- 
matically  in  Fig.  2. 

The  sulfur  is  lowered  by  increasing  the  ratio  of  bases  to  acids,  by 


552 


PYROMETRY    IN  BLAST-FURNACE    WORK 


raising  the  temperature  of  the  metal,  and  by  lowering  the  sulfur  in  the  slag. 
This  is  the  old  story  of  sulfur  control  but  for  the  first  time  in  quantitative 
form,  and  substantiated  by  a  certain  amount  of  proof.  The  ratio  of 
bases  to  acids  is  used  as  an  independent  variable  here  merely  because  it 
gives  more  concordant  results  than  the  ratio  either  of  bases  to  silica,  or  of 
lime  to  silica.  In  the  same  way  the  temperature  of  the  metal  is  used  in 
preference  to  the  temperature  either  of  the  slag  or  of  the  tuyeres,  because 
the  agreement  is  better.  It  must  be  remembered  that  both  equations 
1  and  2  are  offered  merely  as  empirical  equations  found  by  trial  to  agree 
with  the  observed  facts  with  the  consistency  shown  in  Figs.  1  and  2. 
There  will  be  ample  time  to.  theorize  about  these  relations  when  their 
truth  has  been  satisfactorily  established.  It  may  be  of  interest  to  notice, 
however,  that  of  the  three  proposed  expressions  for  the  "basicity" 

of  the  slag  the  ratio  of  bases  to  acids  seems  to  be  the  most  significant. 

• 

DETERMINING  METAL  TEMPERATURE  WITHOUT  PYROMETRIC 
MEASUREMENT 

The  sharply  defined  effect  of  metal  temperature  on  the  sulfur  in  the 
metal  suggests  immediately  the  use  of  equation  2  for  determining  this 
temperature.  If  the  equation  is  rewritten 

t   =  1521  +  30  S  -  61  B  -  1666  s  (3) 

the  temperature  of  the  metal  can  be  directly  computed.  Since  from  Table 
3  the  slag  temperature  averages  45°  hotter  than  the  metal  an  approxima- 
tion to  the  slag  temperature  can  also  be  made.  This  has  been  done  and 

TABLE  5. — Calculated  Sulfur  Content 


Furnace 
Number 

Sulfur, 
Actual 
Per  Cent. 

Sulfur, 
Calculated 
Per  Cent. 

Difference, 
Per  Cent. 

Furnace 
Number 

Sulfur, 
Actual 
Per  Cent. 

Sulfur, 
Calculated 
Per  Cent. 

Difference, 
Per  Cent. 

1 

0.033 

0.033 

0.000 

10 

0.031 

0.026 

+0.005 

2 

0.025 

0.033 

-0.008 

11 

0.024 

0.023 

+0.001 

3 

0.026 

0.035 

-0.009 

12 

0.030 

0.027 

+0.003 

4 

0.034 

0.028 

+0.006 

13 

0.020 

0.016 

+0.004 

5 

0.036 

0.040 

-0.004 

14 

•0.036 

0.038 

-0.002 

6 

0.044 

0.050 

-0.006 

15 

0.034 

0.026 

+0.008 

7 

0.035 

0.038 

-0.003 

16 

0.032 

0.036 

-0.004 

8 

0.049 

0.045 

+0.004 

17           0.041 

0.034 

+0.007 

9 

0.049 

0.047 

+0.002 

18           0.029 

0.033 

-0.004 

the  results  are  given  in  Table  6.  It  is  observed  that  the  maximum  dis- 
crepancy between  the  observed  and  the  calculated  temperatures  is  16° 
and  that  the  average  discrepancy  is  7.4°.  As  the  calculated  value 
involves  all  the  errors  in  chemical  analyses  for  S  in  the  metal,  and  for 
CaO,  MgO,  A12O3,  Si02,  and  S  in  the  slag,  the  agreement  is  remarkably 


P.    H.    ROYSTER    AND    T.    L.    JOSEPH 


553 


close.  Moreover,  as  will  be  mentioned  later,  the  uncertainty  in  the 
pyrometer  readings  on  the  metal  is  easily  7°.  For  any  ordinary  pur- 
pose, calculation  gives  perfectly  satisfactory  metal  temperatures  but 
the  calculated  slag  temperatures  may  be  50°  in  error;  the  average  error 
for  slag  temperature  is  19.5°.  As  was  shown  by  Table  3,  a  change  of  8° 
corresponds,  in  a  general  sort  of  way,  with  a  100°  change  in  tuyere  tem- 
perature. Hence  it  would  be  expected,'  and  it  is  found  to  be  true,  that 
an  attempt  to  trace  back  from  the  sulfur  in  the  metal  to  the  tuyere 
temperature  fails. 


The  facts  presented  and  discussed  above  appear  to  point  to  a  number 
of  conclusions  many  of  which  are  quite  contrary  to  what  has  heretofore 
been  accepted,  and  which  may  be  of  importance  to  furnace  operators. 
In  summary  form  they  may  be  stated: 

TABLE  6. — Calculated  Metal  and  Slag  Temperatures 


Furnace 


Metal  Temperature,  Degrees  C. 


Slag  Temperature,  Degrees  C. 


Number 

Actual 

Calculated 

Difference 

Actual 

Calculated    Difference 

1  . 

1463 

1463 

0 

1526 

1508 

+  18 

2 

1467 

1480       -  13 

1524 

1525 

-  1 

3 

1462 

1478       -  16 

1530 

1523 

+  7 

4 

1470 

1459 

+11 

1506  j   1504 

+  2 

5 

1443 

1449 

-  6 

1473 

1494 

-21 

6 

1426 

1436 

-10 

1531 

1481 

+50 

7 

1444 

1448 

-  4 

1473 

1492 

-19 

8 

1437 

1430 

+  7 

1451 

1474 

-23 

9 

1437 

1434      +  3 

1437 

1479 

4.4. 

10      1456 

1448      +  8 

1449 

1494 

-46 

11 

1467 

1466 

+  1 

1469 

1511 

-42 

12 

1454 

1449 

+  5 

1493 

1494 

j 

13 

1473 

1464 

+  9 

1511 

1509 

+  2 

14 

1437 

1441       -  4     1481 

1486 

-  5 

15 

1468 

1456      +12     1514 

1501 

+  13 

16 

1459 

1465 

-  6     1528 

1510 

-18 

17 

1471 

1459 

+  12     1543 

1504 

-39 

18 

1471 

1477      -  6     1525 

1522      +  3 

1 .  The  silicon  in  the  metal  is  not  dependent  on  the  chemical  composi- 
tion of  the  slag;  the  temperature  of  the  hearth;  the  silica  content  of  the 
ore;  the  viscosity  of  the  slag;  the  blast  temperature. 

2.  The  silicon  in  the  metal  is  dependent  on  the  silica  in  the  coke  ash; 
the  speed  of  operation  expressed  as  tons  of  metal  per  day  per  square 


554  PYROMETRY   IN  BLAST-FURNACE    WORK 

foot  of  hearth  area;  the  slag  temperature,  but  to  an  almost  negligible 
extent. 

3.  It  is  impossible,  practically,  to  estimate  the  temperature  of  the 
combustion  zone  of  the  metal,  or  of  the  slag  from  any  simple  expression 
involving  the  silicon  content  of  the  metal. 

4.  The  sulfur  in  the  metal  is  not,  in  general,  lower  with  high  silicon 
metal. 

5.  The  sulfur  in  the  metal  is  dependent  on  the  sulfur  in  the  slag;  the 
temperature  of  the  metal;  the  ratio  of  bases  to  acid. 

6.  The  temperature  of  the  metal  can  be  calculated  from  the  sulfur 
in  the  slag,  the  sulfur  in  the  metal,  and  the  ratio  of  bases  to  acids  in  the 
slag.     The  maximum  error  in  this  calculation  is  16°  C.  and  the  average 
error  is  7.5°.     This  error  may  easily  be  attributed  to  errors  in  chemical 
analyses,  in  pyrometric  measurements,  to  both,  or  to  the  inaccuracy  of 
the  equation  from  which  the  temperatures  were  calculated. 

7.  The  temperature  of  the  slag  can  be  calculated  from  the  same  vari- 
ables with  a  maximum  variation  of  50°  and  with  an  average  error  of  19.5 
degrees. 

To  the  extent  that  the  furnace  operator  is  satisfied  with  the  production 
of  metal  that  meets  the  silicon  and  sulfur  specifications,  the  pyrometer 
has  little  to  offer.  Knowing  the  temperature  of  the  slag  and  of  the 
metal  it  is  possible  to  predict  the  analysis  of  the  following  cast.  Know- 
ing the  analysis  of  the  metal  and  of  the  slag,  it  is  possible  to  estimate 
the  temperature  of  the  preceding  cast.  The  pyrometer  has  something 
to  tell  about  what  is  going  to  happen  and  that  perhaps  is  an  advantage. 
Often  a  few  hours  mean  much  in  the  operation  of  a  furnace.  Neverthe- 
less it  is  easily  possible  to  overrate  this  advantage. 

There  is  more  to  the  operation  of  the  blast  furnace,  however,  than 
controlling  the  silicon  and  sulfur  content  of  the  metal.  The  real  furnace 
problem  is  the  production  of  a  given  tonnage  of  metal  in  the  shortest 
possible  time  and  with  the  least  expenditure  of  materials.  The  preced- 
ing discussion  has  not  touched  that  problem.  The  existence  of  certain 
temperatures  in  the  hearth  has  been  shown  and  their  effect  has  been 
discussed.  A  complete  pyrometric  study  of  the  blast  furnace  should 
go  further  than  this,  it  should  attempt  to  find  out  the  reason  for  the 
temperatures  observed.  Every  furnaceman  is  trying  to  keep  his  coke 
consumption  at  a  minimum;  the  mark  of  that  minimum,  the  sign  which 
says  he  shall  not  go  further,  is  a  cold  hearth.  It  has  been  shown  that 
a  reduction  in  coke  lowers  the  silicon  in  the  metal  without  indicating 
a  drop  in  temperature.  There  is  a  possibility,  therefore — the  writers 
insist,  however,  that  it  is  only  a  possibility — that  some  furnacemen 
have  stopped  on  the  high  side  of  the  minimum  from  fear  of  low-silicon 
iron.  It  is  where  a  serious  attempt  is  made  for  a  low-coke  campaign 
that  the  pyrometer  promises  to  be  most  useful.  Take  the  case  of  furnaces 


P.    H.    ROYSTER   AND    T.    L.    JOSEPH  555 

15  and  16  in  Table  1,  which  were  burning  1518  and  1502  Ib.  of  carbon  per 
ton  of  metal.  The  pyrometer  readings  show  that  there  was  no  danger 
here  of  a  cold  hearth.  The  metal  and  slag  temperatures  are  above  the 
average,  and  the  tuyere  temperatures  are,  respectively,  the  highest  and 
the  third  highest  observed. 

MANGANESE  IN  THE  BLAST  FURNACE 

Manganese  oxide,  like  silica,  is  reduced  at  a  higher  temperature  than 
iron  oxide.  It  is  generally  believed,  therefore,  that  a  blast  furnace 
making  high-silicon  pig,  spiegeleisen,  or  ferromanganese  must  operate 
with  a  hotter  hearth  than  a  furnace  making  basic  iron.  The  Bureau  of 
Mines  has  published1  a  summary  of  the  temperatures  observed  in  the 
hearth  of  seven  ferromanganese  furnaces  and  five  spiegeleisen  furnaces. 
The  following  figures  will  show  how  the  temperatures  in  the  manganese- 
alloy  furnaces  compare  with  the  temperatures  in  the  iron  furnaces  dis- 
cussed above. 

TUYERE  SLAG  METAL 

FURNACE               TEMPERATURE,  TEMPERATURE,  TEMPERATURE, 

-i'.V  •                   AVERAGE,  AVERAGE,  AVERAGE, 

DEGREES  C.  DEGREES  C.  DEGREES  C. 

Iron 1706  1498  1455 

Spiegeleisen 1597 '  1427  1392 

Ferromanganese 1550  1426  1386 

The  manganese-alloy  furnaces  are  considerably  colder  than  the  iron 
furnaces  in  spite  of  the  higher  temperature  required  to  reduce  manganese 
oxide  and  in  spite  of  the  fact  that  the  average  carbon  per  ton  of  metal 
consumed  is  5323  Ib.  (2414  kg.)  for  the  ferromanganese  furnace  and  3444 
Ib.  (1562  kg.)  for  the  spiegeleisen  furnace. 

It  was  concluded  as  a  result  of  the  Bureau's  investigation  of  manga- 
nese furnaces  that  an  improvement  in  practice  would  result  from  the  use 
of  less  coke  and  from  increasing  the  ratio  of  bases  to  silica  in  the  slag. 
As  an  illustration  of  the  use  of  the  pyrometer  in  furnace  control,  the 
following  figures  may  be  of  interest.  The  writers  observed  the  opera- 
tion of  a  furnace  making  ferromanganese  for  12  days.  Certain  of  the 
daily  operating  quantities  are  shown  in  Table  7.  The  charging  of  this 
furnace  was  done  on  the  basis  of  the  silicon  in  the  metal  and  of  the  opera- 
tor's judgment  of  the  hearth  temperature.  The  ratio  of  bases  to  silica 
was  quite  irregular,  but  in  general  much  too  low.  It  was  only  on  the  fourth 
and  twelfth  days  that  it  was  anything  near  what  it  should  have  been. 


1  P.  H,  Royster:  Production  of  Ferromanganese  in  the  Blast  Furnace.     Bull.  146 
(Feb.,  1919);  Bureau  of  Mines  War  Minerals  Investigation  Series  No.  5  (Dec.,  1918). 
P.  H.  Royster:  Production  of  Spiegeleisen  in  the  Blast  Furnace.    Bureau  of  Mines 
War  Minerals  Investigation  Series  No.  6  (Dec.,  1918). 


556 


PYROMETRY    IN   BLAST-FURNACE    WORK 


TABLE  7. — Daily  Results  of  Furnace  Making  Ferromanganese 


Day 

Pounds  of 
Coke  per  Ton 
of  Metal 

Ratio 
Bases/Silica 
in  Slag 

Silicon 
in  Metal, 
Per  Cent. 

Temperature 
of  Metal, 
Degrees  C. 

Temperature 
of  Tuyeres, 
Degrees  C. 

Per  Cent. 
Manganese 
in  Slag 

1 

7,718 

1.34 

2.20 

1,412 

1,571 

11.0 

2 

8,826 

1.36 

2.30 

1,426 

7.8 

3 

7,974 

1.38 

2.05 

1,450 

1,611 

6.1 

4 

8,109 

1.52 

1.58 

1,475 

1,622 

4.9 

5 

8,333 

1.45 

0.61 

1,456 

9.3 

6 

8,154 

1.48 

0.77 

1,456 

1,598 

6.3 

7 

7,885 

1.48 

1.78 

1,460 

1,614 

6.1 

8 

8,534 

1.45 

0.40 

1,514 

8.8 

9 

10,326 

1.42             0.41 

9.9 

10 

8,669                1.33             0.40 

1,480 

1,546 

8.5 

11 

8,579                1.40 

0.93 

1,480 

1,633 

6.6 

12 

6,797                1.52 

1.85 

.'  •"*• 

.  «-, 

4.9 

On  these  two  days  the  "basicity"  was  1.52,  and  the  manganese  in  the 
slag  was  4.9.  The  coke  consumption  was  in  every  case  much  too  high, 
its  effect  being  to  increase  the  slag  volume,  lower  the  basicity,  and  in- 
crease the  manganese  lost  in  the  slag.  The  coke  consumption  on  the 
twelth  day  was  but  82  per  cent,  of  the  coke  used  on  the  fourth  day,  but 
the  results  were  much  better,  the  per  cent,  manganese  in  the  slag  being 
equally  low  and  the  slag  volume  lower.  The  failure  of  the  silicon  thermo- 
metric  theorem  to  give  the  true  temperatures  or  to  serve  practically  as  a 
guide  to  good  operation  is  evident  throughout.  Bearing  in  mind  that 
the  average  metal  temperature  in  ferromanganese  practice  is  1386°,  it 
will  be  seen  that  this  was  a  very  hot  furnace.  On  the  eighth  day,  the 
metal  was  extremely  hot  (1514°  C.)  and  yet  the  silicon  had  dropped  over 
night  from  1.78  per  cent,  to  0.40  per  cent.  The  operator,  fearing  a  cold 
hearth,  put  on  extra  coke  that  day,  which  took  about  24  hr.  to  pass  down 
the  stack,  running  up  the  coke  consumption  on  the  ninth  day  to  the 
remarkable  figure  10,326  Ib.  per  ton  but  did  not  raise  the  silicon.  The 
tons  of  metal  made  on  the  ninth  day  was  only  38  compared  with  47  on 
the  eighth  day  and  an  average  of  about  50  tons  per  day  for  the  first  seven 
days.  It  can  be  shown  that  it  cost  the  furnace  profits  about  $3000  on 
the  ninth  day  to  conclude  that  the  furnace  was  getting  cold  because  the 
silicon  dropped  from  1.78  per  cent,  to  0.40  per  cent. 

MEASUREMENT  OF  TEMPERATURES  AT  THE  BLAST  FURNACE 

The  pyrometric  measurements  upon  which  the  figures  in  this  paper 
are  based  comprise  over  thirty-six  hundred  readings  at  iron  furnaces  and 
about  forty-one  hundred  readings  at  manganese  furnaces.  The  instru- 
ments used  were  Morse  type  optical  pyrometers,  the  outfit  consisting  of 


P.    H.    ROYSTER    AND    T.    L.    JOSEPH  557 

two  telescopes  and  absorption  screens,  four  lamps,  and  three  Weston 
milliammeters.  The  lamps  and  one  of  the  absorption  screens  were  cali- 
brated by  Leeds  &  Northrup.  The  milliammeters  were  calibrated  at 
intervals  during  the  investigation  in  the  Bureau's  laboratories  against  a 
standard  resistance  and  a  standard  cell.  Effort  was  made  to  avoid 
instrumental,  errors  by  frequent  interchanging  of  the  four  variable 
combinations;  observer,  lamp,  screen,  and  milliammeter.  There  were 
forty-eight  of  such  combinations  possible  and  the  concordance  was 
satisfactory  at  all  times,  except  in  the  case  of  the  Weston  ammeter. 
One  of  them  suddenly  began  reading  110°  C.  high,  and  it  was  necessary 
to  attach  a  shunt  to  it.  All  of  them  persist  in  collecting  particles  of  iron, 
or  magnetic  iron  oxide,  on  the  magnets  obstructing  the  movement  of  the 
swinging  coils. 

The  measurement  of  the  metal  temperature  is  the  simplest  of  the 
three  measurements  the  writers  have  attempted.  The  telescope  can  be 
sighted  from  a  distance  of  about  10  ft.  (3  m.)  on  the  metal  stream  flowing 
over  the  dam  just  below  the  skimmer.  Occasionally  the  presence  of 
traces  of  slag  carried  along  with  the  metal  causes  trouble.  Following 
Burgess,  the  emissivity  of  iron  has  been  taken  to  be  0.40. 

The  measurement  of  the  slag  temperature  is  somewhat  more  difficult. 
The  viscosity  of  the  slags  observed  varied  from  3  to  20  C.  G.  S.  and  the 
arrangement  of  the  slag  runner  is  different  at  each  furnace,  although  in 
general  all  of  them  are  difficult  to  observe.  Considerable  "spitting"  at 
the  cinder  notch,  clouds  of  sulfur-bearing  gases  arising  from  the  stream, 
the  presence  of  small  pieces  of  chilled  slag  on  the  surface  of  the  flowing 
cinder  combine  often  to  tax  one's  ingenuity  to  get  a  fair  sight  on  the  slag. 
The  emissivity  of  the  slag  has  been  taken  to  be  0.65  although  it  seems 
probable  that  0.70  is  more  nearly  correct.  This  change  in  assumed 
emissivity  would  necessitate  subtracting  10°  C.  from  the  slag  tempera- 
tures as  recorded  in  Table  1. 

The  most  difficult  reading  of  all  is  that  of  the  so-called  "tuyere  tem- 
perature." In  -the  first  place,  there  are  several  different  things  visible 
through  the  tuyere  glass.  Coke  in  dancing  lumps,  each  at  a  different 
temperature,  can  be  identified  with  certainty;  very  rarely  can  anything 
resembling  a  liquid  be  seen.  The  falling  "globules"  of  slag  and  metal  on 
their  way  from  the  bosh  to  the  hearth  have  doubtless  been  described 
more  often  than  they  have  been  seen.  About  a  third  of  the  time,  at  the 
average  furnace,  nothing  is  visible  except  a  flame.  The  tuyere  tempera- 
tures recorded  in  Table  1  are  merely  the  average  of  several  hundred  read- 
ings at  each  furnace  taken  at  random  through  unselected  tuyeres  with 
the  hope  that  such  an  average  will  more  or  less  approximate  the  average 
temperature.  There  is  no  combustion-zone  temperature  apart  from  the 
average  combustion-zone  temperature.  The  uncertainty  here  is  not  so 
much  due  to  difficulties  in  reading  as  to  difficulty  in  defining  a  tempera- 


558  PYROMETRY    IN  BLAST-FURNACE    WORK 

ture.  A  correction  to  the  observed  temperature  must  be  made  in  the 
case  of  tuyere  temperatures  on  account  of  the  absorption  of  the  glass 
screen  in  the  tuyere  sight.  This  has  been  taken  to  be  25°  C.,  which  is 
probably  low.  This  absorption  with  a  very  clean  glass  may  be  as  low 
as  15°;  but  due  to  the  flue  dust  in  the  hot  blast,  picked  up  in  its  journey 
through  the  stoves,  the  tuyere  glass  is  never  perfectly  clean.  To  add 
another  25°  to  the  tuyere  temperatures  in  Table  1  would  probably  more 
closely  approach  the  truth. 

DISCUSSION 

A.  L.  FEILD,  Cleveland,  Ohio  (written  discussion*). — In  equation  2, 
B  is  used  to  denote  the  ratio  of  bases  (lime  plus  magnesia)  to  acids 
(alumina  plus  silica)  it  being  stated  that  this  ratio  gives  more  concordant 
results  than  the  ratio  either  of  bases  to  silica,  or  of  lime  to  silica.  I  find, 
however,  that,  by  making  use  of  the  ratio  between  true  CaO  (CaO 
by  analysis  minus  sulfur  calculated  to  CaS)  to  acids,  equation  2  may  be 
modified  to  agree  with  the  observed  facts  within  an  average  error  of 
0.0041  per  cent,  sulfur  instead  of  0.0045  per  cent.  The  revised  equation 
is 

s  =  0.072  +  0.01835  -  0.00061  (t  -  1400)  -  0.04035 

where  B  is  the  ratio  of  true  CaO  to  A12O3  plus  SiO2,  and  the  other  symbols 
have  the  same  significance  as  before. 

I  would  not  appear  to  stress  too  much  this  slightly  better  agreement 
of  the  revised  equation  with  the  observed  sulfur.  Yet  I  believe  that  the 
use  of  this  ratio  of  true  CaO  to  acids  is  to  be  preferred,  if  for  no  other 
reason  than  the  fact  that  it  is  more  in  harmony  with  the  previous  findings 
of  the  Bureau  of  Mines  with  regard  to  the  effect  of  magnesia  and  other 
impurities  on  slag  viscosity.2 

The  empirical  relations  found  for  the  twenty  furnaces  investigated 
should  be  of  great  interest  both  to  the  metallurgist  and  the  furnaceman, 
but  to  establish  firmly  the  validity  of  these  equations,  the  investigation 
should  include  a  somewhat  wider  range  of  practice.  For  instance,  Table 

1  includes  no  pig  iron  with  more  than  0.050  per  cent,  sulfur.     So  far  as 
this  element  is  concerned,  all  the  cases  cited  fall  within  what  is  generally 
designated  as  good  practice  in  the  case  of  foundry,  basic,  or  Bessemer 
iron,  but  the  examples  do  not  cover  the  entire  range  of  good  practice. 
Iron  with  0.060  per  cent,  sulfur  is  quite  commonly  used  in  the  basic 
open-hearth.     Data  for  several  irons  a  little  high  in  sulfur  or  off-grade 
would  permit  a  proper  estimate  to  be  placed  upon  the  value  of  equation 

2  as  an  instrument  in  everyday  works  control.     Similarly,  equation  1, 

*  Received  Sept.  24,  1919. 

2  A.  L.  Feild  and   P.   H.  Royster:  Slag  Viscosity  Tables  for  Blast-furnace  Work. 
Tech.  Paper  187  (1918)  4  et  seq. 


DISCUSSION  559 

dealing  with  the  per  cent,  of  silicon  in  the  metal,  would  be  even  more 
convincing  if  it  were  to  apply  to  a  wider  range  than  from  0.96  to  1.76  per 
cent,  silicon.  Nevertheless,  even  within  the  range  of  practice  covered, 
it  is  remarkable  that  any  arithmetical  relations,  however  empirical, 
have  been  established. 

Probably  the  statement  that  will  meet  the  most  opposition  from 
furnacemen  is  that  the  sulfur  in  the  metal  is  not,  in  general,  lower  with 
high-silicon  metal.  I  believe,  however,  it  is  possible  to  reconcile  the 
accepted  silicon-sulfur  theorem  with  this  radical  conclusion.  The  latter 
is  based  on  observations  on  twenty  different  furnaces,  operated  under 
diverse  conditions  and  with  the  usual  differences  in  dimensions,  tendency 
toward  slips,  uniformity  of  blast  distribution  and  stock  descent.  While 
it  has  been  possible  to  derive  from  such  data  empirical  formulas  that  hold 
remarkably  well  for  the  case  of  silicon  and  of  sulfur,  taken  separately, 
it  would  be  much  more  difficult  to  correlate  silicon  and  sulfur  over  the 
wide  range  of  practice  selected.  It  is  hoped  that  the  validity  of  equations 
1  and  2  will  be  further  proved  by  a  series  of  experiments  on  a  single 
furnace,  covering  a  considerable  range  of  sulfur  and  silicon  values.  By 
confining  the  application  of  the  two  equations  to  a  single  furnace,  a 
relation  may  be  deduced  which  will  show  that,  in  these  circumstances, 
the  sulfur  in  the  metal  is,  in  general,  lower  with  high  silicon  content. 

By  making  a  single  assumption,  which  appears  permissible,  it  can 
be  shown  from  the  data  in  Table  1  that  there  is  a  well-defined  relation 
between  silicon  and  sulfur,  and  of  the  sort  expected.  It  will  be  observed 
that  the  corresponding  temperatures  of  slag  and  metal,  given  in  columns 
3  and  4  respectively,  do  not  differ  on  the  average  by  more  than  51°. 
Measurements  of  slag  and  metal  temperatures  are  subject  to  experi- 
mental errors,  particularly  the  former.  If  10°  is  subtracted  from  the 
slag  temperatures,  as  suggested  by  Messrs.  Royster  and  Joseph,  to 
correspond  to  an  emissivity  of  0.70  instead  of  0.65,  the  average  difference 
between  slag  and  metal  temperatures  will  be  only  41°.  For  our  present 
purpose,  we  will  assume  that  the  temperatures  of  slag  and  metal  are 
equal  for  any  given  furnace.  It  is  then  readily  proved  that  equations 
1  and  2  may  be  combined,  thus 

/FM  \        FM  I  T\ 

Si  +  2s  (  4?  7j  =  4?  ,_,(  0.263  +  0.365  -  0.0732B  +  0.143  ~  j     (a) 

where  Si  is  equal  to  the  silicon  in  the  metal,  S  the  per  cent,  of  sulfur 
in  the  slag,  and  the  other  symbols  have  their  former  significance. 

If  desired,  B  may  be  replaced  by  B',  where  B'  is  equal  to  the  ratio 
of  true  lime  to  alumina  plus  silica,  in  which  case  the  equation  becomes 


/FM  \         FM  I  T  \ 

Si  +  2s  (  47  ?  )   =  47  7(  0.260  +  0.365  -  0.07925'  +  0.143  ^  j 


(6) 


560  PYROMETRY   IN  BLAST-FURNACE   WORK 

An  examination  of  equation  a  shows  that  when  the  pounds  of  coke  per 
ton  of  metal,  the  per  cent,  of  silica  in  the  coke,  the  tons  of  metal  made 
in  24  hr.,  the  sulfur  in  the  slag,  and  the  basicity  of  the  slag  are  held  con- 
stant, the  quantity 

Si  +  2ks  =  constant  (c) 

FM 
where  k  is  a  constant  equal  to  jw-J     In  order  for  (Si  +  2fcs)  to  be 

constant,  it  is  necessary  for  the  silicon  to  go  up  when  the  sulfur  goes 
down,  and  vice  versa.  Such  a  change  might  be  produced  by  a  fall  in 
blast  temperature,  an  increase  or  decrease  in  atmospheric  moisture,  or  a 
change  in  the  regularity  of  movement  of  the  stock.  For  this  particular 
furnace,  therefore,  a  sudden  unexpected  increase  or  decrease  in  silicon  is 
accompanied  by  an  opposite  change  in  the  sulfur  content.  If  lime  or 
coke  is  put  on  or  taken  off,  there  is  another  set  of  conditions  to  which 
equation  c  applies,  but  with  a  different  value  for  the  constant  on  the 
right  hand  side  of  the  equation. 

Since  the  values  for  M,  the  per  cent,  silica  in  the  coke,  and  for  D, 
the  diameter  of  the  hearth  are  not  given,  it  is  not  possible  to  apply 
equations  a  or  6.  Such  a  computation  would  be  interesting  as  it  would 
indicate  what  percentages  of  silicon  would  be  expected  for  various 
arbitrarily  chosen  sulfur  values.  The  silicon-sulfur  curves  for  each 
of  the  twenty  furnaces  might  disclose  some  important  relationships. 

With  regard  to  the  effect  of  slag  viscosity  on  silica  reduction,  it  has 
been  deduced  elsewhere,3  on  theoretical  grounds,  that  a  fluid  slag  is  not 
necessary  because  diffusion  is  a  minor  item.  I  would  like  to  see,  however, 
some  attempt  made  to  correlate  desulfurizing  power  with  slag  viscosity, 
temperature,  and  basicity.  The  sulfur  distribution  between  slag  and 
metal  is  probably  largely  a  matter  of  diffusion,  although  we  do  not  possess 
such  quantitative  proof  of  the  theory  as  Messrs.  Royster  and  Joseph 
have  offered  in  support  of  the  principal  conclusions  of  their  paper. 

C.  P.  LINVILLE,  Elizabeth,  N.  J. — I  am  thoroughly  convinced  that 
silicon  in  pig  iron  is  directly  correlated  with  slag  temperature,  and  I 
want  to  outline,  perhaps  in  a  little  different  way,  some  work  I  did  about 
10  years  ago. 

It  might  be  well,  first  of  all,  to  state  that  the  temperature  obtained 
by  burning  coke  in  a  blast  furnace  is  affected  by  several  things.  That 
temperature  is  merely  a  result  of  heat-forming  reactions,  the  principal 
one  of  which  is  the  burning  of  carbon  to  carbon  monoxide,  and  the 
heat  is  absorbed  by  the  products  of  the  combustion  and  the  materials 
in  the  hearth.  The  temperature  to  which  those  materials  are  raised  is 
directly  proportional  to  their  amount  and  to  the  amount  of  heat  available. 
That,  of  course,  has  all  been  worked  out  before. 

3  Bureau  of  Mines  Tech.  Paper  187,  14-15. 


DISCUSSION 


561 


Now,  in  the  blast  furnace,  the  variables  are  the  hot-blast  temperature 
and  the  composition  of  the  air  used,  particularly  as  regards  moisture. 
We  get  a  hotter  fire  with  a  heated  blast  than  with  cold  blast  and  with 
dry  air  than  with  moist  air  on  account  of  the  heat  of  decomposition  of 
the  water  vapor  present  and  the  variable  specific  heats  of  the  gaseous 
products  of  combustion.  It  seems  to  me,  therefore,  that  data  which 
leave  out  of  consideration  the  matter  of  hot-blast  temperatures  and  the 
moisture  content  of  the  air  blown  neglect  two  very  important  things. 
I  admit  that  this  paper  seems  to  show  that  with  average  conditions  being 
practically  the  same,  there  is  perhaps  no  correlation  between  observed 
slag  temperature  and  percentage  of  silicon  in  the  pig  iron ;  but  if  you  take 
a  single  furnace  and  observe  that  furnace  from  cast  to  cast,  you  will  find 


22  £3  24  25 

August  1510 


27 


28  29 


20  31 


that  slag  temperature  as  it  rises  or  falls  is  accompanied  by  variations  in 
the  silicon  content  which  bear  a  fixed  relationship. 

For  a  period  of  almost  a  month,  I  followed  the  operation  of  a  furnace 
from  which  we  obtained,  hourly,  moisture  readings  of  the  gases  of  the 
air  blown  in.  We  had  a  continuous  record  of  hot-blast  temperatures 
and  got  from  the  laboratory  the  analysis  of  every  cast  of  iron  made. 
From  these  data  the  figures  were  averaged  over  the  period  between  casts. 
For  instance,  we  would  find  an  average  hot-blast  temperature  for  the 
6  hr.  preceding  the  cast,  which  would  be,  we  will  say,  639°  F.  (338°  C.). 
We  found  a  certain  amount  of  average  moisture  content  in  the  air  during 
that  time,  say,  6.9  gr.  per  cu.  ft.  From  data  worked  out,  having  those 
two  figures,  we  can  determine  the  theoretical  combustion  temperature  of 

36 


562  PYROMETRY   IN   BLAST-FURNACE    WORK 

coke  burned  under  those  conditions,  which  is  found  to  be'  3380°  F. 
(I8600  C.). 

If  the  theoretical  combustion  temperature  for  these  periods  and  the 
silicon  content  of,  the  iron  for  each  of  these  casts  are  plotted,  there  is 
found  to  be  a  marked  similarity  between  the  variations  in  the  curves  for 
the  silicon  content,  and  for  the  theoretical  combustion  temperature, 
showing,  it  seems  to  me,  that  there  is  a  relationship  between  combustion 
and  silicon  content.  The  accompanying  chart  will  illustrate  the  above 
contentions. 

I  will  say  that  for  periods  of  10  days,  at  least,  we  found  combustion 
temperatures  almost  identical,  in  their  variations  up  and  down,  with 
the  percentage  of  silicon  found  in  the  pig  iron;  and  in  this  particular 
furnace  under  the  particular  conditions  which  they  were  operating,  with 
the  same  burden  running  along,  we  found  in  general  a  change  of  0.1 
per  cent,  of  silicon  in  the  pig  iron  was  caused  by  a  difference  of  17.5°  in 
the  actual  theoretical  combustion  temperature,  which  might  be  caused 
by  either  a  change  of  20°  F.  in  the  hot-blast  temperature  or  0.7  gr.  in 
moisture  per  cubic  foot  of  air  blown.  These  figures  are  not  to  be  taken 
as  general  for  all  blast-furnace  work  but  for  the  particular  furnace  we  were 
observing  with  the  burden  that  it  was  carrying. 

P.  H.  ROYSTER. — I  agree  with  Mr.  Linville  that  the  moisture  in 
the  blast  has  an  important  effect  on  furnace  operation.  The  aver- 
age furnace  operator  does  not  think  so,  however,  and  hence  makes  no 
systematic  attempt  to  measure  the  humidity  of  the  engine-room  air. 
For  this  reason  the  authors  were  unable  to  obtain  records  of  the  mois- 
ture in  the  blast. 

Mr.  Feild  suggests  that  a  furnace  be  run  in  such  a  manner  that  all 
of  the  operating  factors  remain  constant,  except  the  one  factor  which  is 
under  investigation.  This  is  an  excellent  suggestion.  It  is  impracticable 
in  any  ordinary  plant,  where  two  or  three  operating  quantities  remain 
constant  over  any  considerable  period  of  time.  But  it  cannot  be  sum- 
marily dismissed  as  an  impossible  one.  The  scheme  is  a  promising  one; 
a  few  weeks  of  such  operation  would,  for  investigative  purposes,  be 
worth  a  year's  operation  under  ordinary  conditions. 

C.  P.  LINVILLE. — I  admit  that  all  of  the  pyrometrical  work  I  have 
done  on  blast-furnace  slag  temperatures  is  subject  to  considerable  cor- 
rection. The  work  I  did  was  on  temperatures  of  pig  iron  and  slag  as 
running  from  a  furnace  taken  by  a  pyrometer  with  no  corrections  made. 
I  did  not  consider  accuracy  with  regard  to  temperatures  and  variables 
necessary  but  differences  from  time  to  time.  Nothing  further  has  been 
done  along  the  line  of  actually  determining  temperatures. 

I  felt  that  we  had  a  certain  amount  of  evidence  at  that  time  which  bore 
out  the  perfectly  reasonable  assumption  that  the  actual  slag  tempera- 


DISCUSSION  563 

tures  and  the  actual  iron  temperatures  were  direct  functions  of  the 
theoretical  combustion  temperatures  and  that,  perhaps  on  account 
of  the  difficulty  in  reading  actual  temperatures,  results  could  be  correlated 
a  great  deal  better  on  the  basis  of  theoretical  temperature,  which  might 
be  several  degrees  higher  than  observed  temperature  would  be. 

With  regard  to  the  influence  of  moisture,  a  great  many  blast  furnaces 
take  their  air  supply  from  points  that  have  considerably  higher  moisture 
and  much  more  variable  moisture  content  than  the  local  weather  bureau 
would  show.  Just  to  show  the  variation,  the  moisture  at  7.00  a.m.  on 
this  particular  date,  in  the  air  being  blown  into  a  furnace  was  11.1  gr. 
per  cu.  ft.  That  is  for  the  average  6-hr,  period  ending  at  7.00  a.m.  In 
the  average  6-hr,  period,  ending  at  1.00  p.m.,  the  moisture  had  fallen 
to  7.8  gr.  We  went  from  a  very  high  humidity  to  a  relatively  low  rate, 
so  that  local  weather  reports  taken  from  a  point  some  distance  away 
from  the  furnaces  have  very  little  relationship  between  the  actual  figures 
that  you  would  get  if  you  were  close  to  the  furnace. 

P.  H.  ROYSTER  AND  T.  L.  JOSEPH  (author's  reply  to  discussion*). — 
Mr.  Linville4  has  published  a  record  of  the  temperature  of  the  slag  from  a 
300-ton  furnace  as  measured  by  him  with  a  Fery  radiation  pyrometer 
in  1909.  On  p.  276  he  has  shown  graphically  the  slag  temperature  by 
6-hr,  periods  from  2.00  A.  M.  Feb.  13  to  8.00  p.  M.  Feb.  15.  The  silicon 
in  the  metal  varied  from  2.08  to  1.02  per  cent.  We  have  grouped  his 
casts  into  two  groups,  high-silicon  metal  and  low-silicon  metal,  as  follows: 
High  silicon,  2  A.  M.,  8  A.  M.,  2  P.  M.,  Feb.  13:  8  A.  M.,  2  p.  M.,  8  P.  M., 
Feb.  15;  Low  silicon,  8  P.  M.,  Feb.  13  to  2  A.  M.  Feb.  15. 

In  addition  to  the  values  of  slag  temperature  and  of  silicon  content 
in  the  metal,  he  gives  figures  for  the  "theoretical  combustion  of  car- 
bon," to  which  he  seems  to  attach  importance;  we  do  not  understand 
how  he  calculates  this  temperature,  nor  what  it  means  when  he  has 
calculated  it,  but  we  have  included  these  values  in  the  group  averages 
to  show  that  they  do  not  explain  the  changes  in  the  silicon  content  of 
the  metal.  All  of  his  slag  temperatures  have  been  converted  into  de- 
grees Centigrade,  and  have  been  raised  60°  C. — the  emissivity  correction 
we  have  used  in  our  own  paper.  Mr.  Linville  seems  to  think  that  there 
is  some  disagreement  between  his  observations  and  ours.  For  com- 
parison, therefore,  we  give  below  our  twenty  furnaces  grouped  in  the 
same  manner  in  which  we  have  grouped  his  casts;  namely,  the  ten  furnaces 
showing  the  higher  silicon  and  the  ten  furnaces  showing  the  lower 
silicon. 

The  slag  temperature  is  not  dependent  on  the  silicon  in  the  metal 
based  on  either  Mr.  Linville's  or  our  own  data.  Further,  there  is  no 
relation  between  silicons  in  Mr.  Linville's  casts  and  his  theoretical 


*  Received  Feb.  6,  1920.  4  Trans.  (1910)  41,  268-279. 


564 


PYROMETRY  IN  BLAST-FURNACE  WORK 


Linville 

Royster  and  Joseph 

High  Silicon, 
Per  Cent. 

Low  Silicon, 
Per  Cent. 

High  Silicon, 
Per  Cent. 

Low  Silicon, 
Per  cent. 

Silicon  in  meta 
Observed  slag 
grees  C  

1  

1.72 

1499 
1991 

1.18 
1501 

1988  . 

1.72 
1506 

1.12 

1490 

temperature,  de- 

Theoretical     c 
perature,  deg 

ombustion     tem- 
rees  C  

combustion  temperature  of  carbon.  He  refers  to  some  work  he  did 
"about  10  years  ago,"  which  seems  to  refer  to  the  paper  of  1909  we  have 
quoted.  He  states,  however,  that  his  observations  covered  the  period 
of  "almost  a  month,"  which  indicates  that  he  has  two  sets  of  observa- 
tions only  one  of  which  has  been  published.  He  now  presents  a  curve 
showing  the  results  of  the  operation  of  a  furnace  for  10  days  in  August, 
1910.  Presumably  this  is  10  of  the  30  days  he  refers  to.  We  have 
read  off  from  his  curve  37  years  of  values  for  "theoretical  combustion 
temperature"  and  observed  silicon  content  in  the  metal.  There  is 
definite  evidence  of  a  relationship  between  these  two  quantities  which 
may  be  expressed  by  the  equation 


Si  =  0.009 (T  -  3000)  -  0.50 
Where  Si  =  silicon  content  of  metal,  in  per  cent.; 

T  =  theoretical  combustion  temperature,  in  degrees  F. 


(4) 


The  average  difference  between  the  observed  silicon  and  the  silicon 
calculated  from  equation  4  is  0.32  per  cent,  silicon,  the  maximum  va- 
riation being  0.82  per  cent. 

We  may  summarize  the  evidence  offered  by  Mr.  Linville  as  follows: 
Mr.  Linville  has  shown  that  the  silicon  in  the  metal  was  not  dependent 
on  the  theoretical  combustion  temperature  in  the  case  of  a  furnace  in 
February,  1909.  He  has  shown  that  the  silicon  in  the  metal  was  de- 
pendent on  the  theoretical  combustion  temperature  in  the  case  of  the 
same  (or  another?)  furnace  in  August,  1910. 

It  would  be  difficult  for  us  to  explain  why  Mr.  Linville's  data  do 
not  agree,  as  he  gives  little  information  about  the  other  conditions  of 
furnace.  We  are  not  even  going  to  try  to  explain  it,  because  we  are 
not  greatly  interested  in  any  theoretical  combustion  temperature.  Ap- 
parently Mr.  Linville  has  20  more  days'  worth  of  data  which  he  has 
not  published.  If  these  data  were  available  we  might  be  able  to  draw 
several  more  conclusions,  all  different  and  all  equally  good. 

We  have  felt  in  the  course  of  observing  temperatures  at  furnaces, 


DISCUSSION  565 

in  writing  about  them,  and  in  reading  what  others  have  written  that 
it  is  possible  to  take  a  list  of  observed  temperatures  too  much  for  granted. 
Observed  temperatures  and  the  conclusions  derived  from  them  should 
not  be  accepted  as  correct,  ignoring  experimental  error,  nor  should 
they  be  dismissed  as  incorrect  because  of  experimental  error  unless 
the  correct  attitude  is  really  known.  In  our  present  paper,  we  have 
printed  three  average  temperatures  for  each  of  twenty  furnaces 
and  none  of  these  temperatures  on  their  face  show  any  justification 
for  themselves.  The  metal  temperatures  are  too  nearly  constant  and 
'  the  tuyere  temperatures  are  not  constant  enough.  Our  observed  read- 
ings do  not  bear  out  what  Mr.  Linville  calls  the  "  perfectly  reasonable 
assumption"  that  the  slag  and  the  metal  temperatures  are  a  function 
of  the  combustion  temperatures.  Neither  do  they  bear  out  the  assump- 
tion Mr.  Feild  has  endeavored  to  make,  that  the  slag  temperature 
and  the  metal  temperature  are  identical. 

A  little  elementary  physics  will  show  that  the  temperature  of  the 
slag  cannot  be  the  same  as  the  temperature  of  the  metal.  Part  of  the 
slag  is  formed  from  the  ore  gangue  and  the  flux;  this  part  of  the  slag 
falls  into  the  hearth  with  the  metal.  If  there  were  no  ash  in  the  coke, 
the  slag  and  the  metal  would  come  from  the  furnace  at  the  same  tem- 
perature. The  coke  ash  cannot  fall  into  the  hearth,  however,  until  the 
carbon  in  the  coke  is  burned  at  the  tuyeres.  When  the  coke  ash  does 
fail  into  the  hearth,  it  is  at  the  temperature  of  the  tuyeres.  Since  the 
tuyere  temperature  is  usually  350°  C.  higher  than  the  metal  temperature, 
the  coke  ash  will  heat  the  slag  above  the  metal  temperature.  It  is  a 
simple  matter  to  calculate  how  much  this  temperature  will  be 

If          M  =  total  slag  per  ton  of  metal; 
A  =  coke  ash  per  ton  of  metal; 
Ti  =  temperature   of   coke   in    combustion   zone  —  tuyere   tem- 

perature; 
T2  =  temperature  at  which  gangue,   flux,   and  metal  fall  into 

hearth  —  metal  temperature; 
T3  =  temperature  resulting  from  mixture  —  slag  temperature; 

51  =  specific  heat  of  ash; 

52  =  specific  heat  of  gangue  plus  flux; 
Then 


where  K,  taken  for  simplicity  to  be  a  constant,  is  the  slag's  drop  in 
temperature  due  to  the  heat  loss  to  its  surroundings.  The  difficulty  in 
applying  this  equation  lies  in  the  uncertainty  as  to  just  how  much  coke 
ash  is  charged  per  ton  of  metal,  this  uncertainty  coming  from  the  failure 
of  coke  analyses  to  more  than  hint  at  the  actual  ash  and  from  the  pre- 


566 


PYROMETRY    IN   BLAST-FURNACE    WORK 


vailing  habit  of  charging  coke  without  weighing  it.  Equation  1  in  the 
paper,  however,  shows  the  relation  between  the  silicon  in  the  metal 
made  and  the  coke  ash  and  can  be  used  to  calculate  the  coke  ash  more 
closely  than  we  can  guess  at  this  figure  from  the  furnace  records.  The 
accompanying. table  shows  the  slag  temperatures  calculated  from  the 
equation  here  given,  using  the  ash  as  calculated  from  equation  1. 


Furnace 
.    Number 

Ash  from 
Records 

Ash  from 
Equation 
(1) 

Slag  Tempera- 
ture Calculated 
from  Equations, 
Degrees  C. 

Observed  Slag 
Temperature, 
Degrees  C. 

Differences  Cal- 
culated and 
Observed, 
Degrees  C. 

7 

12.0 

12.6 

1467 

1473 

6 

8 

12.0                  11.2 

1437 

1451 

14 

9 

12.0                  13.4 

1448 

1437 

11 

10 

12.0 

11.1 

1495 

1449 

44 

11 

11.9 

10.6 

1553 

1469 

16 

12 

11.9 

12.9 

1491 

1493 

2 

13 

10.7 

11.2 

1499 

1511 

12 

14 

10.7 

10.2 

1476 

1481 

5 

15 

11.0 

11.7 

1516 

1514 

2 

16 

11.0                  10.0 

1523 

1528                      5 

17 

11.2 

10.6 

1553 

1543 

10 

18 

11.2 

10.0 

1537 

1525 

12 

19 

13.3 

14.2 

1522 

1499 

23 

20 

13.3 

12.8 

1500 

1501 

1 

The  average  difference  between  the  calculated  and  the  observed  slag  tem- 
perature is  12°.  The  maximum  error  occurs  in  the  case  of  furnace  10 
for  which  furnace  the  observed  slag  temperature,  1449°,  is  lower  than 
the  observed  metal  temperature,  1456°.  This  points  to  an  error  in  ob- 
servation or  to  an  irregular  furnace.  If  this  furnace  be  omitted  from 
the  list  the  average  difference  between  the  calculated  and  observed  slag 
temperatures  is  only  10°  C. 

The  significance  of  this  calculation  is  important.  It  ties  the  three 
kinds  of  temperatures  together.  The  actual  slag  temperature  being  a  re- 
sult of  mixing,  on  an  average,  2  Ib.  of  coke  ash  at  1700°  C.  with  8  Ib. 
of  gangue  plus  flux  at  1450°  must  lie  20  per  cent,  of  the  distance  between 
1450°  and  1700°;  i.e.,  1500°.  The  fact  that  the  law  of  mixture  holds 
with  an  accuracy  of  10°  is  surprising.  Leaving  out  the  inaccuracy  of 
the  furnace  records,  we  have  left  only  an  average  error  of  10°  to  be  split 
between  the  slag,  metal,  and  tuyere  temperature  readings.  An  error 
of  10°  in  tuyere  temperature  will  cause  an  error  of  but  2°  in  slag  tem- 
perature. The  probable  division  of  error  would  be  2°  to  the  metal,  5° 
to  the  slag,  and  15°  to  the  tuyeres. 


PYROMETRY    AND    STEEL    MANUFACTURE  567 


Pyrometry  and  Steel  Manufacture 

BY    A.    H.    MILLER,    PHILADELPHIA,    PA. 
(Chicago  Meeting,  September,  1919) 

TEMPERATURE  considerations  are  of  prime  importance  in  the  manufac- 
ture of  steel  products — from  the  time  the  metal  is  produced  in  the  melting 
furnace,  where  the  chemical  reactions  have  a  direct  dependence  on  the 
temperatures  of  the  bath,  to  the  final  machining  of  work  on  which  the 
expansion  due  to  the  rise  in  temperature  of  the  forgings  during 'machining 
must  be  allowed  for.  Methods  employed  and  difficulties  encountered 
in  some  of  .the  more  important  steps  are  here  outlined.  These  methods 
are  the  result  of  years  of  experience,  with  many  methods  of  temperature 
determination,  and  have  been  successful  in  routine  works  practice  in 
the  manufacture  of  high-quality  steels. 

In  the  operations  of  melting,  forging,  and  heat  treatment,  if  the  prod- 
duct  is  to  be  of  the  best  quality  each  operation  must  be  carefully  followed 
through.  A  high-quality  product  cannot  be  obtained  even  with  the  best 
heat  treatment  if  the  steel  is  poorly  melted,  and,  conversely,  the  best 
steel  ever  made  may  be  spoiled  by  inefficient  heat  treatment.  To  per- 
form the  best  work  in  any  of  these  operations,  temperatures  must  be 
accurately  regulated. 

MELTING  TEMPERATURES 

In  the  regulation  of  steel  melting  temperatures,  open-hearth  furnaces 
will  be  taken  as  presenting  typical  difficulties  in  temperature  deter- 
mination. If  accurate  methods  of  temperature  determination  can  be 
evolved  for  use  in  the  open-hearth  furnace,  the  same  methods  will  be  as 
successful  in  the  crucible  or  electric  furnace  process.  Possibilities  as  to 
methods  are:  by  means  of  optical  pyrometers,  by  means  of  thermocouples, 
and  by  means  of  arbitrary  standards  which  may  possibly  employ  either 
of  the  other  two  methods  as  an  auxiliary. 

Optical  Pyrometer. — In  regard  to  optical-pyrometer  determinations, 
the  difficulties  are  the  lack  of  black-body  conditions  in  the  furnace  and 
the  fact  that  it  is  only  with  great  difficulty  that  the  steel  in  the  bath  may 
be  observed. 

Lack  of  black-body  conditions  is  responsible  for  considerable  inac- 
curacy in  taking  measurements,  although  this  source  of  error  may  not  be 
great  enough  to  condemn  the  method.  The  fact  that  the  steel  itself 
cannot  be  observed  is,  however,  of  prime  importance.  Such  data  as  have 


568  PYROMETRY    AND    STEEL    MANUFACTURE 

been  obtained  indicate  that  the  slag  covering  of  the  bath  may  differ  in 
temperature  from  the  steel  by  as  much  as  200°  F.  (94°  C.)  and  that  the 
walls  or  roof  of  the  furnace  may  show  even  a  greater  variation.  Tem- 
perature determinations,  by  means  of  the  optical  pyrometer,  may  be 
made  on  small  samples  of  the  steel  lifted  from  the  furnace  and  poured 
with  the  optical  pyrometer  sighted  on  the  stream.  Having  measured 
the  elapsed  time  between  taking  metal  from  the  bath  and  sighting  the 
optical  pyrometer,  the  temperature  of  the  bath  may  be  deduced  by  ex- 
trapolation. A  series  of  such  measurements  should  be  taken  to  arrive 
at  an  approximation  of  the  temperature  of  the  bath.  Another  method 
of  obtaining  similar  measurements  is  to  lift  a  spoon  of  metal  from  the 
bath  and  pour  it  immediately  into  a  small  test  mold,  readings  being 
taken  on  the  surface  of  the  metal  in  this  test  mold  at  definite  intervals 
of  time  until  the  surface  of  the  metal  freezes.  Having  taken  time  in- 
tervals with  a  stop  watch  simultaneously  with  the  optical-pyrometer 
readings  a  curve  can  be  drawn  which,  extrapolated  back  to  the  time  when 
the  metal  was  removed  from  the  bath,  will  give  an  approximation  of  the 
bath  temperature.  It  is  probable  that  these  two  methods  arrive  at  as 
close  an  approximation  to  an  accurate  bath  temperature  as  we  may  at 
present  obtain. 

An  effort  has  been  made  to  obtain  true  bath  temperatures  by  inserting 
in  the  bath  an  iron  tube,  well  protected  by  fireclay  sleeves,  and  having  a 
thin  wall  graphite  tip.  After  this  tube  has  remained  in  the  bath  for  a 
short  time,  the  interior  surfaces  of  the  graphite  tube  reach  the  tempera- 
ture of  the  bath,  which  temperature  may  be  read  through  the  hollow 
tube  by  means  of  an  optical  pyrometer.  This  method  is  mechanically 
clumsy  and  is  attended  by  considerable  discomfort  to  the  operator  be- 
cause he  is  necessarily  in  close  proximity  to  the  furnace  while  taking 
readings.  It  might  become  successful  but  for  the  fact  that  it  has  been 
impossible  to  obtain  refractories  that  will  withstand  the  temperature 
involved  and  will  give  off  no  fumes  at  this  temperature.  Such  fumes 
cause  variable  pyrometer  readings  that  cannot  be  corrected  for.  In 
using  the  optical  pyrometer  for  pouring  temperatures,  inaccuracies  arise 
because  of  difficulty  in  being  sure  whether  the  point  sighted  on  is  steel 
or  oxide.  Possible  inaccuracies  in  the  emissivity  factors  of  steel,  slag,  and 
oxide  are  also  sources  of  error. 

Thermocouple. — The  use  of  the  thermocouple  in  determining  open- 
hearth  furnace  temperatures  is  attended  by  the  difficulty  of  inserting  the 
thermocouple  to  the  point  at  which  the  temperature  is  desired.  The 
platinum-platinum-rhodium  couple  seems  to  be  the  best  of  any  in  common 
use  but  if  this  couple  is  not  thoroughly  protected  from  furnace  gases  its 
calibration  will  quickly  change.  It  is  probable  that,  even  with  perfect 
protection,  the  calibration  will  change  a  prohibitive  amount  at  the  high 
temperatures  used.  The  best  use  for  the  thermocouple  seems  to  be  the 


A.    H.    MILLER  569 

secondary  one — of  obtaining  temperatures  elsewhere  than  in  the  bath 
itself,  and  deducing  temperatures  of  the  bath  from  these.  The  method 
of  placing  the  thermocouple  in  the  slag  pocket  and  at  various  points  in 
the  checkers  and  at  the  base  of  the  stack  has  been  tried,  but  because 
of  variations  in  the  furnace  operation  it  is  not  believed  that  bath  tem- 
peratures can  be  accurately  deduced  from  data  at  these  points,  although 
useful  knowledge  as  to  the  furnace  working  may  be  obtained. 

Use  of  Arbitrary  Standards. — The  use  of  arbitrary  standards  has  been 
widely  employed.  Metal  may  be  poured  from  a  test  spoon  into  certain 
standard  molds  and  the  temperature  of  the  bath  deduced  from  ob- 
serving this  metal.  This  may  be  done  by  measuring  the  rise  of  tempera- 
ture of  a  protected  thermocouple  inserted  in  the  molten  metal,  by  sighting 
an  optical  pyrometer  on  the  surface  of  the  metal  in  the  test  ingot  or  on  the 
stream  from  the  spoon  while  pouring,  by  measuring  the  length  of  time 
required  for  the  surface  of  the  metal  in  the  test  ingot  to  freeze,  etc.  All 
of  these  methods  present  numerous  inaccuracies,  and  it  is  doubtful 
whether,  in  the  present  state  of  pyrometry,  there  is  a  method  that  can  equal 
the  accuracy  with  which  an  experienced  melter  can  estimate  the  bath 
temperatures.  It  has  been  found  that  such  a  melter  can  estimate  tem- 
perature changes  to  within  an  accuracy  of  10°  to  20°  F.  It  is  true  that 
this  estimation  of  temperature  differences  is  far  from  being  the  same  thing 
as  the  estimation  of  temperature,  as  the  results  cannot  be  recorded  for 
reference  nor  can  they  be  transmitted  from  one  plant  to  another  or  even 
from  one  man  to  another.  It  is  also  true  that  the  "condition"  of  an 
open-hearth  bath  depends  on  other  things  than  temperature;  such 
things  as  the  state  of  the  slag,  the  amount  and  quality  of  the  additions, 
and  the  time  factor,  which  is  of  quite  as  great  importance  as  the 
temperature. 

It  is  fortunate  that  in  the  melting  and  pouring  of  open-hearth  steel 
certain  natural  phenomena  give  temperatures  with  great  accuracy,  that 
are  a  very  close  approximation  to  the  desired  temperatures.  During  the 
first  part  of  the  melting,  while  the  first  elimination  of  impurities  is  in 
progress,  it  is  well  to  have  the  temperature  as  high  as  possible,  with  safety 
to  the  furnace.  The  melting  point  of  the  firebrick  employed  in  the 
furnace  roof  is  therefore  a  very  close  measure  of  the  temperature  to  be 
attained.  A  skilled  operator  can,  by  observation,  determine  the  point 
where  the  firebrick  will  glaze  slightly  but  will  not  actually  melt,  and  will 
run  his  furnace  to  this  heat.  In  teeming  a  heat,  on  the  other  hand,  cor- 
rect temperatures  of  pouring  may  be  accurately  regulated  by  the  appear- 
ance of  the  film  or  crust  that  forms  on  the  surface  of  the  metal  rising  in 
its  molds.  The  appearance  of  this  film  is  quite  characteristic  and,  with 
practice,  the  temperature,  or  rather,  the  fact  that  the  metal  is  at,  above, 
or  below  the  correct  temperature,  may  be  very  accurately  stated.  It  may 
also  be  regulated  at  this  point,  to  a  certain  extent,  by  varying  the  rapidity 


570  PYROMETRY    AND    STEEL    MANUFACTURE 

of  pouring,  according  to  the  appearance  of  the  metal  rising  in  the  mold. 
By  careful  observation  of  such  natural  phenomena,  temperature  differences 
may  be  estimated  to  within  10°. to  20°  F. 

Recapitulation. — During  the  melting,  the  temperature  (or  rather,  the 
correct  temperature)  can  be  estimated  as  closely  as  it  can  be  read  by  any 
pyrometric  methods  now  in  use.  The  best  methods  of  temperature  deter- 
mination are  by  means  of  the  optical  pyrometer  or  a  thermocouple  at 
some  place  other  than  in  the  bath.  During  the  pouring,  the  correct 
temperature  can  be  quite  accurately  estimated,  but  for  record,  optical 
pyrometer  readings  are  of  value. 

FORGING  TEMPERATURES 

In  the  determination  of  forging  temperatures,  both  optical  pyrometers 
or  thermocouples  are  used.  If  the  thermocouple  is  used,  either  plati- 
num-rhodium or  a  nickel-chrome  alloy  couple'  is  valuable.  Because  of 
the  difficulty  in  protecting  platinum-rhodium  couples,  especially  in  com- 
paratively large  work  where  the  thermocouple  must  be  long,  the  nickel- 
chrome  couple  seems  to  be  preferable.  It  is  quite  difficult  to  regulate 
accurately  the  temperature  of  the  pieces  to  be  forged,  because  the  hot 
gases  and  flame  in  the  furnace  affect  the  couple  much  more  rapidly  than 
they  do  the  piece.  For  reasons  of  economy  and  because  of  the  size  of 
ingots  in  heavy  work,  this  difference  is  of  much  greater  importance  than 
it  is  in  heat-treatment  work.  Iron-constantan  couples  have  been  suc- 
cessfully used,  but  the  life  of  this  couple  at  forging  temperatures  is  very 
short,  and  the  accuracy  of  the  couple  is  lower  than  at  temperatures  to 
which  it  is  better  fitted. 

The  optical  pyrometer  seems  to  be  the  best  type  for  measuring  forging 
temperatures.  With  some  of  the  later  improved  types,  temperatures 
can  be  determined  with  a  very  fair  degree  of  accuracy  (we  believe  to 
within  25°  F.)  in  ordinary  works  practice.  To  reach  this  accuracy,  the 
operator  must  have  had  a  certain  amount  of  experience  (2  wk.  is  ample  for 
a  man  of  average  intelligence)  and  must  sight  his  instrument  on  parts  of 
the  forging  that  will  give  an  approach  to  black-body  conditions.  He  must 
avoid  the  influences  that  will  cause  inaccuracies  in  his  readings;  such 
influences,  for  instance,  as  a  flame  playing  so  that  it  will  be  reflected 
directly  from  the  piece  to  his  instrument  or  the  presence  of  smoke  in  the 
furnace.  In  a  good  forge  furnace,  this  is  not  difficult  to  do. 

The  proper  forging  temperatures  of  various  grades  of  steel  differ 
quite  widely.  A  skilled  heater  can,  by  observation,  regulate  his  forging 
temperatures  fairly  well;  he  cannot,  however,  reach  the  uniformity  and 
accuracy  that  may  be  attained  by  the  use  of  the  optical  pyrometer. 
The  difficulties  involved  in  using  such  a  pyrometer  in  forging  work  are 
the  difficulty  in  training  the  operator  to  sight  his  instrument  properly 


A.    H.    MILLER  571 

and  the  mechanical  difficulties  in  the  construction  of  the  instrument 
itself.  These  mechanical  difficulties  are  caused  by  the  necessarily 
rough  usage  that  obtains  in  taking  these  temperatures,  together  with 
the  necessarily  smoky  and  dirty  atmosphere  surrounding  a  forge  and  the 
high  temperatures  to  which  the  instruments  are  exposed  during  use  (by 
exposure  to  radiation  from  open  furnace  doors,  etc.).  Numerous  me- 
chanical repairs  are  necessary  to  keep  the  instrument  in  a  good  working 
order.  To  sum  up,  the  optical  pyrometer  stands  in  a  class  by  itself  for 
the  regulation  of  forging  temperatures. 

HEAT  TREATMENT 

The  temperatures  involved  in  the  heat  treatment  of  steel  objects 
must  be  very  accurately  determined  and  uniformly  followed.  This 
temperature  is  within  the  range  that  is  difficult  to  estimate  by  the  eye 
and  at  the  same  time  it  must  be  most  accurately  followed.  Most  of  the 
higher  grades  of  steel  require  treatments  that  involve  a  quench;  most  of 
the  alloy  steels  require  a  preliminary  treatment  before  the  final  quench 
is  given  and  also  require  a  drawing  treatment  after  the  quench.  With 
the  preliminary  treatment,  which  is  ordinarily  at  a  comparatively  high 
temperature  (in  the  neighborhood  of  1550°  to  1650°  F.~ 843°  to  898°  C.) 
the  limits  of  accuracy  are  not  so  close  as  they  must  be  in  the  final  quench, 
which,  for  the  purpose  of  obtaining  the  best  possible  grain  refinement, 
must  be  above  the  critical  temperature  but  as  close  to  it  as  is  possible. 
The  temperature  of  the  draw,  also,  must  be  quite  accurately  approxi- 
mated, especially  if  the  results  desired  are  to  obtain  the  maximum  degree 
of  softness.  Optical  pyrometers  have  been  used  in  the  determination  of 
heat-treatment  temperatures,  but  with  mediocre  success.  The  general 
tendency  has  been  to  rely  entirely  on  thermocouples  for  this  work,  at  least 
for  temperatures  above  900°  F.  (482°  C.).  For  lower  temperatures,  if  the 
pieces  are  small  enough  to  be  treated  in  a  bath  of  oil  or  melted  salts,  a 
direct  reading  gas  or  vapor  expansion  thermometer  is  simple,  accurate, 
and  gives  but  little  trouble. 

Recapitulation. — It  seems  that  the  estimation  of  temperature  is  entirely 
inadequate  for  an  efficient  heat  treatment,  and  that  the  best  measure- 
ment methods  involve  the  use  of  thermocouples,  except  for  low  tempera- 
tures, attained  in  a  bath,  in  which  case  gas  or  vapor  expansion  instruments 
are  valuable. 

CHOICE  OF  INSTRUMENTS 

Optical  Pyrometers. — The  true  optical  pyrometer  is  better  for  works 
use  than  the  radiation  pyrometer,  because  imperfection  of  black-body 
condition  or  smoky  conditions  affect  its  readings  less.  The  Morse  or 


572  PYROMETRY  AND  STEEL  MANUFACTURE 

Wanner  types  of  optical  pyrometers  are  most  reliable,  and  particular 
care  must  be  exercised  to  select  an  instrument  of  the  simplest  and  most 
rugged  construction  possible. 

Thermocouples. — A  choice  must  be  made  between  various  types  of 
thermocouples  and  various  types  of  instruments  for  measuring  the  elec- 
tromotive force  generated  by  those  couples.  In  choosing  a  couple,  it  is 
necessary  to  consider  the  accuracy  of  the  couple,  the  constancy,  the  life, 
and  the  cost.  Some  years  ago,  platinum-platinum-rhodium  couples 
were  used  almost  exclusively.  It  is  well  known  that  the  calibration  of 
the  platinum-platinum-rhodium  couple -changes  quite  rapidly  wherrthe 
couple  is  subjected  to -the  influence  of  furnace  gases,  but  it  is  difficult  to 
protect  a  platinum  couple  from  this  influence  in  works  practice  and  it  is 
practically  impossible  to  do  so  when  the  thermocouple  must  be  over 
5ft.  in  length.  When  platinum  couples  are  used  in  works  practice,  an 
effort  is  made  to  protect  them  by  either  porcelain  or  silica  containing 
tubes. 

In  steel  works  practice,  where  the  work  is  generally  large,  almost 
all  the  couples  are  at  least  4  ft.  (1.2  m.)  long  and  the  majority  are  from 
5  to  7  ft.,  in  some  they  are  as  long  as  17  ft.  The  practice  with  platinum- 
platinum-rhodium  couples  therefore  involves  the  recalibration  of  the 
couple  after  each  failure  of  the  silica  or  porcelain  tube ;  and  couples  over  5 
ft.  long  must  be  recalibrated  after  every  heat.  Although  this  recalibration 
becomes  a  quite  simple  matter,  the  work  is  considerable  and  the  contami- 
nation of  the  couples  and  consequent  breakage  makes  their  operation  very 
expensive.  The  calibration  is  performed  by  taking  a  check  temperature 
at  the  gold  melting  point,  using  a  small  electric  furnace  and  the  wire 
method. 

With  the  advent  of  base-metal  couples,  of  which  the  iron-constantan 
couple  or  the  nickel-chrome  type  seem  to  be  the  most  popular,  the  use  of 
long  platinum  couples  is  entirely  eliminated.  In  using  base-metal  cou- 
ples, however,  certain  precautions  must  be  taken.  All  couples,  and  par- 
ticularly base-metal  couples,  are  subject  to  errors  due  to  parasite  currents 
induced  by  heterogeneity  in  the  wire  of  which  the  couple  is  made.  This 
lack  of  uniformity  is  almost  invariably  set  up  at  a  point  between  the  hot 
and  cold  junctions  of  the  couple  somewhere  in  the  furnace  wall,  due  to  the 
conditions  of  strain  set  up  at  this  point.  Once  established  this  condi- 
tion is  almost  impossible  to  eliminate.  However,  while  these  parasite 
currents  exist  in  practically  all  couples  that  have  seen  long  service,  the 
error  involved  is  small  enough  to  be  negligible  if  the  position  of  this 
strained  portion  is  unchanged  or  if  it  is  continually  advanced  from  a 
lower  to  a  higher  temperature.  In  practice,  this  means  that  the  accuracy 
of  the  couple  is  unchanged  as  long  as  the  couple  remains  immersed  to 
the  same  depth  in  its  furnace,  or  as  long  as  it  is  advanced  deeper  and 
deeper  into  its  furnace.  Errors,  however,  are  often  caused  if  the  couple  is 


A.    H.    MILLER  573 

partly  withdrawn  from  its  furnace.  Because  of  this  fact,  if  couples  are 
removed  from  their  working  position  and  checked  in  a  laboratory,  the 
checking  often  shows  an  error  whereas  the  couple  in  its  working  position 
was  actually  giving  accurate  readings.  If  careful  attention  is  given  to 
these  provisions  and  to  the  electrical  insulation  of  the  couple,  an  iron- 
constantan  couple  will  give  constant  readings  to  the  approximation  of 
10°  F.  until  it  is  completely  worn  out  by  oxidation  or  other  causes.  In 
the  platinum-platinum-rhodium  couple,  the  condition  of  the  metal  that 
produces  parasite  currents  may  be  destroyed  by  'a  careful  annealing  of 
the  couple.  This  anneal  may  be  carried  on  by  passing  through  the  wire 
an  electric  current  sufficient  to  heat  the  couple  to  a  point  well  above  its 
normal  use.  Parasites  set  up  in  the  heavier  base-metal  couples  are, 
however,  almost  impossible  to  eliminate.  An  iron-constantan  couple 
showing  such  parasites  has  been  heated  to  successive  temperatures  of 
2000°,  1900°,  1800°,  1700°,  etc.,  down  to  500°  F.  (1093  to  260°  C.),  each  of 
these  temperatures  having  been  held  for  approximately  1  hr.  and  each 
temperature  followed  both  by  a  quench  and  a  slow  cool,  without  entirely 
eliminating  the  inaccuracy. 

For  any  approach  to  accuracy  on  the  part  of  any  type  of  thermocouple, 
cold-junction  temperatures  must  be  allowed  for  and  corrected,  or  con- 
trolled. Control  may  be  by  means  of  an  ice  bath  or  other  method  of 
keeping  the  cold  junction  at  a  uniform  temperature.  One  of  the  more 
simple  and  widely  used  methods  is  to  place  the  cold  junction  underground 
to  a  depth  at  which  the  temperature  is  assumed  to  be  uniform.  This 
method  is  quite  desirable,  but  for  the  highest  degree  of  accuracy  is  inade- 
quate. Temperatures  even  5  ft.  (1.5  m.)  underground  may  vary  consid- 
erably with  the  season  of  the  year  and  with  the  previous  working  of  the 
furnace.  An  excellent  method  of  taking  care  of  this  cold-junction  cor- 
rection is  to  employ  lead  wires  of  the  same  composition  as  the  couple 
wires,  which  virtually  lengthens  the  couple  to  the  combined  length  of 
couple  and  leads,  bringing  the  resulting  cold  junction  to  the  measuring 
instrument. 

The  measuring  instrument  is  likely  to  be  at  a  point  where  tempera- 
tures are  more  nearly  uniform  and  where  the  cold  junction  can  be  more 
readily  measured  and  cared  for.  There  is  the  added  advantage  that, 
where  a  number  of  couples  are  being  read  on  one  instrument,  the  cor- 
rection for  these  couples  will  be  identical.  A  number  of  measuring  in- 
struments at  present  on  the  market  take  care  of  this  correction,  either 
automatically  or  by  means  of  a  simple  adjustment  on  the  instrument, 
with  highly  satisfactory  results. 

In  the  selection  of  the  measuring  instrument,  we  have  three  types  of 
instruments  to  consider:  the  galvanometer  type,  the  potentiometer 
type,  and  a  type  combining,  to  a  certain  extent,  the  theory  of  both  of 
these.  The  galvanometer  type  is  much  the  simplest  and  is  less  liable  to 


574  PYROMETRY    AND    STEEL   MANUFACTURE 

personal  error  than  either  of  the  others  because  no  adjustments  are 
required.  It  is  simply  necessary  to  read  the  temperature  directly  from 
the  face  of  a  dial.  However,  there  are  defects  in  the  galvanometer 
method  that  are  inherent  and  can  never  be  eliminated,  even  though  they 
may  be  reduced  to  amounts  said  to  be  negligible.  The  galvanometer 
measures  the  current  produced  in  the  circuit  of  which  the  couple  is  a 
part,  not  the  electromotive  force  generated  by  the  couple;  because  of 
this,  any  variation  in  resistance  of  the  circuit  must  produce  a  variation 
in  the  current  and,  consequently,  in  the  reading  of  the  galvanometer. 
There  must  be  a  change  in  the  total  resistances  of  the  circuit  with  any 
change  in  the  resistance  of  the  couple  itself,  due  either  to  wasting  away 
of  the  wires  by  oxidation  or  to  temperature  changes  in  the  couple,  and 
there  must  be  a  change  in  the  total  resistance  of  the  circuit  due  to  any 
temperature-resistance  change  in  the  lead  wires.  There  is  also  a  change 
due  to  variation  in  the  resistance  of  the  instrument  itself  with  tempera- 
ture change,  because  in  all  high-class  galvanometers,  the  electrical  resist- 
ance is  in  two  parts,  one  of  which  has  a  high-temperature  coefficient  and 
the  other  a  temperature  coefficient  that  approaches  zero.  In  a  high- 
resistance  galvanometer,  the  low-temperature  coefficient  resistance  should 
be  great  compared  with  the  total  resistance.  With  the  use  of  such  a 
high-resistance  galvanometer,  errors  due  to  resistance  changes  in  the 
circuit  are  reduced  to  a  minimum.  Because  of  these  errors,  low-resist- 
ance galvanometers  for  temperature  work  are  not  extensively  used  where 
accurate  work  is  desired.  The  modern  galvanometer,  which  is  jewel- 
pivoted  in  almost  all  cases,  is  also  subject  to  errors  due  to  friction  of  the 
galvanometer  bearings  and  to  changes  of  calibration  of  the  actual  instru- 
ment itself,  due,  for  instance,  to  weakening  of  the  controlling  magnetic 
system. 

The  potentiometer,  in  which  zero  current  passes  through  the  couple 
circuit  when  readings  are  taken,  eliminates  all  errors  due  to  variation  of 
resistance  in  the  line,  and  measures  the  electromotive  force  direct. 
Practically,  the  potentiometer  is  not  so  simple  an  instrument  as  the  galva- 
nometer and  requires  more  attention  to  keep  it  in  efficient  working  order. 
With  very  few  exceptions,  the  disorders  of  potentiometers  that  cause 
errors  in  readings  are  such  that  the  observer  almost  invariably  sees  the 
errors  when  they  occur  and  can  have  his  instrument  repaired,  whereas 
this  condition  does  not  obtain  in  use  of  the  galvanometer. 

RECAPITULATON 

The  determination  of  temperatures  in  open-hearth  furnace  practice 
is  desirable.  At  the  present  state  of  the  art  it  is  doubtful  whether  such 
temperature  determinations  as  can  be  made  are  of  actual  assistance  to  an 
experienced  melter  in  producing  high-class  steel.  The  best  method  of 


A.    H.    MILLER  575 

bath-temperature  determination  seems  to  be  that  of  sighting  an  optical 
pyrometer  either  on  the  surface  of  a  spoonful  of  metal  drawn  from  the 
furnace  or  a  stream  poured  from  a  spoon.  The  elapsed  time  between  the 
drawing  of  this  sample  from  the  bath  and  the  reading  of  the  temperature 
should  be  noted  and  the  temperature  of  the  bath  deduced  from  the  tem- 
perature of  the  sample  and  this  time. 

The  determination  of  temperatures  in  forging  practice  is  also  desirable. 
Present  methods  give  results  greatly  superior  to  the  results  obtained  by 
estimation  by  the  most  skilled  forgeman  and  are  of  great  assistance  in 
forging  work.  The  best  method  is  by  sighting  the  optical  pyrometer 
directly  on  the  article  to  be  forged  while  it  is  in  its  heating  furnace. 

The  determination  of  temperatures  in  heat-treatment  practice  is 
absolutely  essential.  No  amount  of  experience  and  care  on  the  part  of  an 
operator  will  approach  the  results  of  modern  temperature  determinations, 
and,  to  obtain  uniformly  the  best  results,  temperature  control  must  be 
close.  The  best  method  of  making  such  temperature  determinations 
seems  to  be  by  the  use  of  a  thermocouple  of  the  iron-constantan  type, 
using  lead  wires  of  the  same  composition  as  the  thermocouple,  and  meas- 
uring the  electromotive  force  generated  by  that  thermocouple  by  means 
of  potentiometers. 

CALIBRATION  AND  CHECKING 

All  temperature-measuring  apparatus  must  be  frequently  checked  for 
accuracy  in  works  practice.  In  a  large  installation,  it  is  well  to  have  a 
primary  temperature  standard  that  is  used  only  under  the  best  laboratory 
conditions  to  check  one  or  more  secondary  standards.  This  primary 
standard  may  consist  of  a  platinum-platinum-rhodium  thermocouple  with 
an  ice-bath  regulation  of  the  cold  junction,  the  electromotive  force  being 
measured  by  an  accurate  potentiometer.  This  primary  standard  should 
have  its  calibration  made  by  reference  to  well-determined  fixed  points; 
the  melting  point  of  gold  and  the  freezing  point  of  lead  are  convenient 
and  well-determined  points.  The  secondary  standards  may  be  either 
platinum-platinum-rhodium  or  base-metal  thermocouples  and  may  be 
carried  from  point  to  point  of  the  installation  to  check  the  actual  working 
couples  under  working  conditions. 

Optical  pyrometers  should  be  checked  from  time  to  time  using  a 
standard  couple  in  (preferably)  an  electric  furnace  in  which  black-body 
conditions  may  be  made  nearly  perfect. 

If  base-metal  working  thermocouples  are  purchased  with  a  calibration 
from  the  manufacturer,  it  is  well  to  check  this  calibration  to  see  that  it 
corresponds  to  the  works  standard.  If  the  base-metal  couples  are  pur- 
chased (or  manufactured)  as  wire,  and  are  to  be  calibrated,  the  following 
method  of  procedure  is  simple  and  accurate.  A  sample  should  be  cut 
from  each  end  of  each  coil  of  wire  as  received,  one  of  these  samples  selected 


576  PYROMETRY   AND   STEEL   MANUFACTURE 

at  random,  and  each  of  the  other  samples  of  similar  wire  joined  in  turn 
to  it  and  the  electromotive  force  generated  at  approximately  1600° 
noted.  This  electromotive  force  will  be  zero  if  the  wire  is  uniform. 
Having  checked  the  wire  for  uniformity,  several  sample  thermocouples 
should  be  made  up  and  carefully  calibrated,  using  one  of  the  secondary 
standards  in  this  calibration ;  a  calibration  curve  may  be  drawn  from 
the  data  thus  obtained.  One  or  more  of  these  calibrated  base-metal 
couples,  properly  protected,  should  be  inserted  in  a  pot  of  molten  lead 
and  all  sample  couples  made  from  the  coils  to  be  calibrated  inserted  in 
the  same  pot  and  checked  direct  against  the  calibrated  couples;  it  will 
be  found  that  certain  of  the  coils  vary  from  the  calibration  curve  already 
made.  An  arbitrary  allowable  variation  from  this  curve  may  be  selected, 
and  all  samples  that  vary  from  the  standard  by  more  than  this  arbitrary 
amount  discarded.  In  making  such  a  calibration  for  a  lot  of  base- 
metal  couples,  it  is  well  to  have  this  lot  as  large  as  possible,  as  it  is  an 
extremely  difficult  matter  to  reproduce. 

The  instruments  used  in  determining  the  electromotive  force  of  the 
working  thermocouples  should  be  frequently  checked,  even  though  they 
may  be  well  cared  for  and  of  the  most  accurate  type,  not  only  because  even 
the  best  instruments  are  likely  to  go  wrong  but  because  of  the  moral  effect 
such  checking  has  on  the  workmen.  This  frequent  checking  of  instrument 
and  couples  may  seem  to  be  troublesome  and  in  some  cases  superfluous. 
In  a  large  installation  it  will,  however,  pay  well  to  have  one  or  more  men 
whose  sole  duty  it  is  to  perform  such  checks,  as  the  accuracy  of  the  re- 
sulting work  and  the  confidence  inspired  by  such  careful  checking  will 
repay  all  the  effort  involved. 

DISCUSSION 

RICHARD  P.  BROWN,  Philadelphia,  Pa. — Both  the  thermoelectric  and 
the  optical  pyrometer  have  their  field.  With  the  optical  pyrometer 
you  can  secure  an  indication  of  the  temperature  of  the  metal;  with  a 
thermoelectric  pyrometer  installed,  for  instance,  in  the  slag  pocket, 
where  the  temperature  indicates  about  2000°  F.  (1100°  C.)  you  can  secure 
a  record  of  the  temperature  which  cannot  be  obtained  with  an  optical 
pyrometer. 

I  know  of  an  instance,  in  Ohio,  where  the  reversals  were  not  made  for 
about  2  hr.  during  the  night  because  the  furnace  man  fell  asleep.  Even  if 
an  optical  pyrometer  was  used  on  that  furnace  and  the  temperature  was 
up  to  the  required  heat,  the  superintendent  on  the  next  day  would  not 
have  known,  possibly,  that  the  furnace  was  not  reversed  properly  and 
would  have  wondered  what  was  wrong.  In  other  words,  while  the  optical 
pyrometer  can  be  used  to  good  advantage,  at  the  same  time  a  recording 
pyrometer  in  a  slag  pocket  is  a  continuous  check  on  the  temperatures 


DISCUSSION    -\  577 

maintained  by  the  furnace  man.  It  might  be  said  that  the  same  result 
would  be  secured  by  an  instrument  which  records  the  time  of  operation 
but,  unfortunately,  instruments  of  this  kind  are  too  often  tampered  with. 
But  it  is  hard  to  make  a  recording  pyrometer  record  anything  except  the 
actual  conditions.  There  is  a  field  for  both  instruments. 

W.  H.  BRISTOL,  Waterbury,  Conn. — Why  cannot  a  thermoelectric 
couple  recorder  be  tampered  with  just  as  easily  as  a  valve? 

R.  P.  BROWN. — Let  us  assume  that  a  workman  wants  to  take  a  nap; 
what  is  going  to  happen?  The  temperature  is  going  to  fall  off.  If  the 
temperature  is  falling  off,  it  is  rather  difficult  to  pull  out  the  thermocouple 
and  make  the  temperature  read  higher.  It  is  going  to  read  lower.  The 
truth  of  the  matter  is  that  the  average  workman  does  not  feel  competent 
to  tamper  with  a  recording  pyrometer. 


37 


578         ELECTRIC,    OPEN-HEARTH,    AND   BESSEMER   STEEL   TEMPERATURES 


Electric,  Open-hearth,  and  Bessemer  Steel  Temperatures 

BY   F.    E.    BASH,*   CH.    E.,    PHILADELPHIA,    PA. 
(Chicago  Meeting,  September,  1919). 

WHENEVER  electric  and  open-hearth  steel  men  discuss  the  relative 
advantages  of  their  respective  methods,  the  question  of  temperature  is 
always  discussed,  so  that  this  paper  is  written  in  the  hope  that  definite 


3100 
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KX3            2400            C500            2600             2700             2800            2900             3000          3100 
APPARENT    TEMP£RATURE    (FAHR) 

FIG.  1. — EMISSIVITY  CORRECTIONS  FOR  STEEL  AND  SLAG. 
Curve  1. — Steel.     Curve  2. — Slag. 

data  may  settle  some  of  the  questions  and  encourage  further  investiga- 
tions along  these  lines.  The  writer  has  had  the  opportunity  of  taking 
the  tapping  temperatures  of  steel  from  electric  furnaces  of  different  sizes 

*  Research  Engineer,  Leeds  &  Northrup  Co. 


F.    E.    BASH  579 

in  various  plants  and  from  a  number  of  open  hearths  handling  the  same 
kind  of  steel.  All  temperature  measurements  were  made  with  the  same 
disappearing-filament  type  optical  pyrometer  and  the  corrections  for 
emissivity  applied  were  those  worked  out  by  Burgess,1  which  are  0.40 
for  steel  streams  and  0.65  for  slag.  The  correction  is  applied  by  calculat- 
ing the  curve  giving  the  relation  between  the  true  and  apparent  tempera- 
ture in  the  following  formula : 


l         l\ 

Ti  -  TJ 


in  which  E  =  emissivity;  C2  =  14,500;  e  —  base  of  Napierien  log- 
arithms; X  =  wave-length  of  light  used  =  0.65ju;  T$  =  true  temperature, 
in  degrees  absolute;  T\  =  apparent  temperature,  in  degrees  absolute. 
The  curve  showing  the  relation  between  true  and  apparent  temperature 
for  steel  and  slag  are  given  in  Fig.  1. 

For  the  purpose  of  comparison  of  open-hearth  and  electric  furnaces, 
there  are  given  in  Table  1  the  tapping  temperatures  of  two  25-ton  Her- 
oult  electric  furnaces  and  one  6-ton  with  one  50-ton  acid,  one  40-ton  basic, 
and  one  65-ton  acid  open-hearth  furnace,  all  making  nickel  ordnance 
steel  for  guns.  The  two  25-ton  Heroult  electric  furnaces  were  finishing 
steel  refined  by  the  triplex  process  and  the  6-ton  Heroult  finished  steel 
that  was  partly  refined  in  an  open  hearth.  In  this  table,  the  tapping 
temperature  and  the  temperature  of  the  steel  stream  into  the  first  ingot 
mold  are  given  together  with  the  mean  of  each  column.  The  values  for 
each  plant  are  the  average  for  a  number  of  heats. 

TABLE  1. — Tapping  Temperatures  of  Steel 


Tapping 
Steel  Co.                            Temperature, 
Degrees  F. 

First 
Ingot, 
Degrees  F. 

Type  of  Furnace 

A  2867 

2744 

25-ton  Heroult  electric. 

B       2821 

2745 

6-ton  Heroult  electric. 

B  2821 

2721 

6-ton  Heroult  electric. 

C       2842 

2768 

50-ton  open  hearth,  acid. 

D      2877 

2753 

40-ton  open  hearth,  basic. 

E  2895 

2794 

65-ton  open  hearth,  acid. 

Mean                2854 

2754 

Mean  for  electric  furnaces  |         2836 
Mean  for  open-hearth  furnaces         2871 

2737 

2772 

The  table  shows  how  closely  the  temperatures  agree  from   plant  to 
plant  and  in  the  different  types  of  furnaces.     The  greatest  variation  in 

1  Temperature    Measurements    in   Bessemer   and   Open-hearth  Practice.     U.  S. 
Bureau  of  Standards  Tech.  Paper  91  (1917). 


580         ELECTRIC,    OPEN-HEARTH,   AND  BESSEMER   STEEL  TEMPERATURES 


tapping  temperatures,  between  B  and  C,  is  only  74°  F.  (24°  C.)  while 
the  average  for  all  the  electric  furnaces  is  35°  F.  lower  than  for  the  open- 
hearths,  although  it  is  generally  thought  that  open-hearth  steel  is  tapped 
colder  than  electric. 

The  drop  in  temperature  from  the  tap  to  the  first  ingot  depends  on  the 
length  of  time  the  steel  is  held  in  the  ladle,  the  size  of  nozzle,  size  and  pre- 
liminary temperature  of  the  ladle,  and  various  other  factors.  It  is  in- 
teresting to  note,  however,  that  the  mean  drop  in  temperature  of  the 
steel  from  tapping  to  first  ingot  for  both  the  electric  and  the  open-hearth 
furnace  is  100°  F. 

In  Table  2  is  given  a  tabulation  of  temperature  data  taken  on  two 
electric  furnaces  making  nickel  ordnance  steel.  At  the  time  these  tem- 
peratures were  taken,  the  ingots  were  box  poured  so  that  temperature 
observations  were  made  on  the  stream  above  and  below  the  box  on  the 
first  ingot  and  below  the  box  on  all  subsequent  ingots.  Since  there  was 
no  satisfactory  method  of  taking  temperature  of  the  steel  in  the  furnace, 
a  chill  test  was  made  by  taking  out  a  small  spoon  of  steel  and  noting  the 
time  required  for  a  crust  to  form  over  the  surface  of  the  metal  therein. 
These  time  values  are  also  given. 

TABLE  2. — Summary  of  Temperatures  for  Nickel  Gun  Steel 


First  Ingot 

Heat  No. 

Tap 
Tempera- 
ture, 
Degrees  F. 

Over  Box, 
Degrees  F. 

Under 
Box, 
Degrees  F. 

Second 
Ingot, 
Degrees  F. 

Third 
Ingot, 
Degrees  F. 

Chill 
Test, 
Seconds 

Time  Held 
in  Ladle, 
Minutes 

3X707 

2895 

2793 

2737 

2722 

2705 

8 

3X710 

2880 

2842 

2790 

2797 

2770 

26 

5K 

3X711 

2850 

2761 

2730 

2747 

2761 

37 

5 

3X715 

2917 

2843 

2768 

40 

8 

3X719 

2830 

2805 

2737 

31 

6 

3X723 

2805 

2745 

2720 

2680 

35 

5H 

4X651 

2872 

2761 

2752 

2728 

38 

7 

4X659 

2865 

2797 

2745 

2761 

33 

8 

4X666 

2872 

2813 

2768 

2730 

27 

5 

3X727 

2835 

2775 

2705 

2680 

2697 

? 

7 

2X3146 

2910 

2730 

2730 

2730 

Mean  

2867 

2797 

2744 

2732 

2736 

33.5 

6.5 

It  will  be  noted  that  the  tapping  temperatures  vary  from  2805°  to 
2917°  F.  (1540  to  1603°  C.)  but  that  most  of  them  are  within  +  or  -  30°  F. 
of  the  mean.  It  will  also  be  noted  that  the  temperature  of  the  steel  stream 
into  the  second  ingot  mold  is  a  little  higher,  on  an  average,  than  that  into 
the  first  ingot  mold.  The  reason  for  this  is  obvious  as  the  metal  lying 


P.    E.    BASH 


581 


next  to  the  bottom  of  the  ladle  is  colder  than  the  main  mass  of  the  molten 
steel.  Similar  variations  will  be  noted  in  taking  measurements  on  the 
tapping  stream  from  a  furnace,  as  the  metal  in  different  parts  of  the 
hearth  often  varies  25°  to  30°  F.  in  temperature. 

In  an  effort  to  find  the  relation  between  the  tapping  temperature  of 
the  steel  and  the  time  of  the  chill  test,  the  respective  values  were  plotted 
on  a  curve  sheet  with  the  result  shown  in  Fig.  2.  The  curve  is  anything 
but  a  smooth  one  and  shows  how  unreliable  the  chill-test  method  of 
judging  temperatures  is.  In  spite  of  all  care  in  attempting  to  draw  out 
the  spoon  in  the  same  manner  each  time,  the  atmospheric  conditions, 
room  temperature,  and  other  variables  have  an  influence  on  the  time 
required  for  the  crust  to  form. 


fo  2SOO 


H2800 


25  29  33  37  41 

Chill  Test  Time  in  Seconds 

FIG.  2. 

In  Table  3  are  given  some  temperatures  taken  on  manganese  and 
manganese  helmet  steel.  The  observations  in  this  case  were  difficult  to 
make  for  the  reason  that  heavy  clouds  of  smoke  were  given  off  from  the 
manganese  so  that  the  readings  had  to  be  made  from  the  windward  side 
and  a  moment  chosen  for  making  the  reading  when  the  stream  was 
unobscured.  The  manganese  was  melted  in  a  15-ton  Heroult  furnace  and 
tapped  into  a  ladle  which  was  then  transferred  to  the  open-hearth  plant 
where  the  manganese  was  poured  into  the  steel  ladle  at  the  time  that  the 
steel  was  tapped.. 

TABLE  3 


Time,  P.  M. 

Temperature,  Degrees  F. 

Remarks 

6:04 

2790 

Tap  manganese  from  15-ton  Heroult  furnace. 

6:30 

2822 

Tap  open-hearth  furnace. 

6:32 

2549 

Pour  manganese  in  steel  ladle. 

6:34 

2840 

Tap  open-hearth  furnace. 

6:44 

2605 

First  ingot  manganese  steel. 

6:45 

2605 

Second  ingot  manganese  steel. 

6:47 

2647 

Third  ingot  manganese  steel. 

582         ELECTRIC,    OPEN-HEARTH,    AND   BESSEMER    STEEL   TEMPERATURES 


An  opportunity  was  presented  to  take  temperatures  on  steel  made  by 
the  triplex  process,  so  at  the  same  time  blast-furnace  tapping  tempera- 
tures were  taken  and  a  record  made  from  the  blast  furnace  to  the  steel 
ingot.  The  practice  was  to  take  the  molten  pig  iron  to  a  mixer  from  which 
it  went  to  a  Bessemer  converter,  thence  to  an  open-hearth  furnace,  and 
finally  to  an  electric  furnace  for  finishing.  The  mean  of  readings  taken 
on  each  operation  are  set  down  in  Table  4.  *  The  value  for  the  Bessemer 
tap  is  the  mean  for  ten  heats  and  that  for  the  electric  furnace  is  for  eleven 
heats. 

TABLE  4. — Mean  Temperatures  from  Blast  Furnace  to  Finished 

Steel 


Temperature,  Degrees  F. 

Remarks 

1 

2625 

Mean  of  metal  streams  into  ladles  from  blast 

furnace. 

2 

2485 

Mixer  metal  charged  into  Bessemer. 

3 

2909 

Bessemer  tap. 

4 

2797 

Charge  Bessemer  steel  to  open-hearth  furnace. 

5 

2902 

Tap  open-hearth  furnace. 

6 

2872 

Charge  electric  furnace. 

7 

2867 

Mean  electric-furnace  tap. 

8 

2797 

First  ingot  pour  above  box. 

9 

2744 

First  ingot  pour  under  box. 

10 

2732 

Second  ingot  pour  under  box. 

11 

2736 

Third  ingot  pour  under  box. 

Temperature  Differences, 

Degrees  F. 

1 

100 

Temperature  drop  from  Bessemer  tap  to  open- 

• 

hearth  charge. 

2 

25 

Temperature   drop   from  open-hearth  tap  to 

electric-furnace  charge. 

3 

70 

Temperature  drop  from  tap  of  electric  furnace 

to  stream  from  ladle  for  first  ingot  for  average 

of  6J£  min.  in  ladle. 

4 

55 

Temperature  drop  through  box. 

In  Fig.  3,  the  data  in  this  table  are  presented  graphically.  The  great 
increase  in  temperature  in  the  Bessemer  converter  and  other  temperature 
relations  are  distinctly  brought  out. 

In  Table  5  are  given  data  on  tapping  and  teeming  for  three  open- 
hearth  furnaces  in  different  plants  making  nickel  ordnance  steel  and  one 
making  shell  steel.  Six  26-in.  octagon  ingots  were  poured  with  large  end 
up,  each  ingot  was  individually  bottom  poured  from  the  40-ton  basic 
open-hearth  furnace.  One  63-in.,  85-ton  octagon  ingot  was  poured  and 
a  number  of  23-in.  octagons  were  poured  from  two  65-ton  open-hearth 
furnaces  that  were  tapped  simultaneously  and  poured  consecutively. 


P.    E.    BASH 


583 


Beside  the  temperature  measurements  on  nickel  ordnance  steel, 
a  number  of  readings  were  made  on  different  types  of  steel  in  two  10-ton 
Ludlum  electric  furnaces.  These  were  taken  through  the  courtesy  of 
Mr.  P.  A.  E.  Armstrong,  vice-president  of  the  Ludlum  Steel  Co.  The 
practice  was  to  refine  with  two  or  three  slags  so  that  it  was  possible  to 
take  readings  on  the  oxidized  surface  of  the  molten  steel  during  the  skim- 
ming and  also  to  get  readings  on  thin  slag  patches  floating  on  the  metal. 
It  was  found  that  readings  on  the  slag  patches  corrected  for  an  emis- 
sivity  of  0.65  almost  exactly  agreed  with  uncorrected  readings  on  the  iron 
oxide  adjacent.  In  other  words,  if  the  radiation  from  the  oxide  in' the 


3000 


230C 


Fia.  3. — TEMPERATURES  IN  TRIPLEX  PROCESS. 


furnace  with  the  arc  off  and  the  door  open  is  that  of  a  black  body,  then 
the  emissivity  of  slag  is  approximately  0.65,  as  stated  by  Burgess.  In 
Table  6  are  given  a  number  of  readings  on  different  skims  made  in  this 
manner.  It  will  be  noted  that  the  mean  values  for  the  two  columns 
agree  exactly  with  each  other.  If  the  emissivity  of  slag  is  0.65,  then  the 
iron  oxide  under  the  above  conditions  must  give  black-body  radiation  for 
red  light. 

In  Table  7  is  given  a  summary  of  data  taken  on  tapping  and  teeming 
a  number  of  heats.  The  steel  was  tapped  or  poured  from  the  furnace 
into  two  ladles  and  then  very  quickly  teemed  into  ingot  molds. 

In  order  to  make  sure  of  the  steel  temperature  in  the  furnace,  a  Dixon 
graphite  tube  4  ft.  (1.2  m.)  long  by  4  in.  (10  cm.)  outside  diameter,  with 
a  closed  end,  was  pushed  into  the  steel  and  held  there  until  the  end  had 
come  to  temperature,  at  which  time  a  reading  was  made  with  the  optical 
pyrometer  sighted  down  the  axis  on  the  inside  of  the  closed  end,  which 


584         ELECTRIC,    OPEN-HEARTH,    AND   BESSEMER   STEEL   TEMPERATURES 

TABLE  5. — Open-hearth  Steel  Temperatures  on  Nickel  Ordnance  Steel 


Time 

Temperature,  Degrees  F.                                                   Remarks 

50-ton  Acid  Open-hearth  Furnace 

11:55 

2842 

Tap  steel. 

Held  10  min.  in  ladle. 

Four  ingots  bottom  poured  in  sets  of  two. 

12:12 

2768 

Pour  first  two  ingots. 

12:19 

2768 

Pour  second  two  ingots. 

40-ton  Basic  Open-hearth  Furnace 

4:10 

2877 

Tap  steel. 

4:21  • 

2713 

First  ingot. 

4:22 

2722 

Tong  hold. 

2753 

Second  ingot. 

2722 

Second  ingot. 

2722 

Third  ingot. 

2705 

Third  ingot. 

2713 

Fourth  ingot. 

65-ton  Acid  Open-hearth  Furnace 


8:33 

2895 

Tap  one  furnace.     Other  tap  not  recorded. 

8:43 

2835 

Stream  from  ladle  to  runner. 

8:44 

2843 

Stream  from  ladle  to  runner. 

8:47 

2768 

Stream  into  headbox. 

8:47K 

2737 

Stream  into  headbox. 

8:48 

2688 

Stream  from  headbox. 

8:51 

2761 

Stream  into  headbox. 

8:56 

2761 

Stream  into  headbox. 

Second  Ladle 


9:00 

2753 

Stream  into  headbox. 

9:01 

2761 

Stream  into  headbox. 

9:04 

2761 

Stream  into  headbox. 

9:06 

2745 

Stream  into  headbox. 

9:09 

2737 

Stream  into  headbox. 

9:12 

2768 

Stream  into  headbox. 

9:20 

2745 

Stream  for  four  23-in.  octagons. 

9:21 

2620 

Steel   rising    in   mold    one-third    full.     Light 

smoke. 

9:22 

2782 

Stream  to  group  of  23-in.  octagons. 

9:25 

2813 

Stream  to  group  of  23-in.  octagons. 

9:29 

2828 

Stream  to  group  of  23-in.  octagons. 

9:30 

2745 

Stream  to  group,  dark  streak,  good. 

P.    E.   BASH 


585 


Table  5. — Open-hearth  Steel  Temperatures  on  Nickel  Ordnance  Steel 

(Continued) 


Time 


Temperature,  Degrees  F. 


Remarks 


65-ton  Acid  Open-hearth  Furnace, 
Nickel-chromium  Steel 


1:37 

2879 

Tap  steel. 

2895 

Tap  steel. 

2910 

Tap  steel. 

Teem 


1:44 

2835 

First  group  (8  tons  to 

a  group,  four  ingots). 

1:45 

2828 

First  group. 

1:49 

2835 

Second  group. 

1:55 

2790 

Fifth  group. 

2:01 

2775 

Sixth  group. 

2:03 

2753 

Seventh  group. 

NOTE.  —  Acid  furnace  on  shells,  3-in.  nozzle;  carbon,  0.60; 

chromium,  2.25;  nickel 

3.5.. 

TABLE  6. — Summary  of  Data  on  Slag  Skimming 


Optical  Reading  on  Slag  Patch 
Corrected,*  Degrees  F. 


Optical  Reading  on  Iron 
Oxide,  Degrees  F. 


Remarks 


2611 

2618 

H.  S. 

2621 

2629 

XIC. 

2611 

2618 

C.  S. 

2718 
2628 

2709 
2618 

C.  S.  —  refractory  slag. 
C.  S. 

2532 

2562 

C.  S.—  cold. 

2728 

2694 

C.  S.  —  same  heated. 

Mean     2636 

2636 

0  Burgess"  value  of  0.65  for  emissivity  of  slag  used. 

was  in  the  steel.  Under  these  conditions,  the  reading  in  the  tube  should 
give  the  true  temperature  of  the  steel.  One  such  reading  was  taken 
just  before  a  tap  and  is  recorded  in  the  table.  It  is  only  17°  F.  below  the 
temperature  read  on  the  steel  stream,  which  is  a  very  good  agreement. 
Readings  made  on  the  slag  surface  in  the  furnace  before  the  tap  do  not 
agree  so  well  with  readings  on  the  stream,  as  is  also  shown. 

Further  readings  in  a  graphite  tube  were  made  at  another  time. 
The  arc  had  been  off  for  about  10  min.  and  the  door  was  opened  just 


586         ELECTRIC,    OPEN-HEARTH,    AND   BESSEMER    STEEL   TEMPERATURES 


TABLE  7. — Summary  o/Ludlum  Temperature  Data 


Temperature     Ste?  Tapping 
Degrees  F.          DegreesV. 

Reading  on 
Slag  Tapping 
Corrected, 
Degrees  F. 

First          Last 
Ingot,        Ingot, 
Degrees     Degrees 
F.       ;       F. 

Remarks 

Arc  off, 

2815 

2797 

2792 

2746 

2640 

First  ladle—  C.S. 

slag 

2770 

2746 

2625 

Second  ladle—  C.S. 

2737 

2700       2625 

First  ladle—  XIC. 

2686               2652 

2670 

2597 

Second  ladle—  XIC. 

Arc  off, 

slag 

2789               2730               2711 

2653       2580 

First  ladle—  C.S. 

2686               2675 

2617       2550 

Second  ladle  —  C.S. 

Tube 

2673               2690 

2650 

First  ladle—  C.S. 

2633               2646 

Second  ladle  —  C.S. 

2716 

2677       2597 

First  ladle—  XIC. 

2677 

2637       2620 

Second  ladle—  XIC. 

2790 

2723 

2662 

First    ladle—  H.S. 

2759               2751 

2651       2661 

Second  ladle  —  H.S. 

Arc  (slag)        2946               2745 

2732       2699 

First  ladle—  XIC. 

on 

2717 

2692 

2611 

Second  ladle—  XIC. 

DEGREES  F. 

1.  Mean  drop  from  tapping  temperature  to  first  ingot  (max.  108°,  min.  13°) . .  36 

2.  Mean  drop  from  first  to  last  ingot,  for  5-ton  ladle 62 

3.  Mean  temperature  for  first  ladle  tap 2743 

4.  Mean  temperature  for  second  ladle  tap 2704 

5.  Mean  difference  between  first  and  second  ladles .  .          39 


2560 


0123          45678          9       10 
Depth  of  Immersion  -  Inches 

FIG.   4. 

sufficiently  to  allow  the  tube  to  be  inserted.  A  reading  was  made  on  the 
thin  slag  surface  beside  the  tube  and  then  in  the  tube  at  two  different 
depths  which  were  calculated  from  measurements  of  depth  of  immer- 
sion of  the  tube.  The  readings  are  given  in  Table  8  and  are  plotted  in 
Fig.  4;  they  show  the  temperature  gradient  of  the  steel  bath  from  the 
surface  to  a  9-in.  (23  cm.)  depth.  The  top  was  cooler  due  to  the  fact 
that  the  arc  had  been  off  for  a  few  minutes.  This  also  seems  to  show 


F.    E.    BASH 


587 


that  a  reading  made  on  a  thin  slag  surface  in  a  furnace  that  is  enclosed, 
with  the  arc  off,  gives  true  temperatures. 

TABLE  8 


Temperature  Read,  Degrees  F. 

Remarks 

2580 

On  slag  surface,  arc  off  10  min.« 

2606 

In  closed-end  tube,  immersed  4  in. 

2650 

In  closed-end  tube,  immersed  4  in. 

2629 

Slag  surface,  arc  off.6 

2618 

Tube  in  slag. 

0  Slag  was  approximately 

^  in.  thick. 

b  Reading  made  at  a  later  date  than  one  above. 

TABLE 

9.  —  Carbon  Steel  for  Castings 

Time  from  Start  of  Tap 

Temperature, 

i 

Degrees  F. 

Remarks 

Minutes 

Seconds 

17 

2960 

On  slag  tapping. 

28 

2920 

On  slag  tapping. 

46 

2982 

On  slag  tapping. 

64 

2952 

On  slag  finish  tapping. 

2 

30 

.... 

Started  to  skim. 

3 

45 

Finished  skimming. 

4 

45 

Finished  weighing. 

5 

22 

2887 

First  shank. 

7 

30 

2865 

Second  shank. 

8 

20 

2821 

Pouring  second  shank  into  small  mold. 

9 

13 

2835 

Third  shank. 

9 

53 

2797 

Pouring  third  shank  into  mold. 

10 

9 

2797 

Pouring  third  shank  into  mold. 

10 

46 

2828 

Fourth  shank. 

11 

5 

2782 

Pouring  fourth  shank  into  mold. 

11 

43 

2775 

Pouring  fourth  shank  into  mold. 

12 

10 

2835 

Fifth  shank. 

12 

35 

2761 

Pouring  fifth  shank. 

13 

59 

2775 

Pouring  from  ladle  into  first  mold. 

14 

41 

2730 

Pouring  from  ladle  into  second  mold. 

15 

15 

2688 

Third  mold,  small  stream. 

16 

3 

2782 

Fourth  mold,  large  stream. 

16 

21 

2768 

Fourth  mold. 

16 

41 

2761 

Fourth  mold. 

16 

52 

2753 

Fourth  mold. 

17 

12 

2761 

Fourth  mold,  oxidized  stream. 

18 

19 

2753 

Fifth  mold. 

20 

3 

2797 

Sixth  mold,  oxidized  stream. 

21 

2 

2745 

Seventh  mold. 

At  this  point  the  writer  wishes  to  state  that  the  steel  poured  from 
these  electric  furnaces  was  the  coldest  of  any  he  has  had  occasion  to  take 


588         ELECTRIC,    OPEN-HEARTH,    AND   BESSEMER    STEEL   TEMPERATURES 


temperatures  on,  either  open-hearth  or  electric,  and  further  contradicts 
any  supposition  that  electric  steel  is  hotter  than  open-hearth,  at  least 
when  it  is  tapped,  although  it  may  be  hotter  during  the  refining  period. 

In  Tables  9  and  10,  temperatures  are  given  for  tapping  and  teeming 
a  3-ton  basic  Heroult  furnace.  The  steel  was  used  for  castings  and  for 
that  reason  had  to  be  hotter  than  steel  for  large  ingots.  The  tempera- 
tures given  in  Table  10  for  tapping  manganese  steel  are  the  hottest  of 
any  electric  steel  the  writer  has  had  occasion  to  take  measurements  on. 
However,  it  is  no  hotter  than  open-hearth  steel  for  castings  or  Bessemer 
steel  for  the  same  purpose,  as  may  be  seen  by  examination  of  Table  11. 

In  Tables  9  and  10  it  will  be  noted  that  there  is  some  fluctuation 
in  the  teeming  temperatures.  This  is  due  to  the  fact  that  the  steel  was 
poured  over  the  tip  of  the  ladle  and  was  sometimes  large  and  sometimes 
small.  The  small  stream  cooled  much  more  rapidly  and  as  a  consequence 
a  lower  temperature  was  read. 

TABLE  10. — Manganese  Steel  for  Castings 


Time  from  Start  of  Tap 

Temperature, 
Degrees  F. 

Remarks 

Minutes 

Seconds 

15 

3053 

Steel  stream. 

34 

3025 

Steel  stream. 

52 

3025 

Steel  stream. 

1 

14 

3037 

Steel  stream. 

1 

38 

3025 

Steel  stream. 

TABLE  11. — Temperature  Observations  on  3-ton  Acid  Bessemer  Converter 


Time 

Temperature, 
Degrees  F. 

Remarks 

2:07 

2433 

Charging  Bessemer  on  stream. 

Time  after  Start  of  Blow 

Minutes 

Seconds 

2 

50 

2574 

Flame  from  Bessemer. 

3 

10 

2632 

Flame  from  Bessemer. 

3 

37 

2722 

Flame  from  Bessemer. 

4 

10 

2824 

Flame  from  Bessemer. 

•      5 

12 

2860 

Flame  from  Bessemer. 

6 

43 

2747 

Flame  from  Bessemer. 

7 

18 

2752 

Flame  from  Bessemer. 

8 

0 

2527 

Ferromanganese  into  ladle. 

9 

32 

3060 

Tap  Bessemer. 

11 

53 

Finish  tap. 

DISCUSSION  589 

As  a  matter  of  curiosity,  a  set  of  readings  with  the  optical  pyrometer 
on  the  flame  from  a  3-ton  acid  Bessemer  converter  was  taken.  The 
hottest  portion  of  the  flame  was  selected  for  the  balance  for  each  reading 
and  the  time  noted  with  a  stop  watch.  The  temperatures  found  have 
no  particular  value,  as  the  temperature  of  a  flame  as  determined  with  an 
optical  pyrometer  depends  on  its  thickness.  It  is  interesting  to  note 
however,  how  the  temperature  values  increase  to  a  maximum  and  then 
decrease.  The  readings  are  given  in  Table  11. 

In  conclusion  the  writer  hopes  that  the  data  presented  in  this  paper 
will  be  of  some  value  in  starting  further  investigations  along  the  lines  of 
control  of  molten  steel  temperatures  and  the  effects  of  pouring  tempera- 
tures on  quality  of  steel. 

The  writer  also  wishes  to  thank  Mr.  G.  H.  English,  formerly  Lieu- 
tenant of  the  Ordnance  Reserve  Corps,  for  the  opportunity  to  take  tem- 
peratures on  ordnance  steel;  Mr.  A.  H.  Miller,  of  the  Midvale  Steel  Co., 
for  access  to  steel  furnaces  on  many  occasions;  and  Mr.  Knox  Taylor, 
president  of  the  Taylor  Wharton  Iron  &  Steel  Co.,  for  the  privilege  of 
making  tests  at  his  plant. 

DISCUSSION 

E.  S.  TAYLERSON,*  Pittsburgh,  Pa. — What  has  just  been  said  brings 
out  one  of  the  disadvantages  of  the  optical  pyrometer.  It  requires  two 
operators  to  get  satisfactory  readings.  The  method  may  be  simplified 
by  using  a  sensitive  recording  ammeter.  The  operator  can  observe  the 
temperature  and  as  soon  as  the  color  match  is  perfect  a  key  on  the 
telescope  can  be  made  to  record  the  temperature  and  the  time  as  well. 
This  would  be  well  worth  investigating,  especially  in  the  case  of  open- 
hearth  practice  where  the  time  element  must  considerably  affect  the 
results  obtained. 

*  Research  Laboratory,  Amer.  Sheet  &  Tin  Plate  Co. 


590 


Some  Thermal  Relations  in  the  Treatment  of   Steel 

BY    CHARLES    F.    BRUSH,    PH.    D.,    SC.    D.,    LL.    D.,  -CLEVELAND,    OHIO 
(Chicago  Meeting,  September,  1919) 

THE  general  subject  of  accurate  pyrometry,  its  great  development  in 
recent  years,  and  the  importance  of  its  application  in  arts  and  manu- 
factures is  so  ably  treated  in  other  papers  that  this  paper  will  be  confined 
to  a  resume  of  some  research  work  on  certain  temperature  effects  in 
carbon  and  other  steels,  some  of  which  appear  to  be  new.  The  results 
of  these  researches  are  embodied  in  several  papers  presented  to  various 
scientific  societies  during  the  last  few  years,  most  of  them  under  the 
general  title  of  Spontaneous  Generation  of  Heat  in  Recently  Hardened 
Steel.1 

Several  years  ago,  when  studying  the  behavior,  under  certain  condi- 
tions, of  specimens  of  hardened  tool  steel,  I  observed  that  they  all  spon- 
taneously generated  a  small  quantity  of  heat,  the  rate  of  generation 
diminishing  from  day  to  day  for  several  weeks  before  generation  became 
imperceptible  in  the  sensitive  calorimeter  used.  In  each  case  the  steel  had 
been  hardened  only  a  few  days  prior  to  its  use.  It  seemed  highly  prob- 
able that  the  generation  of  heat  was  associated  with  some  sort  of  "season- 
ing" or  incipient  annealing  process,  perhaps  accompanied  by  slight 
changes  of  volume,  and  that  it  would  be  most  rapid  immediately  after 
hardening.  I  subsequently  investigated  this  curious  phenomenon  more 
fully. 

Twelve  3^-in.  round  bars  of  tool  steel,  5  in.  long  and  with  machined 
surfaces,  were  hardened  by  heating  to  high  cherry  red  in  a  reducing 
atmosphere  of  a  gas  furnace  and  quenched  in  cold  water.  The  bars 
then  had  a  thin  strongly  adhering  coating  of  black  oxide.  They  were 
next  stirred  in  a  large  quantity  of  water  at  room  temperature,  to  acquire 
that  temperature,  wiped  dry,  and  oiled  with  heavy,  neutral  mineral  oil  to 
prevent  generation  of  heat  by  further  surface  oxidation,  wiped  free  of 
excess  of  oil  and  placed  in  a  Dewar  jar  in  the  calorimeter.  A  quantity 


*  Proc.  Am.  Phil.  Soc.  (May-July,  1915)  64. 
Phys.  Rev.  [2]  (1917)  9.  . 
Proc.  Am.  Phil.  Soc.  (1917)  56. 

Proc.  Royal  Soc.  Lond.  (1917)  A93;  joint  paper  with  Sir  Robert  A.  Hadfield. 
Proc.  Royal  Soc.  Lond.  (1918)  A95;  joint  paper  with  Sir  Robert  A.  Hadfield  and 
S.  A.  Main. 

Proc.  Am.  Phil.  Soc.  (1918)  57. 


CHARLES    P.    BRUSH 


591 


of  water,  also  at  room  temperature,  just  sufficient  to  equal  the  steel  bars 
in  thermal  capacity  had  already  been  placed  in  another  Dewar  jar  in 
the  calorimeter.  The  testing  device  was  assembled  as  quickly  as  pos- 
sible, and  galvanometer  readings  commenced  within  45  min.  of  the 
time  of  hardening  the  steel.  The  upper  curve  in  Fig.  1  shows  the 
progress  of  heat  generation  in  the  steel  bar,s  during  the  first  150  hr.  after 
hardening.  A  very  slow  generation  of  heat  was  still  easily  observable 
at  the  end  of  a  month. 

The  temperature  of  the  steel  bars  was  rising  rapidly  when  the  galva- 
nometer readings  commenced  and  reached  a  point  (nearly  3°  C.  at  the 
summit  of  the  curve)  where  the  gain  and  loss  of  heat  balanced  each  other 
in  about  8  hr.  The  normal  cooling  curve  was  obtained  5  or  6  wk.  after 
the  other,  and  when  the  generation  of  heat  had  very  nearly  ceased.  For 


Analysis  of  Steel 
Phosphorus  0.012 
Sulphur        0.016 
Silicon          o.2l 
Manganese  0.31 
Carbon 


40       50      DO      70      OO  IOO 

Hours  After  Hardening 
FIG.  1. — CURVES  FOR  TOOL  STEEL. 


120 


140 


this  purpose  the  steel  bars  were  removed  from  the  copper  cylinder,  warmed 
a  few  degrees,  and  replaced;  then  galvanometer  readings  were  made  from 
time  to  time  as  before.  This  curve  is  plotted  in  a  location  convenient 
for  visual  comparison  with  the  heating  curve,  but  might  just  as  well  be 
plotted  to  the  right  of  it. 

From  the  two  observed  curves,  I  have  computed  a  third  curve  (not 
shown)  which  represents  the  progressive  rise  in  temperature  that  would 
have  occurred  if  the  thermal  insulation  of  the  steel  had  been  perfect,  so  as 
to  prevent  any  loss  of  heat.  The  curve  is  strikingly  similar  in  character 
to  the  shrinkage  curve  shown  in  Fig.  3,  and  indicates  a  close  association 
of  heat  generation  and  shrinking,  to  which  I  shall  refer  again.  The  total 
rise  in  temperature  indicated  (about  5°  C.)  is  of  little  quantitative  impor- 
tance because  it  is  highly  probable  that  it  would  have  been  different  if 


592 


SOME   THERMAL   RELATIONS   IN   THE   TREATMENT  OF  STEEL 


the  steel  had  been  hardened  at  a  different  temperature,  or  more  uniformly 
hardened  throughout  each  bar,  or  had  a  different  carbon  content.  Yet 
it  is  interesting  to  note  that  the  observed  quantity  of  heat  spontaneously 
generated  in  the  steel,  measured  by  its  rise  in  temperature  multiplied  by 
its  thermal  capacity,  indicates  internal  work  of  some  sort  sufficient  to 
lift  the  steel  bodily  about  800  ft.  against  the  force  of  gravity. 

The  same  number  of  bars  of  high-speed  tungsten  steel  of  the  same 
dimensions  were  water-hardened  at  white  heat,  not  far  below  the  fusing 
point,  brought  to  room  temperature,  oiled,  and  placed  in  the  copper 
cylinders,  as  in  the  former  case,  and  galvanometer  readings  were  com- 
menced 1  hr.  after  hardening.  Fig.  2  shows  the  curve  of  heat  generation 
in  the  high-speed  steel,  and  the  curve  of  normal  cooling  located  with 
respect  thereto  as  in  Fig.  1.  The  cooling  curve  here  shown  is  the  lower 


140 

120 
IOO 
& 
60 
40 
20 

Analysis  of  Steel 
Phosphorus  0.043 
Sulphur        0.0  1  6 
Silicon          0.16 
Manganese  0.24 
Carbon         0.59 
Chromium    5.45 
Tungsten    16.77 

1 

y 

"S 

X. 

r 

\ 

X 

\ 

"X 

X 

"^ 

-^ 

--—  -  ^ 

-*~^ 

•—•  — 

•»      1  . 

*- 

—  •—  _ 

~     • 

-.  —  * 

10    20    30     40 


50       DO       70 00  IOO 

Hours  After  Hardening 
FIG.  2. — CURVES  FOR  HIGH-SPEED  STEEL. 


120 


140 


part  of  that  used  in  Fig.  1.  It  is  permissible  to  use  the  same  cooling  curve 
for  both  kinds  of  steel  because  the  thermal  capacity  of  the  two  lots  was 
very  nearly  the  same.  It  is  seen  that  heat  generation  in  the  tungsten 
steel  is  the  same  in  character  as  in  the  carbon  steel  of  Fig.  2,  though  much 
less  in  amount  and  somewhat  more  persistent. 

> 
SHRINKAGE  OF  CARBON  STEEL*  WHEN  ANNEALED 

Many  workers  in  steel  are  aware  that  the  metal  expands  a  little  when 
hardened  and  shrinks  when  annealed.  Having  no  more  of  the  carbon 
steel  used  in  the  first  experiment,  I  procured  another  ^-in.  round  bar 
of  the  same  brand,  though  slightly  different  in  composition,  as  the  analyses 
show.  With  a  piece  of  this  bar  23^  in.  long  I  made  a  careful  determina- 
tion of  its  specific  gravity  under  the  conditions  and  with  the  results 
here  shown. 


CHARLES  F.  BRUSH 


593 


SPECIFIC  GRAVITY 

Commercial  condition 7 . 8507 

After  hardening 7.8127 

After  tempering  to  light  blue .  7 . 8350 

After  annealing 7.8529 


ANALYSIS  OF  STEEL,  PER  CENT. 

Phosphorus 0. 015 

Sulfur 0.021 

Silicon 0.16 

Manganese 0 . 33 

Carbon..  1.07 


The  difference  in  density  and  volume  between  the  hardened  and 
annealed  conditions  is  fully  %  per  cent.,  which  is  more  than  I  expected 
to  find;  and  nearly  one-half  of  the  total  shrinkage  was  brought  about  by 
the  very  moderate  heating  necessary  for  tempering  to  light  blue.  The 
annealing  was  very  thorough,  and,  as  the  figures  show,  was  more  complete 
than  in  the  annealed  commercial  condition.  The  shrinkage  incident  to 
tempering  was  large  enough  to  encourage  the  hope  that  if  any  spontaneous 
shrinking,  at  room  temperature,  occurs  during  the  generation  of  heat  that 
follows  hardening,  it  might  be  detected  and  measured.  Apparatus  was 


1 

j  700 

?*" 

Jo  500 

1  400 
co 

^     200 
f 

§    100 

1 

-    •  — 

.«-'  '        ' 

-n   '"- 

—  —  ••  — 

-  1    •  "•- 



An 

_  —     " 
lalysis  of  Si 
isphorus  0 
5hur        o 
:on          o 
nganese   o 
bon         i 

•eel 

^ 

^~-~ 

_  —  •- 

_    i  i-51 

Phc 

on 

f 

Sul 

014 
•23  - 
•39 

1 

Ma 

dr 

.41 

IO      2O      30      40      50      60      70      80                100               120              140 

Hours  After  Hardening 
FIG.  3. — SHRINKAGE  OF  HARDENED  STEEL  BAR. 

designed  with  which  rods  3  ft.  long  and  1  mm.  in  diameter  were  tested. 
After  some  preliminary  experiments,  to  get  acquainted  with  the  appara- 
tus, a  fresh  rod  was  hardened  by  placing  it  horizontally  in  a  wooden  rack 
just  above  a  trough  of  water  at  room  temperature,  quickly  heating  it  to 
bright  redness  by  passing  suitable  electric  current  through  it,  and  plung- 
ing it  in  the  water  beneath,  the  act  of  lowering  the  rod  serving  to  break 
the  electric  circuit.  The  rod  was  kept  straight  while  hot  by  means  of  a 
weak  spiral  spring,  which  took  up  the  expansion.  Preliminary  experi- 
ments had  shown  that  a  rod  could  be  hardened  in  this  way  without  warp- 
ing. The  hardened  rod,  already  at  room  temperature,  was  quickly 
wiped  dry  and  put  in  place  beside  the  comparison  rod.  A  fine  horizontal 
scratch  was  immediately  drawn  across  the  polished  fronts  of  the  bars 
by  means  of  a  straightedge  and  sharp  needle  point  lightly  applied.  A 
microscope,  magnifying  about  200  diameters  and  very  solidly  mounted. 

38 


594         SOME   THERMAL   RELATIONS   IN   THE   TREATMENT   OF   STEEL 

was  brought  into  position  and  focused  on  the  horizontal  scratch,  which 
of  course  consisted  of  an  independent  scratch  on  each  bar,  the  two  halves 
being  initially  in  perfect  register.  Shrinkage  of  the  hardened  rod  was 
detected  within  2  min.  after  scratching  the  brass  bars,  and  was  easily 
observable  at  the  end  of  2  weeks.  Fig.  3  shows  the  progress  of  shrinking 
during  the  first  150  hr.  The  curve  reached  the  500  line  a  day  or  two  later. 
The  hardened  length  of  the  rod  was  assumed  to  be  35  in.,  so  that  its 
actual  shrinkage  at  the  500  line  of  the  curve  was  0.0175  in. 

The  rod  was  next  scoured  clean  and  tempered  to  light  straw  color  by 
electric  warming,  then  to  light  blue  color,  and  its  total  shrinkage  measured 
after  each  operation.  Finally,  it  was  thoroughly  annealed  by  bedding 
in  mineral  wool,  heating  to  low  redness  %  hr.,  and  gradually  reducing 
the  heating  current  to  nothing  in  the  course  of  2  or  3  hr.,  after  which  the 
shrinkage  was  again  measured.  The  rod  shrank  considerably  in  each 
operation,  as  indicated  in  Table  1,  in  which  the  annealed  length  is  taken 
as  unity  or  100  per  cent. 

Linear  Shrinkage  x  100 

••••••••••••••••••••••••••I  Length  of  Rod  After  Hardening 

•••••••••••^^•^•IMHH^HHHB     After  Spontaneous  Shrinking 

••••^^^^^^••^••••••^••1  After  Tempering  to  Light  Straw 

••••^•^•^•••••••••••••••i  After  Tempering  to  Light  Blue 

^•^•^•••••^•••••v  After  Annealing 

FIG.  4. — SHRINKAGE  OF  STEEL  BAR  DUE  TO  HEAT  TREATMENT. 

TABLE  1 

PER  CENT. 

Length  of  rod  after  hardening ^ . '. 100.383 

After  spontaneous  shrinking 100.332 

After  tempering  to  light  straw 100. 182 

After  tempering  to  light  blue 100. 131 

After  annealing 100. 000 

Of  course  the  shrinkage  in  volume  must  have  been  very  nearly  three 
times  the  linear  shrinkage,  or  considerably  more  than  1  per  cent,  from 
the  hardened  to  the  annealed  condition,  which  is  more  than  double  that 
observed  in  the  bar  steel  used  in  the  first  experiment.  Doubtless  this 
was  due  to  the  higher  carbon  content  of  the  small  rod  and  more  uniform 
hardening,  owing  to  its  small  size.  It  is  highly  probable  also  that  more 
heat  was  generated  per  unit  of  mass.  In  Fig.  4  the  stages  of  shrinking 
of  the  small  rod  are  shown  by  magnifying  a  hundred-fold  the  observed 
quantities  in  Table  1 . 

I  have  already  pointed  out  the  close  similarity  in  character  of  the 
spontaneous-shrinkage  curve,  Fig.  3,  and  the  computed  curve  of  total 
heat  generation;  there  seems  little  room  for  doubt  that  the  two  phe- 
nomena are  quantitatively  related.  It  is  equally  clear  that  spontaneous 
shrinking  is  only  incident  to,  and  is  not  the  prime  cause  of  the  generation 


CHARLES    F.    BRUSH  595 

of  heat,  because  the  internal  work  represented  by  the  heat  generated  is 
very  many  times  more  than  sufficient  to  effect,  by  compression,  the  re- 
duction in  volume  that  probably  occurred  while  the  ^-in.  bars  were 
generating  heat.  Sir  Robert  A.  Hadfield,  in  the  second  joint  paper 
referred  to,  treats  the  data  mathematically  and  reaches  the  same  con- 
clusion. More  recent  experiments,  some  of  which  show  no  shrinking 
during  the  generation  of  heat,  further  confirm  it. 

An  attempt  was  made  to  measure  Young's  modulus  of  elasticity  in  the 
small  rod  both  in  the  hardened  condition  (after  spontaneous  shrinking) 
and  after  annealing,  by  hanging  various  weights  to  them  and  measuring, 
with  the  microscope,  the  distortions  produced,  always  far  within  the 
elastic  limit.  But  I  was  unable  to  obtain  reliable  results  because  of  an 
interesting  fact  that  was  brought  to  light :  In  the  annealed  condition,  the 
steel  exhibited  a  small  amount  of  viscosity  or  internal  friction  which 
somewhat  delayed  full  distortion  and  subsequent  restitution;  but  in  the 
hardened  condition  the  viscosity  was  many  times  greater.  This  is  a 
further  illustration  of  the  instability  of  the  hardened  steel. 

I  am  led  to  regard  the  hardened  steel  as  being  in  a  condition  of  very 
great  molecular .  strain  somewhat  unstable,  especially  at  first.  Spon- 
taneous relief  of  a  small  portion  of  the  strain  causes  generation  of  heat 
until  stability  at  room  temperature  is  reached.  Any  considerable  rise 
of  temperature,  as  in  tempering,  permits  further  spontaneous  relief  of 
strain,  or  molecular  rearrangement,  and  is  doubtless  accompanied  by 
more  generation  of  heat,  and  so  on  until  the  annealing  temperature  is 
reached.  It  is  obvious  that  the  process  of  tempering  or  annealing  steel  is 
exothermic  and,  conversely,  that  hardening  is  endothermic. 

NICKEL-CHROMIUM  STEEL 

Subsequently,  Sir  Robert  Hadfield  sent  me  two  specimens  of  nickel- 
chromium  steel  for  further  tests.  Each  specimen  consisted  of  twelve 
3^-in.  round  bars  5  in.  long,  with  machined  surfaces,  so  that  the  results 
obtained  would  be  quantitatively  comparable  with  those  of  the  other 
kinds  of  steel  previously  examined.  Specimen  A  was  gradually  heated 
in  a  gas  furnace,  with  reducing  atmosphere,  while  its  magnetic  behavior 
was  observed  from  time  to  time.  For  this  purpose  the  bundle  of  bars 
was  surrounded  by  a  single  turn  of  asbestos-insulated  platinum  wire, 
the  ends  of  which  were  connected  with  a  ballistic  galvanometer  having 
the  usual  mirror  and  scale.  The  furnace  was  surrounded  by  a  large  coil 
of  heavy  copper  wire  through  which  a  direct  electric  current  could  be 
established  and  broken  at  will  by  means  of  a  switch  and  storage  battery. 
Before  the  steel  bars  were  placed  within  the  platinum  loop  inside  the 
furnace,  closure  of  the  outer  copper  coil  circuit  caused  a  brief  electric 
pulse  in  the  loop  and  a  "kick"  in  the  galvanometer,  giving  a  definite 
minimum  deflection  easily  observed  with  considerable  precision.  With 


596       SOME    THERMAL    RELATIONS    IN   THE    TREATMENT    OF   STEEL 


the  steel  bars  inside  the  platinum  loop  the  galvanometer  deflection  was, 
of  course,  many  times  greater  until,  with  rising  temperature,  the  decales- 
cent  point  was  approached;  then  the  deflection  fell  rapidly  to  the  mini- 
mum value  as  above,  or  very  near  it.  This  simple  induction  apparatus 
was  found  entirely  reliable  and  satisfactory. 

Susceptibility  remained  substantially  constant  until  several  minutes 
after  cherry-red  heat  was  reached,  then  fell  rapidly  to  nothing.  This  was 
taken  to  indicate  complete  decalescence,  and  the  proper  temperature 
for  hardening.  The  furnace  was  immediately  opened  and  the  twelve 
bars  quenched  separately  in  water  and  crushed  ice,  then  stirred  in  much 
water  at  room  temperature,  wiped  dry,  oiled,  and  placed  in  the  calo- 
rimeter used  in  the  earlier  experiments. 


Specimen  A 


Analysis  of  Steel 
Phosphorus  0.018 
Sulphur       0.031 
Silicon         0.13 
Manganese  0.14 
Carbon         0.70 
Chromium    2.75 
Nickel         1.45 


120 


140 


10   20   30   40   50   60   70   80      IOO 

Hours  After  Hardening 
FIG.  5. — GENERATION  OF  HEAT  BY  NICKEL-CHROMIUM  STEEL  AFTER  HARDENING. 

Fig.  5  shows  the  progress  of  heat  generation  following  this  first  hard- 
ening of  specimen  A  of  the  Hadfield  nickel-chromium  steel.  Rapid 
generation  of  heat  is  indicated  in  the  early  stages,  especially  during  the 
first  hour,  and  maximum  temperature,  the  point  at  which  gain  of  heat 
was  balanced  by  normal  cooling,  was  reached  in  about  3^  hr.  Obviously 
the  rate  of  generation  had  diminished  very  much  by  that  time  and  was 
falling  fast,  though  it  still  retained  considerable  value  at  the  end  of  150 
hr.  In  general,  the  curve  of  first  hardening  indicates  that  the  rate  of 
heat  generation  diminished  steadily  from  the  moment  of  quenching  the 
hot  steel.  Hadfield's  analysis  of  this  steel  shows  chromium  2.02  per 
cent.,  nickel  1.98  per  cent. 

After  a  week  of  cooling,  the  bars  were  again  placed  in  the  furnace 
and  heated  to  a  temperature  considerably  higher  than  before  by  continu- 
ing the  heating  15  min.  after  complete  loss  of  magnetic  susceptibility. 


CHARLES   F.   BRUSH 


597 


They  were  then  quenched  and  treated  as  formerly.  The  other  curve  of 
Fig.  5  shows  the  effect  of  the  higher  quenching  temperature.  The  total 
amount  of  heat  generated  was  not  greatly  different  in  the  two  cases. 
Specimen  B  of  the  Hadfield  steel  was  treated  at  first  precisely  like 
specimen  A;  but  in  the  second  hardening  was  not  heated  quite  so  far 
above  the  temperature  of  decalescence.  Fig.  6  shows  the  progress  of 
heat  generation.  The  total  amount  of  heat  generated  was  much  greater 
after  the  second  hardening  than  after  the  first,  in  which  respect  this 
specimen  differs  greatly  from  the  other.  Probably  this  was  due  to  the 
different  carbon,  chromium,  and  nickel  content  as  shown  in  the  analyses 
of  the  two  specimens.  Hadfield' s  analysis  of  specimen  B  shows  chromium 
2.51,  nickel  2.81. 


Specimen   B 


Analysis  of  Steel 
Phosphorus  0.008 
Sulphur        0.028 
Silicon          0.14 
Manganese  0.13 
Carbon         0.54 
Chromium    3.32 
Nickel         2.23 


120 


140 


JO   20   30   40   JO   00   70   &>       100 

Hours  After  Hardening 
FIG.  6. — GENERATION  OF  HE  AT  BY  NICKEL-CHKOMIUM  STEEI^  AFTER  HARDENING. 

The  effect  on  specimen  B  of  a  moderate  increase  in  quenching  tempera- 
ture was  so  great  that  it  was  thought  worth  while  to  quench  it  a  third 
time,  at  a  temperature  somewhat  lower  than  in  the  first  hardening,  so 
the  temperature  of  complete  recalescence  was  chosen.  While  the 
quenching  temperature  of  first  hardening  was  just  above  the  critical 
temperature  of  decalescence,  that  of  the  third  hardening  would 
lie  just  below  the  critical  temperature  of  recalescence.  Specimen  B 
was,  therefore,  again  slowly  heated  in  the  gas  furnace  until  complete  loss 
of  magnetic  susceptibility  was  reached;  then  the  supply  of  gas  and  air 
was  so  far  reduced  that  the  furnace  very  gradually  cooled.  In  about 
20  min.  the  steel  commenced  to  regain  its  magnetic  susceptibility,  and  a 
few  minutes  later  had  completely  regained  it.  The  bars  were  imme- 
diately quenched  and  treated  as  formerly. 

I  had  expected  to  find  that  the  spontaneous  generation  of  heat  would 


598 


SOME  THERMAL  RELATIONS  IN  THE  TREATMENT  OF  STEEL 


be  greatly  modified,  or  possibly  eliminated  by  thus  quenching  the  steel 
below  the  critical  temperature,  because  true  hardening  could  not  have 
taken  place.  But  it  was  not  so.  On  the  contrary,  about  three-fourths 
as  much  heat  was  generated  as  followed  the  first  hardening,  and 
its  curve  (not  plotted)  was  parallel  throughout  with  that  of  the  first 
hardening. 

It  has  been  suggested  that  some,  at  least,  of  the  spontaneously  gene- 
rated heat  found  in  all  my  experiments  may  have  been  due  to  chemical 
action.  To  meet  this  criticism  I  again  slowly  heated  specimen  B  until 
it  began  to  lose  its  magnetic  susceptibility.  This  was  above  the  tem- 
perature of  the  previous  quenching,  and  presumably  obliterated  its 
effects;  it  also  destroyed  the  oil,  and  oily  compounds  if  any,  of  the 
previous  operation.  Then  the  furnace  was  closed  and,  with  its  contents, 
allowed  to  cool  4  hr.  The  steel  bars  were  then  removed  from  the  furnace, 
still  too  hot  to  handle,  allowed  to  air-cool  ^  hr.  longer,  stirred  in  much 
water  at  room  temperature,  wiped  dry,  oiled,  and  placed  in  the  calo- 
rimeter as  before.  The  slow  leakage  of  air  into  the  hot  furnace  after 
closing  had  provided  the  steel  bars  with  a  thin  coating  of  black  oxide 
having  the  same  appearance  as  that  which  followed  the  previous  quench- 
ings.  The  aim  of  all  this  procedure  was  to  reproduce  as  faithfully  as 
possible  all  the  conditions  that  obtained  in  previous  experiments  except 
quenching. 

In  all  former  experiments  the  temperature  of  the  steel  had  risen  con- 
siderably during  the  assembling  of  the  apparatus  immediately  follow- 
ing the  oiling  of  the  bars,  as  shown,  by  the  curve  sheets.  But  in  this 
case  the  first  galvanometer  readings  were  slightly  minus,  showing  that  the 
steel  was  about  3-£o°  colder  than  the  balancing  water  in  the  other  Dewar 
jar;  several  days  passed  before  approximate  equality  of  temperature  was 
reached.  Hence  we  may  safely  say  that  not  the  slightest  trace  of  heat 
was  generated  in  the  steel,  and  that  the  heat  generation  observed  in 
former  experiments  was  surely  due  to  the  quenching,  which  arrested 
transformation. 

TABLE  2 


Length  of  Test  Bar 

Specimen  A 

Specimen  B 

First 
Hardening, 
Inches 

Second 
Hardening, 
Inches 

First 
Hardening, 
Inches 

Second 
Hardening, 
Inches 

After  hardening  

5.0274 
5.0274 
5.0263 
5.0255 
5.0222 

5.0354 
5  .  0342 
5.0293 
5.0293 
5.0250 

5.0180 
5.0180 
5.0172 
5.0161 
5.0139 

5.0151 
5.0143 
5.0093 
5.0091 
5.0063 

After  spontaneous  shrinking.  .  .  . 
After  tempering  to  light  straw  .  . 
After  tempering  to  light  blue  .  .  . 
After  annealing  

CHARLES   F.   BRUSH  599 

Before  commencing  the  experiments,  a  test  bar  of  each  lot  was  pre- 
pared by  machining  its  ends  slightly  convex  and  polishing,  so  as  to  permit 
of  accurate  length-measurements  by  means  of  a  micrometer  caliper 
easily  manipulated  to  constant  end-pressure  and  easily  read  to  0.00005  in. 
The  test  bars  were  mechanically  held  always  in  the  same  position  between 
the  caliper  jaws.  Measurements  of  the  test  bars  were  made  after  various 
treatments,  and  the  results  are  shown  in  Table  2.  The  error  of  measure- 
ment does  not  exceed  one  unit  in  the  last  decimal.  In  neither  case  was 
there  any  spontaneous  shrinking  after  the  first  hardening,  thus  demon- 
strating that  the  heat  generation  which  then  occurred  was  not  due  to 
shrinking. 

In  both  specimens  total  shrinkage  was  about  twice  as  great  after  the 
second  hardening  as  after  the  first  but  was  divided  between  the  several 
treatments  quite  differently.  Comparing  the  second  hardening  shrink- 
ages with  the  first :  Considerable  occurred  spontaneously  (none  in  first) ; 
about  five  times  as  much  followed  tempering  to  light  straw,  and  little  or 
none  followed  the  second  tempering,  to  light  blue,  though  this  caused 
considerable  shrinking  in  both  cases  of  first  hardening. 

In  connection  with  the  first  tempering,  to  light  straw  color,  in  both 
cases  of  second  hardening,  an  interesting  phenomenon  was  observed: 
In  all  cases  of  tempering  to  color,  the  bar  was  sandpapered  bright,  very 
slowly  and  uniformly  heated  until  the  desired  color  appeared,  and  then 
quenched  in  water  at  room  temperature  to  arrest  progress  of  tempering; 
but  the  bars  continued  to  shrink  for  several  hours.  The  final,  stable 
measurements  are  given  in  the  tables.  Why  no  further  shrinking 
occurred  when  the  bars  were  tempered  to  light  blue  is  not  entirely  clear. 

The  anomalous  behavior  of  specimen  B  of  the  Hadfield  nickel-chro- 
mium steel  after  its  third  quenching  prompted  further  investigation. 
In  conducting  these  later  experiments  an  electric  furnace  was  employed 
for  heating,  instead  of  the  less  convenient  gas  furnace  and  the  latest  form 
of  scleroscope  for  testing  hardness  was  installed;  also,  a  most  modern 
industrial  thermoelectric  pyrometer.  The  latter  was  used  as  it  came  from 
the  maker,  without  further  calibration;  hence  the  temperatures  recorded 
may  be  several  degrees  in  error,  though  they  are  thought  to  be  relatively 
consistent. 

In  the  annealed  condition  specimen  B  was  found  to  have  a  scleroscope 
hardness  of  31.  Each  scleroscope  hardness  cited  in  this  paper  is  the 
mean  of  at  least  ten  consistent  measurements,  each  measurement  made  on 
a  fresh  spot  of  surface  carefully  made  smooth  and  flat. 

In  order  to  ascertain  the  critical  temperatures  of  decalescence  and 
recalescence  of  specimen  B,  three  of  the  twelve  bars  were  very  gradually 
heated  until  almost  complete  loss  of  magnetic  susceptibility  was  reached. 
This  occurred  rather  abruptly  at  about  777°  C.  One  of  the  bars  was 
quenched  at  this  temperature,  and  its  scleroscope  hardness  was  found 


600         SOME   THERMAL   RELATIONS   IN   THE   TREATMENT   OF  STEEL 


to  be  74,  which  may  be  taken  as  the  hardness  of  specimen  B  after  the  first 
quenching. 

The  remaining  two  bars  were  allowed  to  cool  very  slowly  in  the  fur- 
nace until  complete  recovery  of  magnetic  susceptibility  took  place  at 
about  660°;  recovery  was  abrupt  in  temperature.  One  of  these  bars  was 
quenched  at  this  temperature,  and  its  hardness  was  found  to  be  only  37, 
which  is  not  much  above  the  annealed  hardness.  This  seems  conclusive 
evidence  that  true  hardening  did  not  take  place  in  specimen  B  on  its 
third  quenching  although  good  spontaneous  generation  of  heat  followed 
the  quenching. 

The  three  bars  were  again  heated  to  complete  decalescence  and 
annealed  in  the  furnace  so  as  to  leave  all  twelve  bars  of  specimen  B  in 
an  annealed  condition. 


Analysis  of  Steel 

Phosphorus  0,008 

Sulphur  0.028 

Silicon  0.14 

Manganese  0.13 

Carbon  0.54 

Chromium  3.32 

Nickel  2.23 


o 


IO   2O   30   40   50   60   70   80       100       I2O 

Hours  After  Hardening 
FIG.  7. — EXTENDED  CURVE  SHEET  OF  SPECIMEN  B. 


140 


Fig.  7  is  the  extended  curve  sheet  of  specimen  B.  The  curve  of 
first  hardening  shows  the  spontaneous  generation  of  heat  that  followed  the 
first  quenching  at  about  777°,  the  curve  of  second  hardening,  indicated 
by  2h,  shows  considerably  greater  generation  of  heat.  Quenching  tem- 
perature and  hardness  were  not  observed;  but  it  is  known  that  the 
quenching  temperature  was  much  higher  than  777°.  The  third  curve, 
showing  spontaneous  generation  of  heat,  is  indicated  by  3q,  meaning 
third  quenching  (not  hardening).  To  make  it  clear  that  heat  was  gene- 
rated in  this  case  the  curve  of  normal  cooling  is  dotted  for  easy  comparison. 
The  quenching  temperature  in  this  case  must  have  been  slightly  below 
660°  and  hardness  only  about  37. 

Specimen  B,  left  in  the  annealed  condition  at  the'close  of  former 
experiments,  with  a  hardness  of  31,  was  next  gradually  heated  to  554°, 


CHARLES   P.   BRUSH  601 

allowed  to  cool  slowly  to  532°  and  quenched.  It  was  then  purposely 
brought  to  a  temperature  slightly  above  room  temperature  and  placed 
in  the  calorimeter.  The  progress  of  cooling  is  plotted  in  the  curve  4q 
(fourth  quenching).  For  easy  comparison  the  normal  cooling  curve  is 
drawn  as  a  dotted  line  through  the  first  station  of  the  4q  curve.  Beyond 
this  point  the  4<?  curve  lies  everywhere  below  the  normal  cooling  curve, 
showing  conclusively  that  the  steel  cooled  abnormally  fast.  In  other  words, 
there  was  spontaneous  disappearance  or  absorption  of  heat  in  the  steel, 
most  notable  during  the  first  few  hours  after  quenching.  Hardness  was 
35.5. 

The  steel  was  next  heated  to  562°  and  quenched.  The  result  of 
this  treatment  is  shown  in  the  curve  5q,  with  its  own  dotted  normal 
cooling  curve.  Absorption  of  heat  is  again  indicated,  even  greater 
than  in  4g  but  somewhat  differently  distributed.  Hardness  was  now 
34.5.  Again  the  steel  was  heated,  this  time  to  594°,  and  quenched. 
Again  there  was  marked  absorption  of  heat.  The  curve  was  almost 
identical  with  4q,  and  is  not  plotted,  to  avoid  confusion  of  lines.  Hard- 
ness was  again  34.5. 

The  seventh  heating  was  carried  to  667°  for  quenching.  This  was 
a  much  larger  temperature  advance  than  in  either  of  the  preceding 
experiments,  and  was  above  the  temperature  of  the  third  quenching, 
which  was  followed  by  very  considerable  generation  of  heat.  But  now 
there  was  very  considerable  absorption  of  heat,  as  shown  in  curve  7q. 
Hardness  was  now  34. 

It  should  be  noted  that  the  quenchings  that  were  followed  by  ab- 
sorption of  heat  were  made  at  rising  temperatures  which  had  not  been 
exceeded  (except  slightly  in  the  case  of  4<?)  since  the  steel  was  annealed. 
But  in  the  case  of  the  third  quenching  the  quenching  temperature  was 
a  falling  one,  reached  by  cooling  from  the  much  higher  temperature  of 
decalescence.  I  can  think  of  no  other  cause  than  this  for  the  radically 
different  results  of  the  third  and  seventh  quenchings,  which  were  made  at 
substantially  the  same  temperature.  The  temperature  difference  be- 
tween complete  loss  and  complete  recovery  of  magnetic  susceptibility 
117°  was  unusually  large;  but  while  this  temperature  drop  brought  about 
almost  annealed  softness,  and  full  restoration  of  magnetic  qualities, 
it  did  not  very  greatly  affect  that  quality  of  the  steel  which  is  responsible 
for  the  spontaneous  generation  of  heat.  Seemingly,  one  or  more  of  the 
several  unstable  compounds  or  mixtures  of  the  constituents  of  the  steel 
formed  at  the  upper  critical  temperature  did  not  have  time  to  wholly 
revert  to  normal  annealed  condition  while  the  metal  was  cooling  to  and 
passing  through  recalescence.  The  time  of  this  cooling  was  about  ^  hr. 

To  confirm  the  curious  result  of  the  third  quenching,  that  is,  gen- 
eration of  heat  without  hardening,  the  bars  were  quenched  the  eighth 
time  as  follows:  Slowly  heated  (nearly  2  hr.)  to  819°,  slowly  cooled 


602 


SOME  THERMAL  RELATIONS  IN  THE  TREATMENT  OP  STEEL 


(nearly  1  hr.)  to  680°  and  quenched.  During  the  heating  complete 
loss  of  magnetic  susceptibility  occurred  at  779°,  which  was  an  excellent 
confirmation  of  the  former  finding  (777°).  But  in  cooling,  full  recovery 
of  magnetic  susceptibility  came  at  680°,  which  is  20°  higher  than  before. 
The  five  intermediate  treatments  may,  perhaps,  account  for  this.  And 


TABLE  3. — -Resume  of  Specimen  B 

Temperature  of  complete  loss  of  magnetic  susceptibility,  777°  C. 
Temperature  of  complete  recover}'  of  magnetic  susceptibility,  660°  to  680° 


C. 


Quenching, 
Temperature 
Degrees  C. 

Hardness 

Remarks 

First  hardening.  .  . 
Second  hardening. 
Third  quenching  .  . 
Annealing  

About  777 
Much  higher  temp. 
About  780-660 

74« 

37° 
31 

Good  generation  of  heat 
Much  larger  generation  of  heat 
Fairly  good  generation  of  heat 

;  ; 

Fourth  quenching. 
Fifth  quenching.  . 
Sixth  quenching.  . 
Seventh  quenching 
Eighth  quenching. 

554-532 
562 
594 
667 
819-680 

35.5 
34.5 
34.5 
34 

47 

Good  absorption  of  heat 
Good  absorption  of  heat 
Good  absorption  of  heat 
Good  absorption  of  heat 
Good  generation  of  heat 

0  Result  obtained  from  a  separate  experiment. 

this  higher  quenching  temperature  may  account  for  the  somewhat 
greater  hardness  produced,  which  was  later  found  to  be  47,  as  against  37  for 
the  third  quenching  (74  for  true  hardening  above  decalescent  temperature). 

Following  the  eighth  quenching  there  was  good  generation  of  heatj 
better  than  after  third  quenching,  but  differently  distributed  in  time — 
not  so  rapid  at  first,  but  much  better  sustained.  This  appears  to  con- 
firm the  third  experiment. 

Absorption  was  rapid  during  the  first  few  hours,  and  nearly  (not 
quite)  ceased  at  the  end  of  50  or  60  hr. ;  while  generation  was  well  marked 
up  to  150  hr.  In  earlier  experiments  generation  of  heat  was  easily 
detected  at  the  end  of  a  month. 

CARBON  STEEL 

As  it  seemed  desirable  to  learn  whether  plain  carbon  steel  would 
show,  like  the  nickel-chromium  steel,  generation  of  heat  without  harden- 
ing, or  absorption  of  heat  when  quenched  at  rising  temperatures  below 
the  lower  critical  temperature,  after  annealing,  the  following  experiments 
were  made  with  the  carbon  steel  used  for  the  first  experiment.  The 
normal  cooling  curve  and  upper  curve  of  heat  generation  shown  in 
Fig.  8  are  taken  from  Fig.  1. 

The  steel  was  quenched  at  very  high  temperature,  temperature  and 
hardness  were  not  then  observed,  but  recently  the  scleroscope  hardness  was 


CHARLES    F.    BRUSH 


603 


observed,  79.  In  the  second  hardening  the  steel  was  quenched  at  802°, 
considerably  above  decalescence,  but  much  lower  than  in  first  hardening. 
Complete  loss  of  magnetic  susceptibility  occurred  at  765°.  Hardness 
was  now  73.  For  the  third  quenching  the  steel  was  heated  to  815°, 


Analysis  of  Steel 
Phosphorus  0.012 
Sulphur        0.016 
Silicon          0.21 
Manganese  0.31 
Carbon 


§  160 

Q  140 

3  120 

1- 

J  &> 

u  6° 

I  40 

.£  20 


10   2p   30   40   50   DO   70   80        100       I2O       I4O 

Hours  After  Hardening 
FiGj  8. — GENERATION  OF  HEAT  AFTER  HARDENING  CARBON  STEEL. 

somewhat  above  preceding  quenching  temperature,  allowed  to  cool 
slowly  to  720°  and  quenched.  This  was  a  little  below  the  temperature 
of  complete  recovery  of  magnetic  susceptibility,  which  had  occurred  at 
729°.  Hardness  was  now  only  28.5,  and  there  was  no  generation  of  heat. 


li 

4>     OO 

§  60 

V 

=   40 


6 


Analysis  of  Steel 
Phosphorus  0.006 
Sulphur  0.010 
Silicon  o.ooo 
Manganese  o.ooo 
Carbon  3.480 


20 


120 


140 


40  60  80 

Hours  After  Hardening 
FIG.  9. — GENERATION  OF  HEAT  AFTER  HARDENING  HIGH-CARBON  STEEL. 


The  nickel-chromium  steel  had  shown  good  generation  of  heat  under 
similar  circumstances.  It  was  annealed  by  heating  to  822°,  to  obliterate 
previous  quenching  effects,  and  cooled  slowly  in  the  furnace.  Hardness 
was  now  25.5.  For  the  fourth  quenching  it  was  heated  slowly  from  the 
annealed  condition  to  633°  and  quenched.  Hardness  was  again  28.5, 


604         SOME   THERMAL   RELATIONS   IN   THE   TREATMENT   OF   STEEL 

and  there  was  no  trace  of  absorption  of  heat.  The  nickel-chromium 
steel  has  shown  good  absorption  of  heat  under  similar  circumstances. 
For  the  fifth  quenching,  it  was  heated  slowly  to  732°,  just  above  the  tem- 
perature of  complete  recovery  of  magnetic  susceptibility,  and  quenched. 
No  generation  or  absorption  of  heat,  nor  change  in  hardness  (28.5). 

For  a  general  check  on  the  performance  of  the  apparatus,  twelve 
H-in.  round  bars  of  Swedish  charcoal  iron,  of  the  aggregate  weight  of 
the  steel  usually  employed,  were  slowly  heated  to  960°  and  quenched. 
Complete  loss  of  magnetic  susceptibility  had  occurred  at  801°.  The  bars 
were  warmed  about  3°  just  before  being  placed  in  the  calorimeter.  There 
was  no  trace  of  heat  generation  following  the  quenching.  Indeed,  the 
curve  of  cooling  followed  the  normal  cooling  curve  with  such  fidelity  that 
nowhere  did  they  differ  as  much  as  the  width  of  the  curve  line.  This  was 
very  gratifying  in  view  of  the  fact  that  observations  for  the  normal 
cooling  curve  were  made  more  than  2  years  prior,  and  checked  only  once 
since  that  time.  Hardness  was  18.5. 

MANGANESE  STEEL 

Sir  Robert  Hadfield  long  ago  suggested  that  interesting  results  might 
follow  similar  experiments  with  manganese  alloy  steel,  so  19  num- 
bered bars,  each  6  in.  long  and  ^  in.  in  diameter,  were  cut  from  the 
same  long  bar  and  ground  to  size  after  treatment.  They  contained: 
carbon,  1.18  per  cent.;  silicon,  0.14  per  cent.;  manganese,  12.29  per  cent. 

Bars  1  to  6,  as  forged,  were  non-magnetic.  Bars  7  to  12,  toughened 
by  water-quenching  at  995°  C.,  were  non-magnetic.  Bars  13  to  18, 
toughened  as  above,  then  reheated  to  500°  and  kept  at  that  temperature 
63  hr.,  were  magnetic.  Bar  19,  treated  like  13  to  18,  then  reheated  to 
995°  and  water-quenched,  was  non-magnetic.  The  scleroscope  hardness 
of  bar  1,  as  forged,  was  37.3;  bar  7,  toughened,  was  28.5;  bar  15,  toughened 
and  reheated,  was  51.6;  bar  19,  toughened,  reheated,  and  retoughened, 
was  39. 

Bar  13  was  subsequently  heated  to  1074°  and  cooled  (annealed) 
in  the  furnace.  Its  hardness,  which  presumably  had  been  about  51.6 
like  its  companion  No.  15,  was  then  28.8,  and  it  was  non-magnetic; 
seeming  to  show  that  quenching  at  high  temperature,  and  annealing 
from  a  still  higher  temperature,  gives  the  same  hardness  and  non-mag- 
netic condition  whatever  the  previous  treatment  may  have  been.  The 
hardness  of  bar  19  seems  to  contradict  this  conclusion,  in  respect  of 
hardness,  but  it  was  quenched  at  a  very  considerably  lower  temperature. 

In  the  following  experiments  ten  of  the  6-in.  bars  were  used,  so  as  to 
approximately  equal  in  weight  the  twelve  5-in.  bars  of  other  steel  employed 
in  former  experiments. 

•    First  Quenching. — Bars  1  to  5  and  7  to  11  were  heated  in  an  electric 
muffle  furnace  to  1013°  C.  and  quenched  in  water;  this  treatment  was 


CHARLES   F.    BRUSH  605 

followed  by  no  appreciable  generation  or  absorption  of  heat  when  tested 
in  the  calorimeter.  Hardness  was  now:  bar  1,  30;  bar  7,  28.3,  showing 
that  the  first  and  the  second  lot  were  brought  to  substantially  the  same 
toughened  condition. 

Second  Quenching. — The  ten  bars  were  again  heated  to  1013°,  aUowed 
to  cool  in  the  furnace  to  800°  and  quenched;  again  there  followed  no 
appreciable  generation  or  absorption  of  heat.  Hardness  was  now; 
bar  1,  27.6;  bar  7,  26.3. 

Third  Quenching. — The  bars  were  heated  to  818°,  allowed  to  cool  in 
the  furnace  to  607°,  and  quenched;  there  was  no  subsequent  generation  or 
absorption  of  heat.  Hardness  was,  bar  1,  26.3;  bar  7,  26.6. 

The  ten  bars  were  next  heated  slowly  to  645°  and  allowed  to  cool 
slowly  in  the  furnace  to  room  temperature.  The  hardness  was,  bar  1, 
26.5;  bar  7,  25.9.  Ah1  the  bars  were  now  very  moderately  magnetic, 
though  in  their  softest  condition. 

The  foregoing  quenching  temperatures  were  falling  ones.  The 
following  quenching  temperature  was  a  rising  one,  from  the  annealed 
condition  last  described. 

Fourth  Quenching. — The  bars  were  heated  slowly  to  615°  and  quenched. 
The  hardness  was  now,  bar  1,  38;  bar  7,  30.3.  Notwithstanding  this  con- 
siderable increase  of  hardness,  there  followed  no  appreciable  generation  or 
absorption  of  heat.  The  bars  remained  very  moderately  magnetic. 

The  results  of  the  foregoing  experiments  make  it  highly  probable 
that  no  spontaneous  generation  or  absorption  of  heat  can  be  brought 
about  by  quenching  this  manganese  steel  at  any  temperature,  rising  or 
falling,  while  in  its  normal,  useful  non-magnetic  condition.  But  it 
was  thought  worth  while  to  make  further  experiments  with  the  steel 
in  its  magnetic  condition  and,  incidentally,  to  study  the  development 
of  this  magnetic  condition  during  the  prolonged  moderate  heating 
necessary  to  bring  it  about. 

The  apparatus  employed  in  the  following  study  consists,  in  part,  of  a 
vertical  cylindrical  electric  furnace  heated  by  small  spirals  of  "nichrome" 
wire  carrying  alternating  current  regulated  by  a  rheostat.  The  heating 
spirals  are  so  disposed  as  not  to  produce  any  magnetic  field  inside  or 
outside  the  furnace.  Instead  of  the  usual  sheet-iron  casing,  this  furnace 
is  cased  with  sheet  brass  slotted  longitudinally  to  prevent  induction 
currents  in  it  when  the  external  magnetizing  solenoid  is  excited.  The 
furnace  is  surrounded  by  a  solenoid  16  in.  inside,  and  20  in.  outside 
diameter,  and  4  in.  long  (high),  consisting  of  860  turns  of  No.  12  in- 
sulated copper  wire  wound  in  two  equal  coils  adapted  to  be  placed  in 
series  or  parallel  relation  by  means  of  a  suitable  switch.  The  axes  of  the 
furnace  and  solenoid  are  coincident.  The  solenoid  is  excited  by  current 
from  a  65-volt  storage  battery,  controlled  by  a  rheostat,  and  the  circuit 


606          SOME    THERMAL   RELATIONS    IN   THE    TREATMENT    OP   STEEL 

is  closed  and  opened  by  a  switch  which  breaks  simultaneously  at  three 
points  in  series,  so  as  to  avoid  the  destructive  arc  that  would  occur  at  a 
single  break.  An  ammeter  and  reversing  switch  are  included  in  the  line. 

A  single  turn  of  asbestos-insulated  platinum  wire  is  located  in  the 
furnace,  and  the  ends  of  this  loop  are  connected  by  a  twisted  cable  with 
a  ballistic  mirror-galvanometer  of  600  ohms'  resistance. 

When  the  solenoid  circuit  is  closed,  a  brief  electric  current  is  induced 
in  the  platinum  loop  in  the  furnace  and  causes  a  minimum  swing  of  the 
galvanometer  scale  easily  read  with  considerable  precision. 

When  a  bundle  of  ordinary  steel  or  iron  bars  is  placed  within  the 
platinum  loop  the  gahanometer  deflection  is,  of  course,  many  times 
greater,  and  is  fairly  proportional  to  their  magnetic  susceptibility,  after 
deducting  the  minimum  deflection  due  to  the  platinum  loop  alone, 
and  when  the  excitation  of  the  solenoid  is  not  too  small  or  too  great.  In 
the  following  experiments  with  the  manganese  steel,  9  amp.  was  found 
to  be  a  suitable  exciting  current  with  the  solenoid  coils  in  series.  Small 
variations  of  exciting  current  were  reduced  to  this  value  in  computing 
results.  Residual  magnetism  was  measured  by  the  usual  method  of 
reversals,  and  allowed  for. 

In  the  following  experiments  galvanometer  deflection,  less  that 
amount  due  to  the  platinum  loop  alone,  are  taken  as  the  measure  of  mag- 
netic susceptibility,  and  the  susceptibility  of  the  Swedish  iron  is  used  as  a 
standard  and  assigned  a  value  of  100.  All  other  values  are  reduced  to 
and  expressed  in  terms  of  this  standard.  As  a  preparatory  measure,  the 
ten  bars  of  manganese  steel  were  brought  to  their  softest  and  toughest 
condition  by  quenching  at  1000°.  The  hardness  was,  bar  1,  26.7;  bar  7, 
25.8.  All  the  bars  were  quite  free  from  any  trace  of  magnetism. 

The  bars  were  placed  within  the  platinum  loop  in  the  electric  furnace 
and  heated  170  hr.  to  a  temperature  fluctuating  between  505°  and  525°. 
The  growth  of  magnetic  susceptibility  is  plotted  in  the  curve  shown  in 
Fig.  10.  There  is  no  doubt  that  the  curve  would  have  been  smoother  if 
the  temperature  had  remained  constant.  It  was  intended  to  use  about 
515°  temperature,  which  was  maintained  during  the  first  few  hours. 
Subsequent  fluctuations  were  due  to  variations  of  voltage  in  the  alterna- 
ting heating  current.  The  higher  temperatures  usually  occurred  in  the 
latter  part  of  the  night,  and  were  always  accompanied  by  more  than 
average  rise  of  susceptibility.  But  the  large  depression  in  the  central 
part  of  the  curve  is  thought  to  be  due  to  some  obscure  cause,  and  not  to 
temperature  variation.  The  entire  absence  of  growth  of  susceptibility 
during  the  last  50  hr.  or  more  prompted  the  belief  that  the  steel  had 
reached  a  stable  condition  at  the  temperature  of  treatment,  and  led  to 
the  discontinuance  oCthis  experiment.  Permanent  magnetism,  which 
had  been  considerable  while  susceptibility  was  rising,  fell  off  very  much 
during  the  last  2  or  3  days. 


CHARLES    F.    BRUSH 


607 


Fifth  Quenching. — At  the  end  of  170  hr.  the  bars  were  quenched, 
after  which  they  exhibited  moderate,  but  typical  and  unequivocal  gen- 
eration of  heat.  The  hardness  was,  bar  1,  48.1;  bar  7,  47.2.  This  great 
increase  of  hardness,  brought  about  by  the  long  heating,  doubtless  ac- 
counts for  the  spontaneous  generation  of  heat  observed. 

During  the  long  heating  the  bars  acquired  a  rather  thick  coating 
of  black  oxide,  which  peeled  off  almost  completely  in  quenching,  leaving 
a  clean  metal  surface.  The  oxide  was  strongly  magnetic;  but  its  weight 
was  so  small,  compared  with  the  total  weight  of  the  bars,  that  it  could  not 
have  affected,  materially,  the  foregoing  magnetic  observations. 

The  bars  were  heated  to  a  higher  temperature  than  before,  fluctuating 
between  590°  and  598°,  for  the  first  90  hr.,  or  from  170  to  260  hr.,  reckoned 
from  beginning  of  treatment.  The  results  are  plotted  in  the  first  curve 


40 
I*30 

t/5 

I20 

s 

10 

Swedish 

I         1         i         1 
Charcoal-Iron  =  100 

/ 

*•*? 

A 

^ 

--< 

™ 

^-- 

*^ 

ir\ 

/ 

T» 

i 

1 

9 

§ 

/ 

<y 

^ 

s 

20                 40                 60                 80                100               120               140               160 

Hours      of     Heating          505*  525*  C 

FIG.  10. — GROWTH  OF  MAGNETIC  SUSCEPTIBILITY. 

of  Fig.  11.  It  is  seen  that  magnetic  susceptibility  started  at  a  very 
considerably  higher  value  than  it  had  at  the  end  of  the  previous  treat- 
ment. The  reason  of  this  increase  during  the  \intervening  few  days, 
without  heating,  is  not  clear.  It  may  have  occurred  at  the  moment 
of  quenching;  or,  more  likely,  during  the  period  of  spontaneous  heat 
generation  which  followed  the  quenching. 

The  curve  shows  a  very  regular,  but  steadily  diminishing,  growth 
of  susceptibility  at  this  higher  temperature,  until  it  reaches  nearly 
double  the  value  it  had  at  the  end  of  the  previous  treatment. 

When  the  temperature  was  next  quickly  lowered  to  its  former  value 
and  then  continued  to  the  end  of  the  experiment,  175  hr.  (340  hr.  total), 
there  was  at  first  a  sudden  rise  of  susceptibility,  followed  by  steady 
growth  as  before. 

Sixth  Quenching. — At  the  end  of  344  hr.,  total,  of  treatment,  the 
bars  were  again  quenched;  this  was  followed  by  very  little,  if  any,  spon- 


608 


SOME  THERMAL  RELATIONS  IN  THE  TREATMENT  OF  STEEL 


taneous  generation  of  heat.  The  hardness  was,  bar  1,  37.4;  bar  7,  36.2. 
This  shows  considerable  softening  since  the  last  quenching,  notwith- 
standing the  large  increase  of  magnetic  susceptibility.  The  softening 
may  account  for  the  absence  of  heat  generation  after  the  quenching. 
The  magnetic  susceptibility  of  the  cold  quenched  bars  was  almost  the 
same  (slightly  lower)  as  before  quenching. 

The  ten  bars  were  again  heated,  slowly  this  tima,  to  590°  and  held 
nearly  at  that  temperature  until  the  381  hr.  of  total  treatment,  as  shown 
in  Fig.  12.  Susceptibility  rose  slightly,  reaching  its  highest  value,  68.5. 
As  this  is  comparable  with  the  susceptibility  of  ordinary  steel,  the  man- 
ganese had  apparently  almost  completely  lost  its  influence. 


a. 


6o 

50 

40 

•^-r 



^ 

t^^-~ 

»- 

^ 

^ 

* 

V 

«*^ 

591 

597° 

X 

^ 

•6I9" 

s 

X 

y 

/ 

r 

-590 

59gr 

'S 

•? 

u 

c 

Composition  of  Steel 

Silicon            0.14 
Manganese   -12.29 
Qrbon            1.18 

0 

10 

l8o  200  220  240  260 

Hours     of     Heating 
FIG.  11. 


280 


300 


320 


At  this  stage  it  was  thought  that  decalescence  might  possibly  be 
brought  about  by  cautiously  raising  the  temperature.  The  effect 
of  doing  so  is  shown  in  the  great  and  nearly  vertical  drop  in  the  sus- 
ceptibility curve.  The  stations  in  this  part  of  the  curve  represent 
observations  at  ^-hr.  intervals,  indicating  2  hr.  for  the  total  drop, 
with  the  temperature  steadily  rising  to  the  maximum  of  692°.  It  seemed 
clear  that  decalescence  was  not  taking  place,  because  loss  of  susceptibility 
was  far  too  slow  in  time  and  the  maximum  temperature  reached  was  not 
sufficiently  high.  Probably  the  manganese  was  simply  resuming  its  sway. 

The  temperature  was  next  rapidly  lowered  to  605°-590°,  bringing 
on  a  rapid  recovery  of  magnetic  susceptibility,  amounting  to  30  points  in 


CHARLES   F.    BRUSH 


609 


21  hr.  as  shown.  Again  the  temperature  was  raised,  but  much  more 
rapidly  than  before,  resulting  in  a  much  steeper  drop  in  the  curve,  the 
observation  stations  shown  representing  only  five-minute  intervals. 

Seventh  Quenching. — At  the  end  of  the  curve  shown  in  Fig.  12  the 
steel  bars  were  quenched  at  687°.  Subsequently  there  was  no  trace 
of  generation  or  absorption  of  heat;  hence  it  is  virtually  certain  there 
had  been  no  decalescence.  The  hardness  was,  bar  1,  42;  bar  7,  41.8. 

Sir  Robert  Hadfield  long  ago  assured  me  that  the  study  of  manganese 
steel  is  full  of  surprises  for  the  investigator.  I  have  experienced  some 
of  them,  and  hesitate  to  draw  conclusions  from  the  results  of  the  ex- 
periments last  described.  That  the  manganese  should  completely 


60 


J* 

n 


30 


—  -590 


z 


360  380  400 

Hours    of    Heating 
FIG.  12. 

obliterate  the  magnetic  quality  of  seven  times  its  weight  of  iron  is  most 
remarkable;  and  the  very  gradual  lifting  of  this  inhibition  at  moderate 
temperatures,  and  the  comparatively  sudden  reversion  when  a  certain 
critical  temperature  (about  600°)  is  exceeded,  afford  an  inviting  field 
for  research.  It  seems  likely  that  the  carbon  present  plays  an  important 
role  in  this  peculiar  alloy,  or  probably  mixture  of  alloys  of  composition 
varying  with  temperature. 

All  interested  in  this  subject  should  read  the  extended  researches 
of  Sir  R.  A.  Hadfield  and  Prof.  B.  Hopkinson  on  the  magnetic  and 
mechanical  properties  of  manganese  steel.2 


1  Jnl.  Iron  and  Steel  Inst.  (1914)  60,  476. 


39 


610  TYROMETRY   IN    THE   TOOL-MANUFACTURING    INDUSTRY 


Pyrometry  in  the  Tool-manufacturing  Industry 

BY  J.   V.    EMMONS,*   CLEVELAND,    OHIO 
(Chicago  Meeting,  September,  1919) 

THE  processes  of  hardening  and  tempering  steel  tools  within  the  past 
15  or  20  years  have  been  so  developed  that  the  forward  strides  of  the 
industry  can  scarcely  be  followed  by  the  average  observer.  No  small 
part  of  the  credit  for  this  phenomenal  advancement  is  due  to  the  improved 
methods  of  measuring  the  high  temperatures  necessary.  Pyrometry 
has  made  possible  the  discovery  and  use  of  the  many  special  alloy  steels, 
the  wonderful  properties  of  which  are  revolutionizing  the  industrial 
world.  So  sensitive  are  these  steels  to  changes  of  temperature  that 
often  a  variation  of  only  10°  F.  at  a  critical  point  of  the  heat  treatment  will 
cause  the  failure  of  the  tool.  In  the  tool-manufacturing  industry  today, 
accurate  control  of  the  many  heating  operations  is  imperative. 

The  principal  processes  in  the  manufacture  of  tools  that  require 
accurate  high-temperature  measurements  are  annealing,  carburizing, 
hardening,  tempering,  and  laboratory  research.  Several  other  operations, 
such  as  forging  and  straightening,  use  rougher  and  correspondingly  less 
accurate  methods.  These  operations  are  usually  performed  in  the 
following  approximate  ranges  of  temperature: 

DEGREES  F.  DEGREES  F. 

Annealing 1000  to  1700  Tempering 200  to  1200 

Carburizing 1400  to  1900  Forging 1400  to  1800 

Hardening 1400  to  2500  Straightening 300  to  1300 

Laboratory  research  covers  the  entire  range. 

Annealing  and  carburizing  are  usually  done  in  muffle  or  semi-muffle 
furnaces,  heated  with  coal,  coke,  oil,  gas,  or  electricity.  Many  of  them  are 
of  large  size,  holding  several  tons  of  steel  at  one  charge.  Both  indicating 
and  recording  pyrometers  are  used.  The  recording  instruments  are 
generally  of  the  thermoelectric  type,  while  for  the  indicating,  both  optical 
and  thermoelectric  are  used. 

Hardening  furnaces  are  of  many  types  and  sizes  and  use  all  the  com- 
mon sources  of  heat.  The  best  known  are  muffles  and  semi-muffles 
fired  with  oil  or  gas,  electric  muffles,  lead  and  salt  baths  heated  by  oil 
or  gas,  and  electrically  heated  salt  baths.  The  pyrometers  used  are 


*  Metallurgist,  Cleveland  Twist  Drill  Co. 


J.   V.    EMMONS  611 

largely  indicating,  although  in  a  few  cases  recording  instruments  have  been 
installed.  For  the  indicating  instruments,  both  thermocouples  and  optical 
pyrometers  are  used.  For  the  high  range  of  temperatures  used  in  harden- 
ing high-speed  steel,  the  optical  pyrometers  are  much  to  be  preferred. 

Tempering  furnaces  are  usually  muffles,  oil  baths,  or  salt  baths  and  are 
heated  by  gas,  oil,  or  electricity.  Temperatures  up  to  600°  F.  (316°  C.) 
are  preferably  measured  by  a  mercury  thermometer.  From  600°  to 
1200°  F.  (316°  to  648°  C.),  a  thermocouple  or  resistance  thermometer  is 
used. 

In  heating  steel  for  forging,  gas  or  oil  fired  furnaces  are  used,  the  tem- 
peratures being  determined  by  the  eye,  with  occasional  checking  up  by 
means  of  optical  pyrometers. 

For  straightening,  open  gas  flames  are  used  and  the  temperatures 
roughly  determined  by  visual  examination  of  the  temper  colors  and  the 
first  appearance  of  redness. 

In  the  laboratory,  it  is  necessary  to  have  accurate  instruments  and 
standards  for  checking  up  all  of  the  types  of  pyrometers  in  use  in  the 
factory  at  frequent  intervals.  Rare-metal  thermocouples  and  the  new 
Leeds  &  Northrup  portable  optical  pyrometers  are  particularly  useful  for 
this  purpose.  The  laboratory  also  requires  pyrometers,  which  may  be 
used  for  experimental  or  research  work  over  the  entire  range  of  tempera- 
tures. Thermoelectric  pyrometers  of  both  fare  and  base-metal  types  and 
several  types  of  optical  instruments  are  used. 

The  Morse  thermogage  has  been  found  particularly  adaptable  for 
tool-manufacturing  purposes  requiring  an  indicating  instrument  only. 
It  consists  of  an  electric  lamp  with  a  hairpin  filament  so  placed  in  a 
telescope  or  tube  that  the  brightness  of  the  filament  may  be  compared 
with  the  brightness  of  the  heated  object,  the  temperature  of  which  is  to 
be  measured.  As  the  incandescence  of  a  heated  object  increases  quite 
rapidly  with  any  increase  of  temperature  above  1200°  F.  (648°  C.)  this 
forms  a  very  sensitive  indication  of  the  temperature.  The  current  for 
the  lamp  must  be  of  very  constant  voltage  and  is  usually  furnished  by 
storage  batteries.  The  temperature  of  the  filament  in  the  lamp  is  con- 
trolled by  means  of  a  rheostat  and  the  current  flowing  through  the  lamp  is 
read  upon  an  ammeter  graduated  to  10  milliamperes.  As  with  the  voltage 
constant  the  temperature  of  the  filament  is  in  proportion  to  the  current 
flowing  through  it,  the  ammeter  may  be  calibrated  in  terms  of  degrees  of 
temperature. 

The  low  limit  of  temperature  for  which  this  instrument  may  be  used  is 
at  the  point  where  a  faint  red  incandescence  begins,  or  at  about  1100° 
to  1200°  F.  (593°  to  648°  C.) .  The  high  range  of  the  instrument  is  almost 
unlimited  as  far  as  practical  operation  is  concerned.  For  high  tempera- 
tures where  the  brightness  of  the  heated  body  causes  discomfort  to  the 
eye,  a  calibrated  absorption  screen  is  inserted  between  the  filament  and 


612  PYROMETRY   IN   THE   TOOL-MANUFACTURING   INDUSTRY 

the  heated  object  to  reduce  the  amount  of  light  entering  the  eye.  For 
cases  when  the  color  of  the  light  emitted  by  the  filament  is  different  from 
the  color  of  the  light  from  the  heated  body,  a  monochromatic  screen, 
usually  red,  may  be  inserted  between  the  filament  and  the  eye  to  assist 
in  the  comparison  of  the  brightness  of  the  two  incandescent  objects. 
For  factory  use  at  temperatures  below  2000°  F.  (1093°  C.),  when  it  is 
possible  to  maintain  black-body  conditions,  it  has  been  found  most 
satisfactory  to  use  a  lamp  with  a  large  carbon  filament  and  a  telescope 
tube  without  lenses  or  absorption  screens.  In  cases  where  the  furnace 
conditions  make  it  possible,  the  telescope  tube  is  permanently  mounted 
as  close  to  the  heated  body  as  possible  and  in  position  for  the  most  con- 
venient observation  by  the  operator.  The  rheostat  and  ammeter  are 
placed  in  a  position  removed  somewhat  from  the  heat  of  the  furnace,  yet 
readily  accessible  to  the  operator.  This  form  of  permanent  installation  is 
particularly  useful  for  lead  and  salt  baths. 

There  are  many  cases,  particularly  in  the  hardening  operation, 
when  it  is  desired  to  hold  a  furnace,  such  as  a  lead  or  salt  bath,  at  a 
constant  temperature  while  a  large  number  of  tools  are  given  a  uniform 
treatment.  For  this  work,  the  Morse  thermogage  has  been  found  par- 
ticularly suitable.  For  this  purpose  the  ammeter  is  set  at  the  required 
temperature  after  which  the  operator  has  only  to  look  through  his 
telescope  tube  occasionally  to  observe  the  condition  of  the  heat  in  his 
furnace.  It  is  instantly  apparent  whether  his  temperature  is  high,  low, 
or  correct.  The  fact  that  the  reading  of  the  temperature  is  almost 
instantaneous  is  one  of  the  great  advantages  of  this  instrument  over  the 
thermocouple.  The  only  lag  present  is  the  time  required  for  the  lamp 
filament  to  respond  to  different  current  changes,  which  is  from  1  to  2 
sec.  at  the  most.  In  hardening  delicate  tools,  the  operations  must  be 
carried  out  with  such  exactness  and  rapidity  that  the  lag  in  the  operation 
of  a  thermocouple  becomes  a  serious  handicap.  Where  a  number  of 
furnaces  are  used,  all  requiring  only  the  occasional  use  of  a  pyrometer, 
the  wires  from  the  current  supply  may  be  carried  to  each  furnace  and  a 
single  instrument  arranged  to  plug  in  whenever  and  wherever  required. 

With  experienced  observers  and  ideal  conditions,  it  has  been  found 
possible  to  obtain  an  accuracy  of  5°  F.,  plus  or  minus,  for  the  range  of 
temperatures  from  1200°  to  1700°  F.  and  an  accuracy  of  10°  F.,  plus  or 
minus,  within  the  range  1700°  to  2500°  F.  Under  commercial  con- 
ditions and  with  average  observers,  an  accuracy  of  10°  F.,  plus  or  minus, 
can  be  obtained  for  the  lower  range  of  temperatures  and  15°  F.,  plus  or 
minus,  for  the  upper  range  above  mentioned. 

The  calibration  of  the  lamps  is  tested  at  regular  intervals  by  com- 
parison with  a  test  lamp  that  has  been  calibrated  by  the  Bureau  of 
Standards.  A  Leeds  &  Northrup  portable  optical  pyrometer  and  a  Le 
Chatelier  rare-metal  thermocouple  are  also  used  for  comparison.  The 


J.    V.    EMMONS  613 

majority  of  the  lamps  are  found  to  hold  their  calibration  very  well 
while  a  few  have  been  in  daily  service  for  from  5  to  10  years  without  any 
appreciable  change  in  calibration. 

The  upkeep  of  the  Morse  thermogage  is  very  small,  consisting  mostly 
of  the  necessary  attention  to  the  storage  batteries  and  occasional  replac- 
ing of  lamps  burned  out  or  accidentally  broken.  The  ammeter  and 
rheostat  rarely  need  attention. 

A  short  period  of  training  is  necessary  to  accustom  a  new  operator  to 
the  use  of  the  instrument,  but  this  is  seldom  longer  than  a  few  days.  A 
few  men  have  been  found  that  through  some  defect  of  eyesight  seem 
unable  to  use  it  accurately;  these  are  very  rare.  A  man  of  average 
intelligence  will  learn  the  principles  of  the  operation  with  a  very  few 
explanations  and  many  become  expert  in  its  use  in  1  day's  time. 

The  limitations  of  the  Morse  thermogage  may  be  summarized  as 
follows:  It  is  inoperative  below  1200°  F.  It  is  suitable  for  an  indicating 
pyrometer  only.  It  is  portable  only  to  the  extent  to  which  connections 
may  be  carried  from  a  storage  battery.  It  is  necessary  to  provide  black- 
body  conditions  for  accurate  work.  A  certain  amount  of  training  is 
necessary  for  the.  most  accurate  use.  It  will  read  the  temperature  of 
the  surface  of  an  object  only  and  cannot  be  used  except  for  such  localities 
as  can  be  made  visible  to  the  eye.  ' 

The  advantages  of  the  thermogage  are:  Its  accuracy,  which  is 
approached  by  few  other  instruments  under  commercial  conditions. 
The  rapidity  with  which  readings  may  be  taken,  the  time  required  being 
a  few  seconds  only;  for  instance,  the  temperature  of  forgings  can  be 
taken  while  they  are  being  worked  under  the  hammer.  The  absence 
of  lag,  the  temperature  indicated  being  the  temperature  existing  at  that 
instant  and  not  the  temperature  present  several  minutes  previously, 
as  with  the  average  thermoelectric  pyrometer.  No  parts  of  the  pyrome- 
ter are  exposed  to  the  heat  of  the  furnace  or  heated  object,  the  tem- 
perature of  which  is  being  measured.  The  high  range  of  temperatures 
for  which  it  is  available,  there  being  no  high  limit  to  its  use  in  commercial 
practice.  The  simplicity  and  ease  of  operation;  there  are  few  parts 
to  get  out  of  order  and  these  being  easily  protected,  the  lamp  may  be 
safely  used  by  unskilled  labor.  The  general  reliability  of  the  instrument; 
the  occurrence  of  difficulties  and  troubles  is  infrequent.  The  low  expense 
of  upkeep,  repairs  and  replacements  being  necessary  only  at  rare  intervals. 

With  regard  to  the  entire  pyrometer  situation  in  tool  manufacturing, 
it  may  be  said  that  the  price  of  success  is  eternal  vigilance.  Small 
defects  and  changes  of  calibration  must  be  discovered  immediately 
and  corrected  before  they  result  in  large  errors  or  serious  loss  may 
result.  A  regular  system  of  inspection  and  testing  will  be  found  the 
best  safeguard  against  unsatisfactory  service  from  all  types  and  makes 
of  pyrometers. 


614  FORGING  TEMPERATURES  OF  LARGE  INGOTS 


Forging  Temperatures  and  Rate  of  Heating  and  Cooling  of  Large  Ingots 

BT   F.    E.   BASH,*  CH.  B.,    PHILADELPHIA,    PA. 
(Chicago  Meeting,  September,  1919) 

IN  recent  years,  there  have  been  a  number  of  experiments  conducted 
to  determine  the  rates  of  heating  and  cooling  of  various  sizes  and  shapes 
of  steel  ingots.  Up  to  date,  however,  most  of  the  published  data  have 
dealt  with  small  sized  ingots,  the  largest  being  an  18-in.  (46  cm.)  cube, 
the  data  on  which  were  presented  before  the  Iron  and  Steel  Institute 
in  May,  1918,  by  E.  F.  Law.  It  is  due  to  this  lack  of  information  on  the 
rate  at  which  large  ingots  absorb  heat  and  come  to  temperature  that 
the  heating  practice  varies  so  widely  in  different  plants.  One  of  the 
questions  over  which  there  is  much  debate  is  the  proper  rate  of  heating 
of  large  ingots  for  forging  and  the  time  actually  required  for  the  center 
of  a  mass  of  steel  to  come  to  forging  temperature. 

As  a  continuation  of  experiments  described  by  M.  E.  Leeds1  and  at 
the  request  of  Mr.  G.  R.  Norton  of  the  Sizer  Forge  Co.  and  Mr.  R.  C. 
Drinker  of  the  Emergency  Fleet  Corpn.,  the  test  described  in  this  paper 
was  carried  out  on  a  24-in.  (61  cm.)  round  ingot  and  the  rate  of  heating 
and  cooling  determined  under  regular  production  conditions.  The 
ingot  had  been  partly  forged  at  one  end  but,  on  developing  a  flaw, 
had  been  scrapped  so  that  it  was  available  for  test.  The  size  and  shape 
of  the  ingot  and  its  position  in  the  furnace  are  shown  in  Fig.  1. 

EXPERIMENTAL  INGOT 

This  ingot  was  prepared  as  follows:  The  end  of  the  24-in.  section 
that  was  the  top  end  when  it  was  cast  was  sawed  off  in  order  to  remove 
the  part  in  which  there  was  segregation,  as  it  was  not  possible  to  drill 
this  end  because  of  its  hardness.  Seven  holes  %  in.  (1.9  cm.)  in  diame- 
ter and  18  in.  (46  cm.)  deep  were  then  drilled  in  the  end  of  the  ingot, 
as  shown  in  the  drawing,  one  hole  }£  in.  (1.27  cm.)  from  the  top  surface, 
one  3^  in.  from  the  bottom,  one  in  the  center,  and  two  evenly  spaced 
between  the  center  and  the  top  and  the  center  and  the  bottom.  The 
holes  were  drilled  18  in.  deep  in  order  to  have  them  farther  from  the 


*  Research  Engineer,  Leeds  &  Northrup  Co. 

1  Some  Neglected  Phenomena  in  Heat  Treatment  of  Steel.    Proc.  Am.  Soc.  Teat. 
Mat.  (1915)  16. 


F.    E.    BASH 


615 


end  than  the  radial  distance  from  the  center  in  order  to  lessen  the  effect 
due  to  the  heat  penetrating  from  the  end. 

To  take  temperatures  along  the  length  of  the  surface  of  the  ingot, 
a  2-in.  (5  cm.)  iron  conduit  pipe  was  half  flattened  under  a  press, 
and  the  end  pinched  shut  and  welded  tight.  The  pipe  was  then  spot 
welded  to  the  ingot  directly  above  the  line  of  holes  in  the  end,  as  shown, 
thus  making  it  possible  to  push  thermocouples  of  different  lengths  down 
the  pipe  so  as  to  take  temperatures  as  near  as  possible  to  the  surface  of  the 
ingot.  As  a  check  on  the  gas  temperatures  near  the  ingot,  two  nichrome 


Coal  54  R.P.M.    Air  2 


No.10  Out  of  Orde 


Coal  i)6  R.P.M.    Air 


2400 

2'JOO 

2000 

1800 

1HOO 

1*H)  Q- 

1*10 

I'-IU 

fc.K» 

MO 


1.1(1 


•  Optical  Pryruii 

!_/  Readings 

4'«  -  K  Target  o»er  Production  Ingot 
°-M     ..         ..  Eiperimental  .. 
0-N-  ..      on  •• 

•  -         Reading  on      ••         » uear 
x-  0-  Back  Wall  by  Couple  12 

•-P- 13 

#-8-      ..        ..    Between  Insets 


Sections  (it  Expciimtntal  Inc< 


Legend  lor  Thermocouple  Curves 
-  No.1  Couple  Ingot  Temp.  — 
—  No.2 


—-,-  H 

Hi 

-— x-  W 


of  Ingot 


FIG.  1. 

targets  2  by  2  by  ^2  m-  were  welded  to  3^-in.  iron  rods  4  ft.  long  and 
spotted  to  the  ingot  in  the  positions  shown. 

FURNACE 

The  furnace  was  of  the  two-door  type  and  had  a  vertical  slot  4  in. 
(10  cm.)  wide  and  30  in.  (76  cm.)  long  in  the  back  directly  opposite 
the  experimental  ingot,  through  which  slot  the  thermocouples  could  be 
pushed  into  the  ingot.  On  both  sides  of  the  slot  and  21  in.  (53  cm.) 
from  its  center  were  mudded  in  two  usalite  porcelain  protecting  tubes 
for  platinum  platinum-rhodium  thermocouples,  which  were  to  record 
gas  temperatures.  One-inch  steel  rods,  with  nichrome  targets  2  in. 
square  and  ^2  in-  thick  welded  on  the  end,  were  suspended  from  the 


616  FORGING  TEMPERATURES  OF  LARGE  INGOTS 

roof  of  the  furnace,  one  directly  over  the  experimental  ingot  and  2  ft. 
above  it,  one  over  the  production  ingot,  and  one  opposite  the  center  of  the 
flue.  Peep  holes  were  arranged  in  the  walls  opposite  these  targets  so 
that  optical  pyrometer  readings  could  be  made  at  intervals  as  a  check  on 
gas  temperatures. 

In  order  to  calculate  the  heat  losses  from  the  furnace,  mercury  ther- 
mometers were  suspended  around  the  outside  of  the  walls  at  intervals 
and  the  bulbs  packed  against  the  brick  with  asbestos  wool.  To  take 
the  temperature  of  the  outside  of  the  roof,  a  thermocouple  of  No.  16 
B.  &  S.  gage  iron  and  constantan  was  prepared  and  the  hot  junction 
buried  in  the  layer  of  dust  on  the  top  of  the  furnace. 

THERMOCOUPLES  AND  APPARATUS 

To  take  the  ingot  temperatures,  thermocouples  were  prepared  as 
follows:  A  length  of  No  14  B.  &  S.  gage  chromel  wire  was  welded  to  an 
iron  plug  %  in.  (9.5  mm.)  in  diameter  and  approximately  1  in.  (2.5  cm.) 
long.  The  wire  was  then  threaded  through  porcelain  insulators  and 
drawn  through  a  length  of  %  in.  (1.5  cm.)  seamless  steel  tubing,  %  in. 
inside  diameter,  until  the  plug  was  flush  with  the  end  of  the  tube.  The 
steel  tube  and  plug  were  then  welded  over  smooth.  The  leads  from 
the  cold  ends  of  the  thermocouples  to  the  Leeds  &  Northrup  poten- 
tiometer recorder  were  of  No.  16  constantan  and  chromel,  bringing 
the  cold  junction  to  the  recorder.  A  calibration  curve  of  millivolts  vs. 
temperature  was  made  for  the  chromel-steel  couples  with  the  aid  of  a 
Bureau  of  Standards  checked  platinum  platinum-rhodium  thermocouple. 
Each  couple  was  checked  against  this  curve  before  the  test.  The  curve, 
which  is  an  unusual  one,  is  shown  in  Fig.  2. 

The  eleven  chromel  steel  couple  temperatures  were  recorded  by  one 
eight-point  recorder  and  three  single-point  curve-drawing  recorders  with 
a  range  of  0-16  millivolts.  The  gas  temperatures  were  recorded  by 
two  curve-drawing  recorders  for  platinum  thermocouples.  The  roof 
temperatures  were  taken  with  a  potentiometer  indicator  and  readings 
on  targets  and  inner  wall  and  ingot  surface  temperatures  were  taken  with 
a  Leeds  &  Northrup  optical  pyrometer,  which  is  the  disappearing  fila- 
ment type. 

TEST 

It  is  the  practice  of  the  Sizer  Forge  Co.  to  run  the  forging  furnaces, 
which  are  fired  with  powdered  coal,  all  week  and  to  shut  down  over 
Sunday.  On  Sunday  night,  the  furnaces  are  lighted  and  are  ready  to 
charge  on  Monday  morning.  The  test  was  ready  to  run  by  Monday 
morning  so  the  furnace  was  lighted  Sunday  night.  The  pulverized  coal 
is  fed  into  the  furnace  by  a  screw  operated  by  an  electric  motor,  the 


F.    E.    BASH 


617 


speed  of  which  may  be  varied  by  means  of  a  field  rheostat.  To  increase 
the  air,  a  gate  could  be  raised  in  the  air  pipe.  This  gate  was  roughly 
calibrated  so  that  the  heater  knew  approximately  where  to  set  his  gate 
opening  for  a  certain  coal  feed. 

The  experimental  ingot  was  charged  at  10 :30  A.  M.  and  the  door  was 
open  %  hr.  during  the  charging.  The  couples  were  inserted  in  the  ingot 
and  a  1-in.  (2.5  cm.)  conduit  pipe  18  in.  (46  cm.)  long  was  slipped  over 


2400 
2300 
2200 
2100 
2000 
1900 
1800 
1700 
1600 
1500 

«  140° 
8  1300 
0  1200 
1100 
1000 
900 
800 
700 
600 
500 
400 
300 
200 
100 
\i 

1 

1 

/ 

c 

HRC 

)ME 

L- 

RO 

1 

/ 

/ 

/ 

/ 

' 

^ 

^ 

/ 

/ 

/ 

/ 

> 

/ 

/ 

/ 

/ 

/ 

/ 

1 

\ 

\. 

6      7      8      9     10    11    12    13    14    15     16 
Millivolts 


FIG.  2. — CHROMEL-IRON  THERMOCOUPLE  CALIBRATION. 


each  couple  up  to  the  end  of  the  ingot.  The  space  between  the  couples 
was  filled  with  bricks  and  clay  to  separate  them.  The  1-in.  sleeves  used 
to  protect  the  couples  from  the  corrosion  of  the  furnace  gases  furnished 
an  opening  through  which  the  couples  could  be  withdrawn  at  the  close  of 
the  test.  The  production  ingot  was  charged  3  ft.  (0.9  m.)  at  11  A.  M.,  but 
as  they  were  not  able  to  procure  a  weight  to  balance  it,  it  was  not  com- 
pletely charged  until  12.45.  In  the  time  intervening,  the  door  was  closed 


618  FORGING   TEMPERATURES    OF   LARGE    INGOTS 

down  to  the  ingot  but  the  open  spaces  at  the  sides  of  the  ingot  were  not 
bricked  up.  The  furnace  was  started  at  a  coal  feed  of  39  r.p.m.  and  a 
gate  opening  of  2^  in.  (5.6  cm.).  Afterward,  these  were  increased  to 
54  r.p.m.  and  2%  in.  and  later  to  69  r.p.m.  and  3^  in.;  this  maximum 
feed  was  later  reduced. 

The  coal  leaves  a  fine  ash  that  settles  all  over  the  interior  of  the  furnace 
and,  where  it  comes  in  contact  with  iron  oxide,  slags  with  it.  The  ash  settles 
on  an  ingot  and  may  appear  a  good  deal  brighter  than  the  ingot  itself  and 
may  actually  be  hotter.  This  is  shown  by  a  reading  of  2202°  F.  (1207°  C.) 
on  the  end  of  the  production  ingot  with  the  ash  on  and  2061°  F.  (1128°  C.) 
on  the  surface  after  the  ash  was  scraped  off.  This  fact  may  cause  a 
heater  to  overestimate  the  temperature.  When  the  ingot  was  charged 
this  fine  ash  had  settled  all  over  the  iron  rods  suspended  from  the  roof 
and  made  them  appear  %  in.  larger  in  diameter  than  they  really  were; 
also  the  two  nichrome  targets  suspended  over  the  ingots  had  burned  off 
while  the  target  at  the  entrance  to  the  flue  was  partly  burned.  Readings 
with  the  optical  pyrometer  made  on  the  suspension  rods  on  the  side 
in  the  shadow  from  the  flame,  with  the  ash  clinging  to  them  and  with  the 
ash  scraped  off,  agreed  to  5°  F.  (2.7°  C.).  Readings  on  the  side  toward 
the  flame  were  considerably  higher  due  to  reflections. 

The  flame  was  a  pulsating  one  and  reached  beyond  the  experimental 
ingot  so  that,  on  making  a  reading  with  the  optical  pyrometer,  the  fila- 
ment appeared  alternately  light  and  dark.  The  readings  taken,  which 
appear  in  the  log,  show  the  effect  of  the  flame  on  the  temperature  read, 
this  flame  effect  amounted  to  from  12°  to  22°  F.  (6.6°  to  12.2°  C.). 
A  balance  was  made  with  the  optical  pyrometer  on  one  of  the  rods  through 
the  fine  ash  haze  in  the  furnace  and  then  the  coal  and  air  were  cut  off  and 
another  reading  made  directly  afterward;  this  agreed  exactly  with  the  first 
reading.  For  that  reason  we  can  say  that  there  is  no  noticeable  error  due 
to  the  ash  when  making  a  reading  through  it  with  the  optical  pyrometer. 
The  log  of  the  test  and  data  taken  in  addition  to  the  ingot  temperatures 
are  given  in  the  following  tables. 

According  to  the  curve  shown  in  Fig.  1,  after  7  hr.  heating  the  ingot 
had  reached  a  temperature  of  2370°  F.  (1299°  C.)  at  the  surface  and 
2287°  F.  (1253°  C.)  at  the  center;  this  was  considered  a  good  forging  tem- 
perature. The  cooling  curve  shows  that  if  forging  were  started  15  min. 
after  the  ingot  was  pulled  from  the  furnace  the  outside  temperature 
would  be  2125°  F.  (1163°  C.)  and  the  center  would  be  2280°  F.  (1249°  C.) ; 
the  temperature  5  in.  (12.7  cm.)  from  the  surface  would  be  2260°  F. 
(1238°  C.).  As  the  outside  temperature  drops  so  rapidly  and  the  cen- 
ter so  slowly,  there  can  be  a  temperature  difference  of  150°  F.  (83°  C.) 
between  the  two  in  the  furnace  and  by  the  time  it  is  ready  to  forge  or 
shortly  after  starting,  the  center  will  be  the  hottest  part  of  the  ingot. 


F.    E.   BASH 


619 


TABLE  1. — Log  of  Test 


Time 

Temperature,                                                        •R«~,O,L-. 
Degrees  F.                                                          Remarks 

A.  M. 

8:45                 2070 

O.  P."  Reading  in  porcelain  tube,  gas  temperature. 

10:30 

Charged  experimental  ingot. 

11:00 

Charged  production  ingot  (3  ft.),  not  bricked  up. 

11:15 

1845 

0.  P.  on  N1,  dark. 

11:15 

1857 

O.  P.  on  N1,  bright;  flame  effect,  12°  F. 

11:25 

1881 

O.  P.  on  M,  (rod),  dark. 

11:25 

1903 

O.  P.  on  M,  (rod),  bright;  flame  effect,  22°  F. 

P.  M 

12:30 

Changed  coal  feed  to  54  r.p.m. 

Changed  gate  opening  to  2%  in. 

2044 

O.  P.  011  N1,  light. 

2020 

O.  P.  on  N1,  dark. 

2005 

O.  P.  on  M,  dark. 

12:45 

Pushed  production  ingot  clear  in,  and  bricked  up. 

1:05 

Changed  coal  to  69  r.p.m. 

Changed  air  to  3%  in. 

1:50 

Coal  and  air  off  for  2  min. 

1:57 

2191 

O.  P.  on  N  just  before  power  off. 

1:58 

Power  off. 

2:01 

Power  on. 

2:02 

2080 

O.  P.  on  N  just  after  power  on. 

3:06 

Power  off. 

3:08 

Power  on. 

3:50 

2202         i  O.  P.  on  loose  ash  on  end  of  production  ingot. 

3:51 

2061           O.  P.  on  end  production  ingot,  clean. 

4:03 

Coal  to  56  r.p.m.,  air  to  2%  in. 

4:22 

Coal  to  47  r.p.m.,  air  to  2%  in. 

5:30 

Water  flowing  through  door  10^  lb.  in  20  sec.,  heated 

from  25°  to  55°  C. 

6:05 

Ingot  taken  out. 

0  Optical  pyrometer. 


TABLE  2. — Optical  Pyrometer  Readings,  in  Degrees  F. 


Time, 
Hours 

Target 
Center 
Flue 
J 

Center 
End 
Prod. 
Ingot 
Q 

Target 
Over 
Prod. 
Ingot 

Target 
Over 
Exp. 
Ingot 

Target 
on  Exp. 
Ingot 
Front 

N» 

Target 
on  Exp. 
Ingot 
Back 

N 

Back 
Wall  by 
Couple 
No.  12 
O 

Back 
Wall  by 
Couple 
No.  13 
P 

End 
Wall 
near 
Flue 
R 

Back 
Wall 
Center 

S 

Floor 
Com- 
bustion 
Cham- 
ber 
T 

3:17 

1993 

1773 

2120 

2229 

2191 

4:17 

2075 

1965 

2209 

2328 

2305 

2305 

2277 

2241 

2124 

2196 

2343 

* 

Front 

On 

wall 

ingot 

5:34 

2107 

2061 

2233 

2378 

2449 

2385 

2178 

2325 

2473 

6:34 

2140 

2164 

2265 

2350 

2346 

2309 

2174 

2265 

2370 

620  FORGING   TEMPERATURES   OF  LARGE   INGOTS 

TABLE  3. — Outside  Brick  Temperatures,  in  Degrees  F. 


Time, 
Hours  from  Start 

Between 
Doors 
A 

End  Wall 
C 

Back  Wall 
Com- 
bustion 
Chamber 
D 

Back  Wall 
near  End 
F 

Top  over 
Com- 
bustion 
Chamber 
G 

Top  over 
Exp.  Ingot 
H 

Top  over 
Prod. 
Ingot 

S 

2.50 

211 

222 

192 

177 

3.00 

360 

3.17 

... 

.  .  . 

.  .  . 

379 

3.50 

234 

236 

204 

190 

4.17 

... 

.  .  . 

.  .  . 

.  .  . 

366 

350 

4.50 

244 

246 

213 

196 

5.34 

... 

.  .  . 

.  .  . 

360 

5.50 

257 

255 

220 

206 

6.34 

.  .  . 

... 

.  .  . 

373 

6.75 

270 

265 

220 

212 

TABLE  4. — Outside  Ingot  Temperatures,  in  Degrees  F. 


Time, 
Hours 
from 
Start 

Top  Exp. 
Ingot  1  Ft. 
from  Door 
U 

Top  Exp. 
Ingot  2  Ft. 
from  Door 
V 

Top  Exp. 
Ingot  3  Ft. 
from  Door 
W 

Top  Prod. 
Ingot  1  Ft. 
from  Door 
X 

Top  Prod. 
Ingot  2  Ft. 
from  Door 
Y 

Top  Prod. 
Ingot  3  Ft. 
from  Door 
Z 

3:83 

249 

157 

123 

5:50 

.  .  . 

227 

143 

117 

6:25 

338 

191 

•  142 

COAL  RECORD 

POUNDS 
10:30-12:30:  39  r.p.m.  =  9.7  Ib.  per  min.,  120  min.  at  9.7  Ib.  per  min.. . .  =  1164 

12:30-1:05:  54  r.p.m.  =  14.4  Ib.  per  min.,  35  min.  at  14.4  Ib.  per  min =     504 

1:05-4:03:  69  r.p.m.  =  21.53  Ib.  per  min.,  178  min.  (shutdowns  at  1:58- 

2:11  and  3:06-3:08,  15  min.),  163  min.  at  21.53  Ib.  per  min =  3509.39 

4:03-4:22:  56  r.p.m.  =  15.1  Ib.  per  min.,  19  min.  at  15.1  Ib.  per  min =    286.9 

4:22-6:05:  47  r.p.m.  =12.1  Ib.  per  min.,  103  min.  at  12.1  Ib.  per  min =  1246.3 


Total  coal 6710. 59 

The  heating  curves  of  the  ingot  from  the  room  temperature  to,  ap- 
proximately, 950°  F.  (510°  C.)  are  estimated  and  dotted  in.  In  the  case 
of  couples  No.  1  and  7  it  is  possible  that  the  temperature  came  up 
more  rapidly  than  shown.  The  time  after  charging  the  ingot  to  the 
beginning  of  the  record  was  taken  up  in  inserting  the  couples  and  brick- 
ing up  the  slot  through  which  they  were  inserted. 

The  platinum  platinum-rhodium  thermocouples  with  the  usalite  pro- 
tecting tubes  registered  gas  temperatures  very  accurately  and  did  not 
seem  to  be  affected  by  the  proximity  of  the  ingot.  This  is  shown  by 
the  readings  with  the  optical  pyrometer  on  the  suspended  targets.  The 


F.    E.   BASH 


621 


heating  curves  show  that  optical  readings  on  the  target  above  the  experi- 
mental ingot  show  gas  temperatures  about  20°  F.  higher  than  that  re- 
corded by  the  thermocouple  at  the  side  of  the  ingot  next  to  the  flame, 
and  that  readings  on  the  target  spotted  to  the  ingot  are  about  20°  F. 
lower  than  the  thermocouple.  This  is  good  proof  that  the  thermocouple 
indicates  the  gas  temperature  as  it  is  the  mean  of  the  two.  Optical 
pyrometer  readings  on  the  target  suspended  over  the  26-in.  production 
ingot  check  very  closely  with  the  temperatures  recorded  by  the  platinum 


2400 
2300 
2200 
2100 
2000 
1900 
1800 
1700 
1600 
1500 


\   \N 

\    \ 


A 


\ 


Cool  ng  Curves  for  24 


inches    ngot 


\ 


Cooled  in 


Air 


\ 


1300 
1200 


1000 
900 
800 
700 
600 
500 
400 
300 
200 
100 
0 


Time  Hours 


3456 

FIG.  3. — COOLING  CURVES. 


10 


thermocouple  at  the  side  of  the  experimental  ingot  away  from  the  flame, 
which  is  reasonable  to  believe.  Optical  readings  made  on  the  top  of 
the  experimental  ingot  near  the  end  of  couple  No.  9  check  very  closely  to 
the  temperature  recorded  by  the  couple;  this  indicates  that  the  optical 
pyrometer  reads  the  true  surface  temperature  of  the  ingot. 

Two  optical  readings  made  on  the  front  wall  after  the  rod  suspended 
from  the  roof  had  burned  out  do  not  show  any  relation  to  the  ingot  tem- 
perature, but  readings  made  on  the  back  wall  between  the  two  ingots 


622  FORGING   TEMPERATURES   OF   LARGE   INGOTS 

and  near  couple  No.  13  check  the  couple  readings  fairly  closely,  the 
greatest  difference  being  20°  F.  Readings  on  the  back  wall,  on  the  flame 
side  of  the  experimental  ingot,  agree  with  the  gas  temperature  as  shown 
by  thermocouple  No.  12  to  40°  F.  It  would  seem  from  this  that  a 
reading  with  the  optical  pyrometer  on  the  back  wall  at  the  height  of  the 
ingot  from  the  floor  will  give  the  gas  temperature  at  that  point  to  within 
30°  to  40°  F. 

With  the  exception  of  the  time  at  which  the  ingot  was  going  through 
the  critical  period,  the  temperatures  along  the  surface  of  the  ingot 
always  fell  between  the  temperatures  of  the  gases  on  both  sides.  The 
point  nearest  the  door,  naturally,  was  the  coolest,  the  points  about  the 
middle  of  the  furnace  were  the  hottest,  and  the  point  nearest  the  back 
wall  was  somewhat  cooler  than  the  center  of  the  furnace.  The  drop  in 
the  temperature  of  the  gases  from  one  side  of  the  ingot  to  the  other 
averages  about  125°  F. ;  this  represents  the  heat  lost  by  the  gas  and  ab- 
sorbed by  the  ingot.  The  temperature  drop  of  the  gas  from  the  combus- 
tion chamber  to  the  flue  averaged  from  230°  to  266°  F. ;  the  heat  in  the 
flue  gases  was  used  to  evaporate  water  in  waste  heat  boilers  and  so  was 
conserved.  The  temperature  gradient  along  the  ingot  from  the  portion 
just  outside  the  door  to  the  inside  is  very  great,  as  is  shown  in  Table  4. 
The  stresses  set  up  at  this  section  must  be  very  large  and  injurious. 

The  coal  used  amounted  to  6710  Ib.  (3043  kg.).  This  was  calculated 
from  the  revolutions  of  the  screw  feed  which  had  previously  been  cali- 
brated. Taking  the  weight  of  the  experimental  ingot  heated  as  13,083 
Ib.  (5934  kg.)  and  the  production  ingot  as  18,050  Ib.  (8187  kg.),  which  we 
will  estimate  was  0.8  heated,  the  total  weight  of  steel  this  amount  of  coal 
will  heat  when  fired  in  this  manner  would  be  27,533  Ib.  (1262  kg.),  or  the 
rate  of  using  coal  will  be  0.244  Ib.  (0.110  kg.)  of  coal  per  pound  of  steel. 
The  rate  was  really  less  than  this  as  the  ingot  was  hot  in  6^  hr.  and  the 
coal  is  calculated  to  7^  hr.  To  heat  a  24-in.  (61-cm.)  ingot  in  7  hr.,  in 
this  type  and  size  of  furnace  with  this  kind  of  fuel,  means  that  the  coal 
should  be  fired  at  the  rate  of  16  Ib.  (7.25  kg.)  per  min.  To  bring  it  to 
temperature  in  a  longer  time,  the  coal  must  be  fed  more  slowly  and  more 
coal  will  be  needed;  how  much  more  will  depend  on  the  radiation  losses. 
The  heating  value  of  the  coal  used  was  13,000  B.t.u.  per  Ib.  so  that  the 
heat  developed  was  87,230,000  B.t.u.  The  heat  absorbed  by  the  steel 
was  approximately  7,270,000  B.t.u.,  the  percentage  of  the  total  heat 
absorbed  by  the  steel  being  8.32  per  cent. 

When  the  ingot  was  pulled  from  the  furnace  and  laid  on  the  ground, 
four  couples  were  replaced  in  the  ingot  and  the  cooling  curve  taken.  The 
center  of  the  ingot  showed  the  passage  through  the  critical  point  more 
markedly  than  any  other  point,  as  was  also  the  case  in  heating.  The 
recalescence  and  decalescence  points  were  at  practically  the  same  tem- 
perature.' After  the  ingot  had  cooled  to  750°  F.  (399°  C.)  all  the  points 


F.    E.   BASH  623 

in  it  from  %  in.  (1.27  cm.)  under  the  surface  to  the  center  cooled  together 
and  were  all  at  the  same  temperature  as  they  cooled. 

FORGING  TEMPERATURES 

In  Table  5  are  given  the  temperatures  read  on  three  four-door  furnaces 
at  the  Sizer  Forge  Co.,  with  the  Leeds  &  Northrup  optical  pyrometer. 
The  furnaces  were  lettered  J,  K,  and  L,  and  were  so  arranged  that  the 
flames  were  from  left  to  right  on  L,  right  to  left  on  K,  and  left  to  right  on  J. 
J  had  a  bridge-wall  in  the  combustion  chamber,  but  K  and  L  had  none. 
The  doors  were  numbered  consecutively,  beginning  with  the  one  next  to 
the  burner. 

TABLE  5. — Temperatures  on  Forging  Furnaces 

TTU..  TEMPERATURE,  PwiwAnK-a 

TlME     DEGREES  F. 

11 :55  1525  Finished  forging  26  in.  ingot  to  13  in.,  surface  reading. 

.    12:08  2477  On  surface  No.  1  K,  roll  ingot. 

12:10  2511  Back  wall  right  of  ingot  No.  1  K,  flame  right  to  left. 

12: 11  2350  Back  wall  left  of  ingot  No.  2  K,  flame  right  to  left. 

12: 12  2221  Back  wall  right  of  ingot  No.  3  K,  flame  right  to  left. 

12 : 12  Pull  ingot  from  No.  2  L. 

12 : 13  2169  Back  wall  right  of  ingot  No.  4  K. 

12:14  Charge  above  ingot  in  No.  1  L. 

12 : 17  2245  Back  wall  No.  2  L,  door  open  from  12 : 12. 

12:19  Charge  cold  ingot  in  No.  2  L. 

12:22  Coal  off  of  K  furnace,  air  about  1%  in.  opening. 

12:35  Bricking  up  Nos.  1  and  2  L,  coal  off  and  air  on. 

12:32  2505  Floor  of  combustion  chamber  of  K  furnace,  coal  on 

12 : 34  2508  Wall  of  combustion  chamber  of  K, 

12 :36  2529  Right  of  No.  1  K,  wall;  flame  right  to  left. 

12:36  2392  Left  of  No.  1  K,  wall;  flame  right  to  left. 

12:37  2392  Right  of  No.  2  K,  wall;  flame  right  to  left. 

2346  Left  of  No.  2  K,  wall;  flame  right  to  left. 

12 :38  2346  Right  of  No.  3  K,  wall;  flame  right  to  left. 

2289  Left  of  No.  3  K,  wall;  flame  right  to  left. 

12 :39  2289  Right  of  No.  4  K,  wall;  flame  right  to  left. 

2265  Left  of  No.  4  K,  wall;  flame  right  to  left. 

1 : 09  2403  Ready  to  pull  26  in.  ingot  in  No.  1  K. 

1 : 10  2580  Right  N.IK,  wall.. 

2343  Scale  on  ingot  30  sec.  after  being  drawn  from  furnace. 

2111  Scale  in  groove  under  press. 

1:12  2140  Clean  spot. 

1 : 16  2015  Clean  spot. 

1:19  1851  Groove. 

1:20  1919  Second  groove,  clean. 

1:22  1886  Third  groove,  clean. 

1 :24  1990  Deep  groove  under  cutter,  clean;  approximately  6  in.  deep. 

1 :26  2015  Deep  groove  under  cutter,  clean;  approximately  6  in.  deep. 

1 : 32  1930  Groove,  light  scale. 

1:41  1685  Small  groove. 

1:44  1706  Deep  groove. 

1:50  1741  Deep  groove  near  end. 


624  FORGING   TEMPERATURES   OF  LARGE  INGOTS 

TABLE  5. — Temperatures  on  Forging  Furnaces — (Continued) 

T          TEMPERATURE,  PKMAWICB 

1IME       DEGREES  F. 

1:50       1767  Deep  groove  third  from  end. 

1:56       1663  Corner. 

2:00       1373  Outside  24  in.  section. 

1576  Corner. 

2:08       1315  Finish  24  in.  section. 

1576  Finish  13^  in. 

2:14       1407  Finish  24  in. 

1407  Finish  13^  in. 

2:22  Stop  working. 

1:58       2460  Right  No.  2  K,  No.  1  empty,  flame  right  to  left. 

2:03       2423  Left  of  No.  2  K,  flame  right  to  left. 

2:26       2385  Left  of  No.  1  L;  flame  left  to  right. 

2285  Right  of  No.  1  L. 

2233  Right  of  No.  2  L. 
No.  3  empty. 

2233  Left  of  No.  4  L. 

2 : 30       2221  Right  of  No.  4  L. 

2:32       2442  Combustion  chamber  of  K. 

J  Furnace 

3:56       2403     Bridge-wall;  flame  left  to  right. 
2321     No.  2  J  left. 
2249     No.  2  J  right. 
2124     No.  3  J  right. 

No.  1  J  and  No.  4  J  empty. 
2148    No.  4  J,  back  wall. 

L  Furnace 

4: 00       1863  Left  of  No.  4  L,  flame  left  to  right. 

1913  Right  of  No.  4  L. 

2039  Right  of  No.  3  L. 

2148  Right  of  No.  2  L. 

2128  Right  of  No.  1  L. 

4:05       2221  Combustion  chamber,  wall. 

The  drop  in  temperature  from  one  end  to  the  other  of  the  four-door 
furnaces  on  which  readings  were  taken  range  from  164°  to  308°  F.,  de- 
pending on  the  length  of  time  the  ingots  have  been  in,  how  fast  the  coal 
is  being  fired,  and  the  type  of  furnace. 

CONCLUSIONS 

It  is  possible  to  heat  a  24-in.  ingot  from  room  temperature  to  forging 
temperature  in  7  hr.  but  the  question  is  raised  whether  this  fast  rate  is  not 
injurious  to  the  steel,  especially  while  the  steel  is  still  comparatively 
cold,  large  stresses  being  set  up  which  may  cause  internal  fissures.  The 
rate  at  which  an  ingot  can  be  heated  without  injury  depends  on  the 
kind  of  steel,  chrome  steel  being  very  tender  while  low-carbon  steel 
will  stand  more  abuse.  Opinions  differ  on  the  length  of  time  necessary 
to  bring  this  size  of  ingot  to  temperature,  but  the  best  practice  appears  to 
indicate  slower  heating  up  to  the  critical  temperature.  The  question  is  a 
difficult  one  to  settle,  however,  as  it  depends  on  a  number  of  variables. 

It  is  difficult,  if  not  impossible,  to  calculate  the  stresses  set  up  at  any 


F.    E.    BASH  625 

one  point  in  an  ingot,  due  to  unequal  expansion,  and  in  that  way  deter- 
mine what  the  maximum  allowable  difference  in  temperature  between 
the  outside  and  the  center  will  be.  If  this  could  be  done,  it  would  be  a 
simple  matter  to  prescribe  the  rate  of  heating  that  any  size  ingot  should 
have.  The  main  source  of  information  is  the  experience  of  steel  men 
through  years  of  practice  on  forging  large  ingots. 

The  results  of  the  test  show  that  an  optical  pyrometer  can  be  used 
to  determine  when  an  ingot  is  ready  to  forge.  To  make  sure  that  there 
is  no  error  due  to  loose  scale  or  ash,  it  is  well  to  push  a  bar  in  the 
furnace  ati.d  clean  the  spot  on  which  the  pyrometer  is  to  be  sighted. 
This  being  done,  if  the  surface  temperature  is  100°  F.  higher  than  the 
temperature  at  which  it  is  desired  to  forge,  the  ingot  is  ready  to  pull  out. 
For  instance,  if  the  forging  temperature  is  2250°  F.  (1232°  C. ),  should  the 
optical  pyrometer  read  2350°F.(1287°C.),the  ingot  is  ready  to  forge .  The 
outside  temperature  of  the  ingot  will  drop  very  rapidly  in  the  air  and 
leave  the  center  the  hottest  portion;  for  a  24-in.  ingot,  the  difference 
between  the  outside  and  center  is  about  100°  F. 

The  optical  pyrometer  can  be  used  to  determine  gas  temperatures 
approximately  by  sighting  on  the  bricks  of  the  back  wall  or  by  sighting 
on  a  target  or  in  a  tube.  Usalite  porcelain  stands  the  forging-furnace 
temperature  very  well  and  quickly  changes  temperature  with  the  gases, 
as  will  a  target  of  thin  metal.  The  target,  however,  does  not  stand  the 
corrosive  action  of  the  gases  for  any  length  of  time,  a  nichrome  target 
lasting  to  a  temperature  between  2200°  F.  (1204°  C.)  and  2300°  F.  (1260° 
C.)-  The  error  due  to  making  a  reading  through  a  light  flame  is  ap- 
proximately 20°  F.  Readings  through  light  ash  from  a  powdered-coal 
flame,  with  an  optical  pyrometer,  do  not  appreciably  affect  the  tem- 
perature read. 

A  couple  on  the  surface  of  a  large  ingot  may  indicate  a  temperature 
approximately  that  of  the  gas  and  much  above  that  of  a  point  in  the 
steel  %  in.  under  the  surface. 

The  temperature  gradient  along  the  ingot  from  the  door  to  a  short 
distance  inside  the  door  is  very  large  and  must  create  serious  stresses  at 
that  point. 

From  the  rate  of  cooling  of  the  24-in.  ingot,  it  appears  that  it  can  be 
worked  for  2  hr.  without  working  it  too  cold.  This  may  not  actually  be  the 
case  as  the  thick  scale  drops  off  and  the  piece  is  worked  down  to  smaller 
dimensions  where  it  will  cool  more  rapidly.  On  the  other  hand,  working 
heats  the  piece  and  will  tend  to  counterbalance  more  rapid  cooling. 

The  amount  of  coal  used  in  this  test  was  approximately  0.25  Ib.  per 
pound  of  steel.  The  average  amount  of  coal  used  for  production  work 
at  this  plant  was  1  Ib.  per  pound  of  steel.  This  shows  that  the  furnaces 
can  be  fired  much  more  efficiently. 

In  conclusion  the  writer  wishes  to  thank  Mr.  G.  R.  Norton  and  Mr. 
R.  C.  Drinker  for  their  hearty  cooperation  and  interest  in  this  investigation. 

40 


626 


FORGING  TEMPERATURES  OF  LARGE  INGOTS 


DISCUSSION 

LAWFORD  H.  FRY,  Burnham,  Pa.  (written  discussion*). — As  a  sup- 
plement to  the  information  given  by  Mr.  Bash,  a  diagram  is  submitted 
showing  the  results  of  a  somewhat  similar  experiment,  carried  out  at  the 
ordnance  plant  of  the  Standard  Steel  Works  Co.  In  this  case  the  ingot 
to  be  heated  was  a  nickel-steel  octagon  ingot,  weighing  14,700  Ib.  (6667 
kg.).  This  ingot  was  to  be  forged  into  two  255-mm.  howitzer  jackets. 
A  hole  was  bored  half  way  of  the  length  of  the  ingot,  extending  into  the 
longitudinal  axis,  and  a  base-metal  thermocouple  inserted  in  this  hole. 


1         2         34         5        6         1  •      8         9        10       11       12       13      14       15       16       17 
Time  in  Hours 

FIG.   4. — HEATING  EXPERIMENT  WITH  30-iN.  CONCAVE  OCTAGON  INGOT  N-85-B  IN 

FORGE   SHOP  FURNACE. 

In  order  to  obtain  the  temperature  of  the  exterior  of  the  ingot,  a  similar 
couple  was  placed  in  the  hole  in  a  cylindrical  lag  block,  11  in.  (28  cm.)  in 
diameter,  weighing  300  Ib.  This  couple  is  the  A-couple  in  the  diagram, 
the  couple  in  the  ingot  being  the  B-couple.  The  exterior  or  A-couple 
reached  2000°,  which  was  the  maximum  temperature  registered  by  the 
pyrometer  in  10  hr.,  while  the  couple  at  the  center  of  the  ingot  took 
approximately  2^£  hr.  longer  to  come  to  this  temperature.  The  exterior 
temperature,  measured  by  an  optical  pyrometer,  was  approximately  2 100° 
at  the  end  of  17  hr.  The  experiment  was  carried  out  in  connection  with 
the  discussion  of  the  proper  time  for  heating  large  ingots  for  gun  forgings 
and  the  results  were  presented  to  the  Gun-Howitzer  Club  in  September, 
1918. 

*  Received  Sept.  30,  1919. 


TEMPERATURES    OF   INCANDESCENT-LAMP    FILAMENTS  627 


Temperatures  of  Incandescent-lamp  Filaments 

BY  BBNJ.    E.    SHACKELFORD,  *    PH.   D.,   BLOOMFIELD,    N.    J. 
(Chicago  Meeting,  September,  1919) 

THE  present  paper  is  concerned  with  typical  temperature  values 
experienced  in  lamp-filament  measurements  as  made  on  regular  factory 
and  engineering  products.  It  deals  with  ^he  relations  existing  between 
temperature,  efficiency,  lamp  size,  and  life  of  incandescent  lamps, 
in  so  far  as  they  affect  the  rating  of  the  product  and  its  use  by  the  indi- 
vidual consumer. 

The  temperatures  measured  lie  in  that  range  covered  only  by  the 
general  method  of  radiation  pyrometry,  as  opposed  to  direct  methods 
used  under  contact  conditions.  The  range  of  temperatures  ordinarily 
experienced  extends  from  about  2125°  K.,  for  the  now  almost  extinct 
carbon-filament  lamp,  to  3200°  K.  for  the  comparatively  new  tungsten- 
filament  motion-picture  lamp.  Most  of  the  more  common  sizes  of 
tungsten  lamps  have  temperatures  ranging  from  2500°  to  3000°  K. 
Because  of  the  high  temperatures  involved  and  the  relatively  small 
area  of  the  sources  used,  we  are  practically  restricted  to  two  methods 
of  measurement,  both  of  which  depend  on  the  light  radiated  from  the 
filament.  The  first  and  more  usual  method  is  that  of  the  Morse  pyrome- 
ter, dependent  on  the  variation  in  temperature  of  the  amount  of  light 
of  a  given  color  range  yielded  by  the  filament.  In  this  case,  the  measured 
temperature  is  that  of  a  small  part  of  the  incandescent  body,  that  is, 
the  part  which  is  focused  on  the  comparison  filament.  The  second 
method,  known  usually  in  this  country  as  that  of  "color  match"  and  in 
England  as  "color-identity"  is  based  on  the  color  of  the  total  light 
yielded  by  the  filament,  the  comparison  being  facilitated  by  the  use  of 
an  ordinary  photometer  head. 

-A  more  definite  idea  of  the  temperatures  involved  in  the  case  of 
filaments  of  various  vacuum  and  gas-filled  lamps  is  given  by  Fig.  1. 
The  size  of  the  lamp,  in  watts,  is  plotted  along  the  horizontal  axis  and 
the  temperature  of  the  filament  is  plotted  along  the  vertical  axis.  The 
lamps  concerned  are  regular  product,  rated  for  1000  hr.  life.  The  tem- 
perature values  for  the  old  16-candlepower  carbon  lamp  and  the  new 
motion-picture  lamp  are  shown  by  the  cross  and  circle  respectively. 
The  life  of  the  latter  lamp  is  100  hr.  Lamps  for  ordinary  lighting 
service  yield,  on  the  average,  1000  hr.  of  useful  life;  that  is,  they  continue 

*  Physicist,  Westinghouse  Lamp  Works. 


628 


TEMPERATURES    OF   INCANDESCENT-LAMP    FILAMENTS 


to  give  at  least  80  per  cent,  of  their  original  light  for  that  length  of  time. 
A  vacuum  lamp  usually  fails  from  blackening,  that  is,  from  the  deposit 
of  the  tungsten  on  the  bulb  surface.  Since  the  smaller  filaments  are 
more  affected  by  the  loss  of  material  than  are  the  larger  ones,  it  is  neces- 


2100 


200 


300 


700    800 


900   1000 


400     600     600 
Wattage 

FIG.  1. — TEMPERATURE  vs.  WATTAGE  FOR  1000  HR.  LIFE. 

sary  to  run  the  former  somewhat  cooler  than  the  latter,  in  order  that 
they  may  not  fail  before  they  reach  the  required  hours. 

In  the  case  of  gas-filled  lamps,  conditions  are  somewhat  more  acute. 
The  presence  of  the  gas  lowers  the  vaporization,  and  carries  the  de- 


au 
19 
18 
17 
16 

2  »S 
*14 

I* 

a13 
S« 

V 

en 

9 
->  10 

9 
8 
7 

/ 

/ 

f 

MAZDA-C/ 

/ 

X 

/ 

/ 

/ 

/ 

MAZDA--B 

/ 

X 

X 

2300 


2400 


2500 


28*0 


2900 


3000 


FIG.  2. — TEMPERATURE  vs.  EFFICIENCY,  100-WATT  MAZDA  B,  500- WATT  MAZDA  C 

(ARGON). 

posited  tungsten  into  a  part  of  the  bulb  where  it  is  relatively  unobjec- 
tionable. Therefore,  the  filaments  of  most  lamps  are  operated  at  such 
a  temperature  that  they  will  have  a  burnout  life  of  1000  hr.  Since 


BENJ.    E.    SHACKELFOBD 


629 


these  lamps,  therefore,  are  operated  at  a  higher  temperature  than  the 
vacuum  lamps,  any  loss  of  material  becomes  more  effective  in  hasten- 
ing burnout.  The  small  filaments  are  less  able  to  stand  this  effect  than 
they  were  in  the  vacuum  lamps,  and  consequently  the  curve  is  steeper. 

With  a  given  lamp,  the  higher  the  temperature  of  the  filament,  the 
higher  is  the  efficiency  of  the  lamp.  This,  of  course,  is  due  largely  to 
the  fact  that  as  the  temperature  is  raised  a  larger  part  of  radiated  energy 
comes  within  the  visible  spectrum.  Fig.  2  shows  the  relation  between 
the  temperature  and  efficiency,  in  lumens  per  watt,  for  one  size  of  vacuum 
and  one  size  of  gas-filled  lamp.  Where,  as  here,  we  are  considering  the 
efficiency  of  the  lamp  as  a  whole,  this  curve  will  shift  somewhat  in  going 
from  one  lamp  size  to  another.  In  the  gas-filled  lamps  there  is  also  a 
shift  from  lamp  to  lamp,  due  largely  to  the  dependency  of  the  cooling 

we 


600 


500 


3  400 


£300 


200 


100 


X 


\ 


10  11  12  13  14  15  16  17 

Lumens  per  Watt, 

FIG.  3. — VARIATION  OF  LIFE  WITH  EFFICIENCY. 


18 


effect  of  the  gas  on  the  pressure  of  the  gas  and  the  concentration  of  the 
filament.  This  relation  between  the  efficiency  and  temperature  gives 
another  method  of  measuring  temperature,  which  is  very  useful  to  the 
lamp  manufacturer. 

Since  with  increased  temperature  comes  increased  efficiency,  the 
obvious  tendency  is  to  raise  the  temperature  of  the  filament  to  the 
highest  point  consistent  with  proper  life.  Fig.  3  shows  the  relation 
between  efficiency  and  life  for  ordinary  sizes  of  vacuum  lamps.  The 
actual  operating  temperature  is  governed  by  a  proper  balancing  of  the 
two  relationships,  the  increase  of  efficiency  and  the  shortening  of  life 
with  increasing  filament  temperature. 

Another  important  practical  fact  is  that  as  the  size  of  the  lamp 


630 


TEMPERATURES    OF   INCANDESCENT-LAMP    FILAMENTS 


is  increased,  the  temperature  necessary  for  a  given  efficiency  is  lessened. 
An  instance  of  this  is  shown  in  Fig.  4.  Particularly  in  the  case  of  gas- 
filled  lamps  it  is  noticed  that  for  the  low  wattages,  the  temperature 
necessary  for  this  given  efficiency  is  very  much  higher  than  in  the  case 
of  the  higher  wattages.  This  is  due  to  the  fact  that  the  relative  energy 
loss  due  to  gas  cooling  is  much  greater  with  small  filaments  than  with 
large.  Practically  this  means  that  there  is  a  size  below  which  the  gas- 
filled  lamp  is  less  efficient  than  the  vacuum  lamp.  This  is  shown  explic- 
itly in  Fig.  5,  where  the  ordinary  efficiencies  of  the  various  wattage 
lamps  are  indicated.  It  will  be  noticed  that  in  the  case  of  the  gas-filled 
lamps  particularly,  the  larger  sizes  are  more  efficient.  For  this  reason, 


Temperature,  Degrees  1C. 

\ 

\ 

\ 

s 

s 

\ 

\ 

* 

\ 

\ 

s 

"X 

^ 

"*••• 

-  — 

2660 

100    200    300    400    500    600    700    800    900 
Wattaee 

1(KX 

FIG.  4. — VARIATION  OF  TEMPERATURE  WITH  LAMP  SIZE. 

among  others,  large  lamps  are  much  more  economical,  for  the  same 
amount  of  light,  than  are  small  ones. 

The  100-watt  vacuum  lamps  operate  at  a  temperature  of  2500°  K. 
when  set  up  at  an  efficiency  of  10  lumens  per  watt,  or  1  watt  per  hori- 
zontal candle.  A  nitrogen-filled  lamp  at  the  same  efficiency  will  oper- 
ate at  a  temperature  of  2900°  K.,  and  yet  will  have  a  somewhat  longer 
life  than  the  vacuum  lamp.  When  the  nitrogen  is  replaced  by  argon, 
the  temperature  at  the  same  efficiency  is  only  2800°  K.  and  the  life 
is  much  better  than  in  either  of  the  other  two  cases — because  of  the 
lower  thermal  conductivity  of  argon.  This  gas,  however,  is  much  more 
expensive  than  nitrogen  and  the  lamp  manufacturers  expend  annually 
hundreds  of  thousands  of  dollars  additional  for  the  more  useful  gas. 
This  expenditure  results  in  a  saving  to  the  consumer  of  about  twice 
the  above  mentioned  amount. 


BENJ.    E.    SHACKELFORD 


631 


The  lead-in  wires  and  supports  naturally  cool  the  filament  a  great 
deal  and  lessen  the  efficiency  of  a  lamp.  The  distribution  of  tempera- 
ture along  a  filament  is  shown  in  Fig.  6.  These  measurements  were 


100    200    300     400     500     600    700     800 
»  '  Watts 

FIG.  5. — VARIATION  OP  EFFICIENCY  WITH  LAMP  SIZE,  1000-HR.  LIFE. 


made  on  the  comparatively  short  coils  of  street  series  lamps  and  show 
the  different  amount  of  cooling,  due  to  varying  amounts  of  contact 
by  the  support  wires. 


1  —  -^ 

r 

"*  , 

, 

\ 

/ 

s-  • 

\ 

/" 

^ 

\ 

/I 

i 

^ 

i 

\ 

/ 

\ 

/ 

i 

\ 

\ 

/ 

j 

/ 

Q 

Lead 

. 

i. 

/ 
1 

"3 

i 

I 

2550 

Distance  along  Filament 
FlG.    6. — 6.6  AMP.  STREET  SERIES  LAMP,  FOUR  SECTIONS. 

Incandescent-lamp  filaments  offer,  not  only  some  of  the  most  in- 
teresting fields  for  optical  pyrometry  investigation  of  various  types, 
but  also  they  offer  one  of  the  most  available  and  satisfactory  groups 
of  sources  for  such  investigations.  In  other  words,  they  offer  both 
the  problem  and  the  means  of  solution. 


632      TEMPERATURE   MEASUREMENTS   OF   INCANDESCENT  GAS   MANTLES 


Temperature  Measurements  of  Incandescent  Gas  Mantles 

BY    HERBERT   E.    IVES,    PH.    D.,    PHILADELPHIA,    PA. 
(Chicago  Meeting,  September,  1919) 

THE  incandescent  gas  mantle  is  of  considerable  interest  from  the 
standpoint  of  temperature  measurement  because  it  presents  a  series  of 
apparent  contradictions  to  the  established  laws  of  radiation  on  which 
are  based  some  of  our  best  methods  of  temperature  determination. 
One  of  these  contradictions  is  that  the  mantle  of  least  brightness  (of  the 
commercial  thoria-ceria  group)  is  the  one  having  the  highest  temperature; 
this,  though  explicable  without  any  violation  of  radiation  laws,  was 
long  a  stumbling  block  to  the  understanding  of  the  performance  of  the 
mantle.  Another  anomaly  is  that  the  energy  radiated  by  the  mantle 
decreases  with  the  rise  of  temperature,  thus  apparently  invalidating  total 
radiation  methods  of  pyrometry,  based  on  the  fourth-power  law.  The 
discussion  of  methods  of  measuring  mantle  temperature  which  follows 
is  largely  taken  from  an  extensive  study  of  the  physics  of  the  Welsbach 
mantle.1 

The  incandescent  gas  mantle  consists  of  a  skeleton  of  refractory 
oxide  of  very  light  weight  and  open  structure,  formed  by  the  ignition  of 
a  cotton  or  silk  "stocking"  previously  thoroughly  impregnated  with 
salts  of  certain  rare  earths.  The  mantle  of  commerce  is  a  mixture  of 
approximately  99  parts  of  thorium  oxide  with  1  part  of  cerium  oxide. 
This  mixture,  discovered  largely  by  accident,  gives  luminous  radiation 
many  times  greater  than  that  from  either  of  the  constituents  taken 
alone.  It  is  customary  to  speak  of  the  thoria  as  the  "base"  and  the 
ceria  as  the  "colorant,"  and  the  commercial  mantle  represents  one  of  a 
family  in  which  an  oxide  of  low  emissive  power  is  employed  as  a  base, 
which  will  assume  a  high  temperature,  while  a  small  amount  of  some 
other  oxide  of  high  visible  emissive  power  is  added,  which  will  reduce 
the  temperature  of  the  mantle  but  little.  This  is,  in  general,  the  most 
efficient  way,  from  the  standpoint  of  visible  emission,  to  utilize  a  sub- 
stance of  high  emissive  power  in  the  visible  spectrum.  But  it  is  not 
necessary  that  two  substances  should  be  employed  to  produce  a  radiator 
of  high  visible  and  low  general  emission.  Some  substances,  of  which 
lanthana  is  the  best  example,  possess  these  characteristics  naturally; 
it  merely  happens  that  the  Welsbach  mixture  is  as  yet  the  most  efficient 
radiator  of  this  type  known. 

1  Ives,  Kingsbury  and  Karrer:  A  Physical  Study  of  the  Welsbach  Mantle.  Jnl. 
Frank.  Inst.  (Oct.  and  Nov.,  1918)  186,  401,  585. 


HERBERT    E.    IVES  633 

The  methods  of  temperature  measurement  studied  in  the  investiga- 
tion above  referred  to  were  three:  optical,  total  radiation,  and  by 
thermocouples.  They  will  be  taken  up  here  in  that  order,  which  is 
their  order  of  utility,  from  least  to  greatest. 

OPTICAL  PYROMETRY  APPLIED  TO  MANTLES 

The  optical  method  of  measuring  temperature  was  employed  by 
Rubens  in  his  study  of  the  mantle.  He  used  it,  however,  in  a  manner 
that  could  lead  to  correct  results  only  with  a  completely  opaque,  com- 
pletely absorbing  body,  which  the  mantle  is  far  from  being.  Rubens' 
results  were,  however,  substantially  correct  because  he  confined  his 
observations  to  mantles  rich  in  ceria  and  to  the  blue  end  of  the  spectrum, 
where  the  absorption  band  due  to  ceria  is  of  high  saturation.  With 
mantles  of  low  visible  emissive  power  this  method  would  have  been 
inapplicable. 

The  optical  method  consists,  in  general,  in  measuring  the  black- 
body  temperature  by  the  usual  method  of  equality  of  brightness  used 
in  the  Holborn-Kurlbaum  and  Henning  pyrometers,  and  then  deriving 
the  true  temperature  from  a  knowledge  of  the  optical  properties  of  the 
radiator. 

If  TX  =  reflecting  power  of  body  at  wave-length  X; 
«7X  =  radiant   emission   of   black   body  at  same  temperature; 
EI  =  radiant  emission; 
£x  =  transmitting  power; 

from  Kirchhoff's  law; 


providing  the  surface  under  study  is  continuous.  If  it  is  discontinuous 
(as-  a  grid  of  fine  fibers  would  be),  if  s  represents  the  fractional  part  of  the 
area  occupied  by  the  solid  material, 

#x  =  Jxs  (1  -  rx  -  fx) 

Using  Wien's  law,  this  gives  for  the  true  temperature  T,  in  terms  of 
the  apparent  or  black-body  temperature  Ta 


In  this  formula,  the  constants  s,  r,  ajid  t  refer  to  the  properties  of  the 
hot  body,  which  are  usually  different  from  those  of  the  cold. 

It  is  obvious  that  accurate  temperature   determinations  by  this 
method  would  demand  very  elaborate  measurements  to  establish  the 


634        TEMPERATURE   MEASUREMENTS   OF   INCANDESCENT   GAS   MANTLES 

hot  porosity,  reflecting,  and  transmitting  powers.  In  the  case  of  the 
mantle,  such  measurements  are  practically  out  of  the  question.  The 
best  condition  for  applying  the  method  holds  at  the  edge  of  the  mantle, 
where  it  appears  continuous  by  projection,  so  that  s  approximates  unity. 
Also,  by  working  near  the  edge  of  the  mantle  r  reaches  its  nearest  approxi- 
mation to  the  reflecting  power  of  a  thick  opaque  layer  of  the  mantle 
oxide  in  powdered  form.  The  latter  is  capable  of  accurate  measurement 
in  the  cold  condition,  and  it  is  not  difficult  to  obtain  the  ratio  of  hot  to 
cold  reflecting  power  by  measuring  the  brightness  of  the  image  of  an 
arc  crater  focussed  on  the  hot  and  cold  mantle. 

The  possibility  thus  exists  of  obtaining  accurate  results  from  the 
edge  of  the  mantle,  if  we  can  safely  assume  t  =  0  and  r  =  reflecting  power 
of  a  solid  layer.  We  may  so  assume  in  the  case  of  strongly  absorbing 
substances,  but  experiments  with  various  mixtures  of  thoria  and  ceria 
show  that  these  assumptions  hold  only  for  mantles  rich  in  ceria,  and  are 
quite  unwarranted  with  mantles  in  general.  This  point  was  tested  in 
several  ways,  one  of  which  was  by  making  up  mantles  with  continuous 
patches  formed  by  sewing  small  rectangles  of  filter  paper  on  the  mantle 
fabric.  Certain  of  these  were  selected  which,  when  viewed  normally, 
copied  the  behavior  of  the  edge  of  the  ordinary  mantle  structure.  These 
patches  were  found,  on  measurement,  in  the  case  of  pure  thoria  to  have 
a  reflecting  power  of  only  0.55,  while  the  solid  layer  has  a  reflecting  power 
of  0.85.  Moreover,  upon  examining  the  brightness  of  the  patch  when 
illuminated,  it  was  found  that  the  unilluminated  side  was  over  half  as 
bright  as  the  illuminated,  showing  that  a  very  large  part  of  the  incident 
light  is  transmitted,  even  by  the  edge  of  the  mantle. 

It  therefore  appears,  as  is  borne  out  by  our  complete  data,  that  the 
optical  method,  which  should  give  accurate  results  for  opaque  layers 
of  radiating  material,  is  not  applicable  to  mantles  in  general.  In  the 
regular  99-per  cent,  thoria,  1-per  cent,  ceria  mantle,  the  black-body 
temperature  differs  by  an  amount  less  than  the  errors  of  measurement 
from  the  temperature  as  derived  from  the  introduction  of  the  hot  re- 
flecting power,  assuming  the  transmission  zero,  as  long  as  the  measure- 
ments are  made  in  the  extreme  blue.  Therefore  no  test  of  the  method 
is  afforded  by  such  measurements.  In  the  case  of  the  pure-thoria  mantle, 
however,  the  nearest  approach  to  the  true  temperature  (as  obtained  by 
graduated  thermocouples)  that  the  complete  optical  method  gives,  even 
if  r  and  t  are  assumed  to  amount  together  to  the  reflecting  power  of  an 
opaque  layer,  namely,  85  per  cent.,  falls  short  by  as  much  as  100°.  This 
failure  is,  in  part,  due  to  the  error  in  the  value  of  r  and  t  assumed  for  the 
cold  mantle  and,  in  part,  perhaps  to  an  actual  increase  of  transparency 
with  increase  in  temperature.  In  any  event,  it  illustrates  clearly  the 
inapplicability  of  the  method. 


HERBERT    E.    IVES 


635 


Measurements  of  the  total  radiation  from  all  the  mantles  studied 
were  carried  through,  by  means  of  a  surface  thermopile.  It  was  early 
found  that  when  the  temperatures  measured  by  graduated  thermocouples 
were  assumed  as  the  correct  ones,  the  radiation  for  constant  gas  con- 
sumption was  not  constant,  but  decreased  with  increasing  mantle  tem- 
perature. Thus  a  black-bulb  thermometer  placed  so  as  to  be  heated 
by  the  radiation  from  mantles  of  various  compositions  would  exhibit  the 
apparent  anomaly  of  showing  the  greatest  temperature  rise  for  the  mantle 
of  lowest  temperature.  In  the  thoria-ceria  series,  the  black-bulb  ther- 
mometer, or  any  total  radiation  pyrometer,  would  show  the  highest  reading 
for  the  mantle  of  pure  ceria,  and  steadily  decreasing  readings  until  pure 


£.5 

p 


1000°  IZOO'  1400°  1600'  1600°  2000'  ZZOQ' 

r 

Fia.  1. — CONVECTION  LOSSES  AS  CALCULATED  FROM  SPECIFIC  HEAT  OF  PRODUCTS: 

C,  CONVECTED  ENERGY;  P,  APPLIED  ENERQYJ  R,  RADIATED  ENERGY.  FULL  LINE  IS 
COMBUSTION  IN  AIR.  BROKEN  LINE  IS  COMBUSTION  IN  OXYGEN.^ 

thoria  was  reached,  with  no  reflection  whatever  of  the  enormous  lumi- 
nous maximum  at  the  99-per  cent,  thoria  point.  A  relationship  of  this 
sort,  besides  demanding  explanation,  offers  a  possible  method  of  tempera- 
ture measurement. 

The  explanation  lies  in  this  fact :  that,  for  a  given  constant  consumption 
of  gas  (rate  of  supply  of  energy),  the  portion  of  the  total  power  dissipated 
by  convection  and  conduction  is  greater  the  higher  the  temperature. 
Consequently  the  rest  of  the  applied  energy,  which  can  escape  only  as 
radiation,  must  be  smaller  the  higher  the  temperature.  The  convection 
losses  (beside  which  the  conduction  losses  are  small)  may  be  calculated 
with  considerable  accuracy,  by  making  a  heat  balance  for  the  mantle 
and  burner.  If  it  is  assumed  that  the  products  of  combustion  leave  the 
mantle  at  the  temperature  of  the  latter,  we  can,  by  knowing  the  specific 
heats  of  these  products  and  their  amount,  calculate  the  amount  of  energy 
carried  away  by  them.  The  difference  between  this  and  the  energy  input 
will  be  the  radiation. 

The  result  of  carrying  through  these  calculations  for  a  mantle  of 
several  different  temperatures  is  shown  in  Fig.  1.  A  gas  of  630  B.t.u.  per 
cu.  ft.  was  assumed,  and  the  specific  heats  of  the  products  of  combustion 
in  B.t.u.  per  cu.  ft.  of  gas  consumed  per  degree  centigrade  at  various 


636 


temperatures  were  plotted  from  standard  tables.  The  calculation  then 
consisted  in  comparing  this  specific  heat  with  the  initial.  It  appears 
from  the  figure  that  the  convection  loss  rises,  in  a  nearly  linear  manner, 
from  45  per  cent,  for  a  mantle  at  1350°  K.  to  75  per  cent,  for  a  mantle  at 
1900°.  The  radiation  must  correspondingly  decrease. 

Fig.  2  exhibits  the  relation  actually  found,  between  temperature  T 
(centigrade  absolute)  and  the  radiation  R,  in  arbitrary  units,  for  a  set  of 
mantles  composed  of  various  refractory  oxides.  It  is  clear  that  the  gen- 
eral linear  relationship  holds,  but,  at  the  same  time,  that  total  radiation 
methods  give  only  a  comparatively  rough  measure  of  temperature. 


2000* 
1600* 
1600' 
WOO' 
1200' 
\nnrf 

•FL 

AM 

E 

'5\ 

.Nd 

^> 

5?C 

e. 

i"/l 

Nd* 

7> 

& 

Be 

. 

^"*i 

-< 

B^CJ 

Z.Z 

0% 

le 

Mrf 

> 

3Q 

$ 

X*' 

FIG.-  2. — EXPERIMENTALLY  FOUND  RELATION  BETWEEN  TEMPERATURE  AND  RADIANCE. 
/?,  RADIANCE  IN  ARBITRARY  UNITS;  T,  TEMPERATURE. 


It  is  obvious  that  the  relationship  established  between  temperature 
and  total  radiation  for  the  mantle  is  entirely  conditioned  by  the  escape 
of  energy  by  convection.  Where  the  heating  is  done  in  vacuo,  as  by 
cathode  discharge,  no  such  relation  holds.  It  is  to  this  fact  indeed  that 
the  enormously  greater  brilliancy  of  thoria  over  ceria,  when  heated  in  the 
cathode  stream,  is  due. 

MANTLE  PYROMETRY  BY  THERMOCOUPLES 

Quite  the  most  satisfactory  method  of  mantle  temperature  measure- 
ment, on  the  whole,  is  that  which  utilizes  a  series  of  thermocouples  of 
graduated  size,  first  utilized  by  Nichols  for  flames,  and  later  by  White 
and  Travers  for  the  incandescent  mantle.  The  theory  in  its  simplest 
form  is  that,  while  a  single  thermocouple  will  not  give  correct  readings, 
due  to  the  heat  it  radiates  and  conducts  away,  this  error  is  less  the  smaller 
the  couple.  So  by  using  a  series  of  decreasing  size,  the  value  that  would 
be  given  by  a  couple  of  zero  mass  may  be  fixed  by  extrapolation. 

The  temperatures  of  the  ordinary  mantles  and  the  Bunsen  flame  are 
fortunately  within  the  range  of  the  platinum  platinum-rhodium  couple. 
We  have  used  successfully  a  series  of  diameters  0.35,  0.25,  0.15,  and  0.05 
mm.,  secured  from  Engelhardt  and  calibrated  by  the  Bureau  of  Standards. 
Certain  precautions  were  observed  in  their  use,  some  obvious  and  some 
learned  by  experience.  As  great  a  length  as  possible  of  the  couple 
should  lie  against  the  mantle.  The  bead  should  be  as  nearly  as  possible 


HERBERT    E.    IVES 


637 


of  the  same  diameter  as  the  wires  it  connects.  After  continued  use,  the 
couples  may  give  inconsistent  results,  perhaps  due  to  contamination; 
when  this  condition  is  evident,  the  beads  may  be  cut  off  and  new  ones 
fused. 

It  is  ordinarily  assumed  that  the  points  given  by  such  a  series  of 
thermocouples  lie  on  a  straight  line.  Our  results  on  the  couples  de- 
scribed indicate  consistently  that  these  points  lie  on  a  curve.  Some 
representative  results  are  shown  in  Fig.  3  for  mantles  of  different  com- 


FIG.  3. — MEASUREMENT  OF  MANTLE  TEMPERATURE  BY  THERMOCOUPLES  OF  GRADU- 
ATED DIAMETER.        d,    THERMOCOUPLE  DIAMETER;    T,   TEMPERATURE. 

positions  and  temperatures.  The  curvature  is  well  shown,  and  it  appears 
as  well  that  with  the  lower-temperature  mantles  the  curve  is  more  nearly 
perpendicular  to  the  temperature  axis,  that  is,  that  the  couples  differ 
less  in  their  readings. 

The  accuracy  attainable  by  the  thermocouple  method,  while  far 
greater  than  that  given  by  the  preceding  methods,  is  not  comparable 
with  what  thermocouples  will  do  under  less  trying  conditions.  A  good 
set  of  readings,  using  the  potentiometer,  will  usually  fix  the  temperature 
of  the  mantle  within  20°  to  30°,  but  great  care  must  be  taken  to  place 
the  bead  upon  exactly  the  same  point  in  the  mantle  with  each  couple. 


638 


PROBLEMS    OF    LAMP    DESIGN    AND    PERFORMANCE 


Application  of  Pyrometry  to  Problems  of  Lamp  Design  and  Performance 

BY   I.    H.   VAN    HORN,*  B.    S.,    CLEVELAND,    OHIO 
(Chicago  Meeting,  September,  1919) 

IN  the  development  of  the  incandescent  electric  lamp ,,one  aim  of 
the  investigators  K.  has  been  to  establish  the  fundamentals  of  lamp 
design,  so  that  the  performance  of  any  new  lamp  may  be  accurately 
predicted.  The  study  of  .the  temperature  relations  in  lamps  has  done 
much  toward  establishing  these  fundamentals.  Fig.  1  gives  the  typical 


50  WATT  -  1  15  VOLT 
MAZDA  B_LAMR 

-Brass  Contact 

Glaus 

Insulation 

Brass  Screw 

Contact 


150  WATT  -  115  VOLT 

MAZDA  C  LAMP 
Contact; 
Olas: 
Insulation 


Glass  Stem 

Bulb  N«-ck 

Air  Tight  Seal 

Lead  in  Wires 

Mica  Disc 

Filament 


FIG.  1. — ARRANGEMENT  OP  PARTS  IN  MAZDA  B  AND  MAZDA  C  LAMPS. 

arrangement  of  the  lamp  parts  in  both  the  Mazda  B  (Vacuum)  and 
Mazda  C  (gas-filled)  lamps.  The  outstanding  differences  are  the 
filament  form  and  the  bulb  shape. 

The  ideal  lamp  is  one  in  which  the  filament  operates  at  a  uniform 
temperature  throughout  its  length.  In  practice,  it  is  necessary  to  disturb 
this  uniformity  by  the  introduction  of  supports  and  terminals.  The 
amount  of  cooling  at  the  supports  and  terminals  affects  the  over-all 
efficiency  of  the  filament  as  a  light  producer.  The  life  of  the  lamp  is 
ordinarily  proportional  to  the  maximum  and  not  to  the  average  filament 
temperature.  The  Holborn-Kurlbaum  type  of  optical  pyrometer  has 


*  Physicist,  National  Lamp  Works,  General  Elec.  Co. 


I.    H.    VAN    HORN 


639 


been  used  for  determining  the  temperature  gradients  in  lamp  filaments.1 
The  cooling  effects  of  supports  and  terminals  have  been  evaluated.2 

Fig.  2  illustrates  the  cooling  effect  of  supports  in  a  vacuum  lamp. 
The  terminals  a  and  b  are  16-mil  copper;  the  supports  c,  d,  and  e  are  10-, 
20-,  and  40-mil  copper,  and  the  supports  /,  g,  h,  and  i  are  2-,  4-,  8-,  and 
16-mil  molybdenum.  The  length  of  filament  cooled  depends  on  the  size 
and  material  of  the  supports  and  terminals,  the  size  and  material  of  the 
filament,3  and  the  maximum  filament  temperature.  The  cooling  of  the 
filament  at  terminals  and  supports  is  a  very  important  factor  in  the  de- 
signing of  low-voltage  lamps.  The  filament  used  in  the  ordinary  pocket 
flashlight  lamp  is  about  3  mm.  long  and  the  effect  of  end  cooling  extends 
over  its  whole  length.  In  lamps  of  this  type  the  average  filament  tem- 
perature is  much  lower  than  the  maximum. 


FIG.  2. — SPECIAL  VACUUM  LAMP  WITH  SUPPORTS  OP  DIFFERENT  MATERIALS  AND  SIZES 

The  control  of  temperature  distribution  in  the  vacuum  lamp  is  much 
simpler  than  in  the  gas-filled  lamp.  In  the  gas-filled  lamp  the  filament 
is  cooled,  not  only  by  the  conduction  of  the  supports  and  terminals  but 
also  by  gas  conduction  and  convection.4  The  arrangement  of  the  fila- 
ment therefore  becomes  of  prime  importance  in  obtaining  the  most  uni- 
form filament  temperature  as  well  as  the  lowest  maximum  temperature 
for  a  given  efficiency  of  light  production.  Fig.  3  is  a  photograph  of  a 
100-watt  Mazda  B  construction  lamp  filled  with  gas.  The  lamp  was 
burning  tip  down  when  photographed.  The  upper  portion  of  the  fila- 
ment is  operating  at  a  much  higher  temperature  than  the  lower.  This 
shows  the  necessity  for  a  different  mount  design  for  the  gas-filled  lamp. 


1  Hyde,  Cady  and  Worthing:  Trans.  111.  Eng.  Soc.  (1911)  6,  238. 

2  Amrine:  Trans.  111.  Eng.  Soc.  (1913)  8,  385.     Worthing:  Phys.  Rev.  [2]  (1914) 
4,  524. 

3  Worthing:  Phys.  Rev.  [2]  (1914)  4,  535.     4Langmuir:  Phys.  Rev.  (1912)  34,  401. 


640 


PROBLEMS  OF  LAMP  DESIGN  AND  PERFORMANCE 


It  is  advantageous  to  coil  the  filament  wire  in  the  form  of  a  helix  in  order 
to  reduce  the  energy  loss  due  to  the  gas  and  to  give  a  more  uniform  fila- 
ment temperature.  The  curves  of  Fig.' 4  show  a  much  lower  gas  loss  for 
the  coil  filament  than  for  a  straight  filament  of  the  same  diameter. 


FIG.  3. — 100-WATT  MAZDA  B  CONSTRUCTION  LAMP  WHICH  HAS  BEEN  GAS-FILLED. 

The  gas  loss  may  also  be  different  for  variations  in  the  pitch  and  mandrel 
of  filament  coiling. 

The  optical  pyrometer  has  been  of  great  assistance  in  studying  these 
effects.  The  temperature  at  any  point  on  the  filament  can  be  measured 
quickly  and  accurately  and  different  observers  find  no  difficulty  in  ob- 


r  cent  Energy  Loss  in  Gas 

S  §  S  S  g 

Energy  loss  from  Tungsten 
Filaments  of  Different  Dia- 
meters in  Argon  Gas 

\ 

A 

K 

£ 

i 

\ 

\ 

;    N 
= 

\ 

=3 
^- 

X 

Co 

In  I 

St 

•niif 

0 

.01        .02        .03        .04        .05        .06        .07         .08        .09        .!( 
Diameter  in  Inches 

FIG.  4. — PEE  CENT.  GAS  LOSS  FOR  STRAIGHT  AND  COILED  FILAMENTS  IN  GAS. 

taining  results  that  agree  closely  when  they  use  the  same  temperature 
scale  as  a  basis  for  calibration.  The  errors  and  limitations  in  optical 
pyrometry  have  been  thoroughly  studied  and  discussed  by  various  re- 
search laboratories5  in  connection  with  high-temperature  investigations. 

6  Worthing  and  Forsythe:  Phys.  Rev.  [2]  (1914)  4,  163. 


I.    H.   VAN   HORN 


641 


The  determination  of  the  average  filament  temperature  by  the  optical 
pyrometer  method  is  rather  tedious  since  it  requires  a  large  number  of 
observations.  An  optical  pyrometer  apparatus  has  been  developed  in 


Temperature  Degrees  Kelvin 

/ 

v 

\ 

/ 

v 

\ 

/ 

I 

\ 

1 

1 

I 

|MM 

1C. 

1 

s 

law 

ICo 

1 

« 

i 

rnii 

il 

Su 

purt 

T 

Tin 

rial 

Distance  'along  Filament 
}.  5.  —  TEMPERATURE  DISTRIBUTION  IN  A  MAZDA  C  LAMP. 

the  laboratory  with  which  the  author  is  connected,  which  facilitates  the 
measurements  on  a  lamp  filament.  The  motion  of  the  lamp  carriage  is 
controlled  by  the  observer  as  he  watches  the  image  of  the  filament  through 


i.o 


o.s 


\ 


10 


30 


10 


Filament  Length  in  Cm. 
FIG.  6. — EFFICIENCY  FOR  DIFFERENT  FILAMENT  LENGTHS   IN  A  MAZDA  C  STREET 

SERIES     LAMP. 

the  telescope.     He  can  move  it  up  or  down,  to  the  right  or  left,  and  for- 
ward and  backward,  and  can  rotate  it. 

Fig.  5  shows  the  temperature  distribution  curve  for  a  common  form 
of  Mazda  C  lamp.     The  filament  coil  was  arranged  in  the  form  of  a  W 

41 


642 


PROBLEMS  OF  LAMP  DESIGN  AND  PERFORMANCE 


with  the  support  at  the  upper  central  point  of  the  W.  The  temperature 
distribution  in  a  coil  mounted  vertically  is  less  uniform  than  in  a  coil 
mounted  horizontally.  The  maximum  temperature  in  the  vertical  coil 
is  usually  toward  the  upper  end.  The  gas  loss  is  slightly  greater  for  the 
horizontal  coil.  The  curve  in  Fig.  6  shows  the  relation  between  effi- 
ciency and  filament  length  for  the  same  maximum  filament  temperature. 
This  serves  to  emphasize  the  fact  that  the  watts  per  mean  spherical  can- 
dlepower  does  not  necessarily  indicate  the  temperature  of  the  filament. 
However,  for  lamps  of  the  same  wattage  and  design,  the  watts  per  mean 
spherical  candlepower  may  be  taken  as  proportional  to  the  average 
filament  temperature. 

One  of  the  very  simple  methods  of  determining  the  average  filament 
temperature  is  the  use  of  the  filament  as  a  resistance 
element  of  a  resistance  pyrometer.  The  ratio  of  the 
resistance  at  high  temperature  to  the  resistance  at  room 
temperature  gives  a  measure  of  the  absolute  tempera- 
ture for  a  given  quality  of  filament  wire.6  This  method 
has  the  disadvantage  that  it  is  not  independent  of  the 
wire  quality  nor  is  it  quite  as  sensitive  as  some  other 
methods. 

The  average  filament  temperature  may  also  be 
measured  by  the  color-match7  photometer  method. 
The  ordinary  Lummer-Brodhun  photometer  sight-box 
is  suitable  for  this  work.  The  lamp  of  unknown 
filament  temperature  is  placed  in  the  test  socket  of 
the  photometer  and  the  comparison  lamp  voltage  is 
varied  until  there  is  no  apparent  difference  in  the 
color  of  the  two  fields.  Temperature  measurements 
made  by  the  color-match  method  are  reliable  only 
when  the  character  of  the  filament  radiation  is  known  to  agree  with 
that  for  which  the  calibration  is  made.  The  radiation  from  a  straight 
filament  or  from  the  outside  of  a  coiled  filament  has  been  found  to  be 
of  a  different  quality  than  the  radiation  from  the  inside  of  a  coiled 
filament.  The  light  from  the  inside  is  redder8  than  that  from  the  outside. 
The  temperature  of  a  coiled  filament  may  be  measured  with  an 
optical  pyrometer  by  sighting  upon  the  outside  of  the  helix,  since  the 
quality  of  the  radiation  from  the  outside  is  unaffected  by  coiling.  Fig. 
7  shows  a  lighted  coil  filament  and  shows  the  difference  in  brightness  on 
the  inside  and  the  outside  of  the  helix. 


FIG.  7. — LIGHTED 
COIL  FILAMENT. 


6Langmuir:  Phys.  Rev.  [2]  (1916)  7,  306. 
7  Hyde,  Cady  and  Forsythe:  Phys.  Rev.  [2]  (1917)  10,  395. 
8Langmuir:  Op.  cit.,  152. 
Coblentz:  U.  S.  Bureau  of  Standards  Bull.  14  (1918)  115. 


I.    H.   VAN   HORN 


643 


The  maintenance  of  a  vacuum-tight  seal  is  essential  to  the  successful 
operation  of  all  incandescent  electric  lamps.  The  stem  seal  is  therefore 
one  of  the  vital  points  of  the  lamp.  While  glass  is  a  very  good  electrical 
insulator  at  room  temperatures,  it  becomes  somewhat  conducting  at 


FIG.  8. — GLASS  STEMS  FROM  MAZDA  C  LAMPS  CRACKED  AS  A  RESULT  OF  ELECTROLYSIS. 

the  temperature  attained  in  incandescent  lamps.  The  lamp  design 
must  be  such  that  the  stem  temperature  will  be  safe  from  the  standpoint 
of  electrolysis  of  the  stem.  The  stem  temperature  may  be  measured 


CONTOUR  TEMPERATURE  GRADIENT 
OF   100-WATT    MAZDA  C    LAMP 

P.  8.  -25  BULB                         NO  REFLECTOR 

—  i. 

I 

N 

i 
1 

N 

x 

N 

X 

3 

\ 

^ 

^ 

^ 

•^-. 

——  „ 

V 

.-  — 

^l  — 

^ 

^=: 

fc- 

\    ' 

\ 

^ 

+*** 

5 

/ 

/ 

S/J 

/ 

/ 

/ 

/ 

/ 

/ 

s 

KXT       120'       140        160J       180D       200J       220°      240°       260J 
Degrees  Fahrenheit 

fIG     9 BULB    TEMPERATURE     DISTRIBUTION    IN     A     MAZDA    C    LAMP. 

by  inserting  a  small  thermocouple  in  the  stem  press  when  making  the 
stem.  Fig.  8  shows  the  results  of  stem  electrolysis;  the  glass  has  cracked 
along  the  weld  seal  wires. 

In  the  gas-filled  lamp  the  stem  temperature  ts  affected  by  the  bulb 


644  PROBLEMS    OF  LAMP    DESIGN    AND    PERFORMANCE 

shape  and  size,  the  distance  of  the  stem  seal  from  the  filament,  the 
wattage  of  the  lamp,  the  deflectors  used,  and  the  condition  of  opera- 
tion, whether  in  enclosed  or  open  lighting  fixtures,  and  whether  with 
base  up  or  down,  or  horizontal,  or  at  an  angle.  It  is  therefore  necessary 
to  make  stem-temperature  measurements  under  conditions  equivalent 
to  those  met  with  in  the  most  severe  service  for  which  the  lamp  is 
designed. 

Fig.  9  shows  the  bulb  temperature  distribution  in  a  certain  type  of 
Mazda  C  lamp  operated  base  uppermost  with  no  enclosing  fixture.  The 
bulb  temperature  may  be  conveniently  measured  with  the  resistance 
pyrometer  in  which  the  resistance  element  is  wound  around  the  bulb 
at  the  point  to  be  investigated.  The  upper  limit  for  bulb  temperature 
of  good  lamp  performance  is  not  ordinarily  the  softening  or  devitrifica- 
tion of  the  glass.  It  is.  difficult  to  remove  all  the  moisture  from  the- 
glass  parts  at  the  time  of  exhaust.  The  moisture  may  be  given  up 
later,  if  the  bulb  temperature  is  very  high,  and  result  in  an  inferior  lamp. 
Change  in  the  bulb  size  or  shape  may  give  a  more  favorable  temperature 
distribution. 

Temperature  measurements  have  played  an  important  part  in  fixing 
the  present  designs  of  the  Mazda  lamp.  Some  of  the  methods  found 
useful  in  studying  the  temperature  relations  have  been  discussed  and 
some  of  the  results  indicated. 


A.  G.  WORTHING,  Nela  Park,  Cleveland,  0. — There  is  a  very  definite 
relation  that  is  helpful  in  the  measurement  of  tungsten-filament  tempera- 
tures by  means  of  a  resistance.  Plotting  the  log  of  the  resistance  against 
the  log  of  the  temperature,  for  the  range  from  1300°  to  3200°  K.,  gives 
a  straight-line  relation.  A  great  many  points  taken  in  a  careful  test  lie 
very  close  indeed  to  the  straight  line.  This  is  different  from  the  relation 
Mr.  Northrup  found  for  tin. 


TEMPERATURE   OF   A  BURNING   CIGAR  G45 


Temperature  of  a  Burning  Cigar 

BY  T.  S.  SLIGH,   JR.,*  M.  S.,   AND  HENRY  R.   KRAYBILL,  f  PH.  D.,  WASHINGTON,  D.  C. 
(Chicago  Meeting,  September,  1919) 

OF  all  the  qualities  that  are  essential  in  a  good  cigar  tobacco  none  is 
quite  so  important  as  the  burn.  This  term  is  general  and  includes  many 
points,  the  most  important  of  which  are  evenness  of  burn,  color  of  ash, 
firmness  and  coherence  of  ash,  and  fire-holding  capacity.  The  fire- 
holding  capacity  refers  to  the  length  of  time  the  leaf  or  cigar  will  continue 
to  glow  after  ignition. 

Chlorides  tend  to  prevent  complete  combustion  and  products  are 
formed  thereby  that  are  injurious  to  the  flavor  and  aroma.  On  the  other 
hand,  the  carbonates  of  the  alkalies,  particularly  of  potassium,  aid  the 
combustion  and  increase  the  fire-holding  capacity.  Barth1  thought 
that  the  harmful  effect  of  the  chlorides  was  due  to  their  fusing  and  coat- 
ing the  tobacco,  thereby  preventing  complete  combustion.  Schlosing,2 
Nessler,3  and  Garner,4  suggested  widely  different  theories  to  account 
for  the  favorable  action  of  potassium  salts.  In  order  to  study  carefully 
the  action  of  the  various  salts  upon  the  course  of  combustion  of  the 
cigar  a  knowledge  of  the  temperature  of  the  burning  cigar  is  necessary. 
Lehmann,5  who  seems  to  be  the  only  investigator  who  has  made  any 
attempt  to  determine  the  temperature  of  a  burning  cigar,  gives  480°  C. 
as  the  maximum  temperature  which  he  recorded.  It  is  evident,  however, 
that  his  measurements  were  in  error,  probably  on  account  of  the  method 
and  apparatus  used,  since  the  lowest  visible  red  corresponds  to  a  tem- 
perature of  about  575°  C. 

The  object  of  the  present  investigation6  was  to  develop  a  method  of 

*  Assistant  Physicist,  U.  S.  Bureau  of  Standards. 

t  Assistant  Physiologist,  Bureau  of  Plant  Industry,  U.  S.  Dept.  of  Agriculture. 

1  Max  Barth:  Untersuchungen  von  im  Elsass  gesogenen  Tabaken  und  einigen 
Beziehungen  zwischen  der  Qualitat  des  Tabaks  und  seiner  zusammensetzung.     Landw. 
Ver.  Stat.  (1891)  39,  81-104. 

2  Th.  Schlosing:  t)ber  die  Verbrennlichkeit  des  Tabaks.    Landw.  Ver.  Stal.  (1891) 
9,98. 

3  J.  Nessler:  Dungomgsversuche  zu  Tabak.    Landw.  Ver  Stat.  (1881)  29,  309-312. 

4  W.  W.  Garner:  Relation  of  the  Composition  of  the  Leaf  to  the  Burning  Qualities 
of  Tobacco.     Bull.  105,  Bureau  of  Plant  Industry,  U.  S.  Dept.  of  Agriculture. 

6  K.  B.  Lehmann:  Chemische  und  Toxikologische  Studien  uber  Tabak,  etc.  Ar- 
chives fur  Hygiene  (1908-09)  68,  319. 

6  The  work  presented  in  this  paper  was  performed  in  1916.  Determinations  upon 
cigars  of  varying  ash  and  moisture  content  were  planned  with  a  view  to  ascertaining 
the  effect  of  these  factors  upon  the  temperature  attained  but  up  to  the  present  time 
it  has  not  been  found  feasible  to  carry  out  this  program. 


646 


TEMPERATURE    OF    A   BURNING    CIGAR 


determining  the  maximum  temperature  within  the  burning  cigar  which 
would  eliminate  the  theoretical  objections  to  the  methods  employed 
previously  (i.e.,  a  possibility  of  low  readings  due  to  heat  conduction 
along  the  thermocouple  wires  and  to  a  leakage  of  cold  air  into  the  junc- 
tion) and  to  determine  approximately  the  maximum  temperature 
attained  in  cigars  smoked  under  ordinary  conditions  or  as  near  to  such 
as  the  method  of  taking  readings  would  permit. 

The  thermocouples  were  composed  of  the  following  wire:  platinum 
0.01  cm.  and  0.015  cm.  in  diameter  and  platinum  10  per  cent,  rhodium 
of  the  same  diameters  supplied  by  J.  Bishop  Platinum  Works.  The 
potentiometric  method  of  measurement  was  used.  The  set  up  was  as 
shown  in  Fig.  1;  a  diagram  of  the  electrical  connections  is  given  in  Fig.  2. 


FIG.  1. — APPARATUS  USED  TO  OBTAIN  TEMPERATURE  OF  BURNING  CIGAR. 

In  order  to  eliminate  conduction  and  leakage,  it  was  decided  that 
only  couples  composed  of  very  small  wires  should  be  used,  so  the  plat- 
inum-rhodium couple  was  chosen  as  offering  greater  reliability  and  less 
chance  of  trouble  due  to  brittleness  of  the  wire  in  the  smaller  sizes. 

A  small  glass  capillary  tube  drawn  down  to  a  point  was  thrust 
through  the  cigar  at  a  point  about  2.5  cm.  from  the  tip  of  the  cigar  as 
shown  in  Fig.  3.  One  of  the  wires  of  the  couple  was  then  passed  into  this 
tube  and  so  through  the  cigar,  the  tube  drawn  through  the  cigar  and  re- 
moved from  the  wire,  and  this  wire  joined  to  the  other  wire  of  the  couple 
by  arc  welding.  The  junction  having  been  made,  it  was  trimmed  down 
and  pulled  back  to  the  longitudinal  axis  of  the  cigar  and  the  small  holes 
around  the  wire  plugged  with  paper  pulp.  In  this  way  the  junction 
was  located  in  the  region  of  highest  temperature,  the  filler  of  the  cigar 


T.    S.    SLIGH,    JR.,    AND    HENRY    R.    KRAYBILL 


647 


was  disturbed  to  only  a  very  small  extent  and  good  insulation  between 
the  wires  was  secured  without  the  necessity  of  introducing  additional 
heat-absorbing  material  into  the  cigar.  The  smaller  couple  No.  1  was 


A/WWWWWWWWW\ 
0  or  2800  -0_ 


0 


Potentiometer 


FIG.  2. — ELECTRICAL  CONNECTIONS  OF  APPARATUS. 

located  about  2.5  cm.  from  the  tip  with  the  larger  couple  No.  2  about 
2  cm.  farther  back. 


FIG.  3. — PLACING  JUNCTIONS  IN  CIGAR. 

The  apparatus  was  adjusted  and  the  cigar  lighted.  As  soon  as  a 
temperature  near  300°  C.  was  indicated,  readings  were  taken  of  the 
highest  temperature  reached  during  the  puff,  and  of  the  temperature  in 
the  coal  or  cigar  about  30  sec.  after  the  puff.  These  latter  readings 
were  taken  to  determine  whether  the  point  of  highest  temperature  had 


648 


TEMPERATURE    OF   A   BURNING    CIGAR 


been  reached  as  indicated  by  a  rising  or  falling  temperature.  The  time 
interval  between  puffs  was  1^  min.  and  the  duration  of  the  puff  was 
from  5  to  8  sec.  An  attempt  was  made  to  keep  the  draft  and,  conse- 
quently, the  rate  of  combustion,  normal.  When  the  temperature 
indications  began  to  decrease  on  successive  puffs,  the  other  junction 
was  switched  into  the  circuit  and  a  similar  set  of  readings  taken;  see 
Table  1.  The  couples,  as  taken  from  the  ash,  were  usually  check 
standardized  with  boiling  sulfur  as  a  reference  point. 

.  TABLE  1. — Data  from  Single  Cigar 


Temperature,  Degrees  C. 

No. 

Couple  No.  1 

Couple  No.  2 

Puff 

After 

Puff 

After 

1 

708 

472 

2 

415 

426° 

3 

853 

594 

4 

532 

536C 

5 

878 

730 

6 

620° 

634C 

7 

832 

793 

8 

693° 

708° 

9 

859 

829 

10 

753" 

738 

11 

892 

904 

12 

803° 

772* 

13 

817 

910 

14 

769 

707d 

15 

797 

16 

609 

a  Holds  this  temperature.       *  Cools.         e  Rising.         d  Couple  dropped  out  of  cigar. 

Readings  as  indicated  were  taken  upon  a  number  of  different  cigars 
designated  by  letter.  Table  1  is  typical  of  the  data  obtained  on  each 
cigar.  Table  2  is  a  resume  of  maximum  temperatures  obtained  during 
the  puff  and  in  the  coal.  Complete  data  for  each  cigar,  standardiza- 
tion and  check  points  for  thermocouples,  etc.,  are  omitted  as  unneces- 
sary detail. 

It  is  noted,  upon  comparing  the  readings  taken  on  different  cigars, 
.that  the  maximum  temperatures  vary  considerably  from  one  cigar  to  the 
next.  This  probably  can  be  accounted  for  by  a  variation  in  the  moisture 
present  and  a  variation  in  the  compactness  of  the  cigar,  which  would 
influence  both  the  rate  of  combustion  by  limiting  the  draft  and  the  avail- 
ability, or  rather  the  suitable  arrangement,  of  the  material  present  for  free 
combustion.  Another  source  of  uncertainty  is  the  possible  presence  of 
voids  in  the  cigar  at  or  near  the  junction.  Again,  since  puffing  is  not 


T.    S.    SLIGH,    JR.,    AND    HENRY   R.    KRAYBILL 


649 


continuous  it  might  well  be  that  in  some  cases  the  junction  is  not  located 
just  at  the  point  of  highest  temperature  during  a  puff;  this  would  explain 
the  small  differences  existing  between  the  readings  indicated  by  couple 
No.  1  and  couple  No.  2  in  the  same  cigar,  see  Table  2. 

TABLE  2. — Maximum  Temperatures  in  Cigars 


During  Puff,"  Degrees  C. 


In  Coal,6  Degrees  C. 


Couple  No.  1       Couple  No.  2 

Couple  No.  1 

Couple  No.  2 

I 

835 

839 

658 

J 

865 

842 

717 

668 

K 

886 

887 

584 

629 

L 

925 

813 

670 

657 

M 

807 

825 

803 

708 

'  N 

837 

802 

O 

892           910 

a  Corresponding  readings  in  the  same  cigar  with  different  couples. 

6  Readings  taken  at  random  where  temperature  in  coal  was  stationary  or  approxi- 
mately so.  These  readings  were  not  taken  consistently  as  their  value  was  not  fully 
appreciated  at  time  tests  were  run. 

A  comparison  of  the  maximum  temperatures,  as  indicated  in  Table  2, 
shows  that  the  readings  with  the  two  couples  in  the  same  cigar  vary  much 
less  than  the  readings  of  any  two  cigars  that  might  be  compared.  This 
indicates  that  the  moisture  content,  chemical  composition,  and  physical 
condition  of  the  cigar  may  influence  the  maximum  temperatures  attained. 

The  platinum-rhodium  couples  were  selected  of  different  sizes  of  wire 
in  order  that  any  great  lowering  of  temperature  due  to  conduction  along 
the  wires  might  show  up  as  a  consistently  low  reading  of  the  larger  couple. 
As  the  indicated  maximum  is  greater  first  in  one  couple  and  then  in  the 
other,  it  is  only  reasonable  to  assume  that  the  conduction  effect  in  these 
small  couples  is  negligible-.  Finally,  though  it  may  seem  that  the  tem- 
peratures shown  in  Table  2  are  rather  high,  a  comparison  of  the  color 
brightness  of  the  tip  of  a  lighted  cigar  with  the  color  of  the  walls  of  a 
furnace  known  to  be  at  some  temperature  near  900°  C.  will  go  far  toward 
removing  any  doubt  one  may  have  as  to  the  possibility  of  the  existence  of 
such  temperatures  in  the  cigar. 

SUMMARY 

1.  A  method  of  determining  the  temperature  of  the  burning  cigar 
which  seems  to  give  satisfactory  results  is  described. 

2.  The  maximum  temperature  recorded  is  925°  C.,  if  we  may  disregard 
the  reading  of  950°  C.  as  doubtful  because  the  readings  taken  before 
and  after  the  puff  in  that  case  indicate  a  much  lower  maximum,  because 
a  decided  and  well-supported  maximum  occurs  later  on  and  because  the 
readings  taken  with  couple  No.  2  in  the  same  cigar  gave  a  very  much 


650 


TEMPERATURE    OF   A   BURNING    CIGAR 


lower  maximum  temperature.     The  maximum  temperatures  recorded 
were  as  follows: 


Couple  No.  1, 
Degrees  C. 

Couple  No.  2, 
Degrees  C. 

Couple  No.  1, 
Degrees  C. 

Couple  No.  2, 
Degrees  C. 

Cigar  I 

835 

839 

Cigar  ]V1 

807 

82*5 

Cigar  J  

865 

842 

Cigar  N 

837 

802 

Cigar  K  

886 

887 

Cigar  O 

892 

910 

Cigar  L  

925 

813 

3.  The  highest  stationary  temperature  recorded  in  the  coal  was  803°  C. 
The  average  temperature  will,  of  course,  depend  on  the  zone  over  which 
the  average  is  taken.  The  maximum  stationary  temperatures  in  the 
coal  were  as  follows : 


Couple  No.  1, 
Degrees  C. 

Couple  No.  2, 
Degrees  C. 

Couple  No.  1, 
Degrees  C. 

Couple  No.  2, 
Degrees  C. 

Cigar  I  

658 

Cigar  L 

670 

657 

Cigar  J  

717 

668 

Cigar  M 

803 

708 

Cigar  K  

584 

629 

4.  The  temperature  gradient  becomes  very  steep  as  the  coal  approaches 
the  junction,  that  is,  the  temperature  a  few  millimeters  ahead  of  the  coal 
is  comparatively  low. 

5.  The  data  obtained  indicate  that  such  factors  as  moisture  content, 
chemical  composition,  and  compactness  of  the  cigar  affect  the  maximum 
temperature  attained  during  the  combustion. 

Acknowledgments  are  due  to  the  Department  of  Physics  of  the 
Pennsylvania  State  College  for  the  use  of  its  laboratories  and  apparatus 
in  this  work. 

DISCUSSION 

W.  P.  WHITE,*  Washington,  D.  C.  (written  discussion f).  —  The 
authors  seem  to  have  proved  that  for  a  phenomenon  as  irregular  as  the  one 
they  were  investigating  there  was  no  perceptible  conduction  effect  in  the 
platinum  wire.  It  should  be  mentioned,  however,  that  the  conduction 
of  heat  to  and  from  small  wires  is  not  proportional  to  their  surface  and 
might  give  unexpected  values,  varying  largely  with  the  medium  in  which 
the  wire  was  situated.  Attempts  to  eliminate  a  conduction  effect  in  the 
wire  by  extrapolating  the  curve  obtained  by  using  different  sizes  have  in 
some  cases  given  incorrect  results,  so  that  the  authors'  result  should  be 
applied  with  great  care  as  a  guide  in  other  cases.  The  difference  in  size 
of  wire  employed  by  them  is  really  rather  small.  If  the  desire  had  been  to 
find  out  how  great  the  effect  was  instead  of  merely  to  demonstrate  its 
absence,  a  greater  difference  of  diameter  would,  of  course,  have  been 
selected. 


*  Physicist,  Geophysical  Laboratory. 


t  Received  Sept.  25,  1919. 


DISCUSSION  651 

The  sharp  gradient  immediately  in  front  of  the  burning  portion  of  the 
cigar  is  quite  surprising  at  first  sight.  The  weight  of  air  coming  from  a 
flame  is  considerably  greater  than  the  weight  of  material  burned,  and  it 
would  be  thought  that  this  stream  of  heated  air  would  heat  very  con- 
siderably the  material  not  yet  reached  by  the  zone  of  combustion.  Pos- 
sibly the  heat  is  exhausted  in  evaporating  moisture  from  the  material. 
In  that  case  the  cigar  is  really  a  sort  of  regenerative  furnace,  except 
that  it  is  not  air  but  the  material  which  is  preheated,  and  the  pre- 
heating produces  dryness  rather  than  increase  of  temperature.  Whether 
or  not  this  is  the  case  would  be  shown  by  finding  how  hot  a  thoroughly 
dry  cigar  would  get. 

T.  S.  SLIGH  (author's  reply  to  discussion*). — The  authors  attempted 
only  to  obtain  some  measurements  of  temperature  attained  in  cigars 
selected  at  random  and  since  these  individual  temperatures  would 
necessarily  vary  considerably  among  themselves  the  method  was  not 
subjected  to  a  rigid  examination  as  to  the  attainable  accuracy. 

The  authors  were  aware  that  attempts  to  determine  flame  temperature 
by  extrapolation  of  the  readings  of  couples  of  varying  diameters  had 
led  to  inconsistent  results.  It  should  be  noted,  however,  that  in  the 
present  work  the  couple  is  in  contact  with  a  considerable  mass  of  burning 
material  for  about  50  to  70  diameters  of  the  wire  on  each  side  of  the 
junction,  thus  supplying  the  larger  part  of  the  heat  conducted  away 
by  the.  leads  from  a  portion  of  the  coal  more  or  less  remote  from  the 
junction;  the  section  of  the  couple  in  the  cigar  is  swept  by  a  stream  of  hot 
gases  and  the  junction  is  completely  shielded  from  radiation  losses. 
We  may  calculate,  using  experimentally  determined  emission  constants, 
that  the  heat  loss  from  the  leads  is  about  0.01  cal.  per  sec.  and  0.03  cal. 
per  sec.  from  the  smaller  and  larger  couple,  respectively.  We  have  no 
data  on  the  degree  of  thermal  contact  secured  between  the  couples 
and  the  burning  cigar,  but  if  we  assume  the  same  degree  of  contact  for 
each  couple,  in  view  of  the  fact  that  heat  transfer  between  small  wires 
arid  a  gas  is  practically  independent  of  the  diameters  of  the  wires,  we 
find  that  the  lowering  of  the  temperature  of  the  larger  couple  should  be 
approximately  three  times  that  of  the  smaller  couple.  If,  on  the  other 
hand,  we  assume  solid  contact  between  the  couple  and  the  coal,  the 
heat  transfer  would  be  proportional  to  the  surface  areas,  of  the  wires 
and  we  find  that  the  lowering  of  temperature  of  the  larger  couple  is 
twice  that  of  the  smaller  couple.  If  this  lowering  of  temperature  were 
appreciable  one  would  expect  to  note  its  effects  on  the  temperatures 
indicated  by  the  two  sizes  of  couples  employed. 

The  authors  agree  with  Doctor  White  in  his  explanation  of  the 
cause  of  the  sharp  gradient  ahead  of  the  coal.  An  examination  of  a 
section  of  a  partly  burned  cigar  shows  that  the  tobacco  is  dried  but  for 
only  a  short  distance  ahead  of  the  coal. 

*  Received  Jan.  20,  1920. 


652  MANUFACTURE    OF    GAS-MASK    CARBON 


Application   of   Pyrometry  to   the   Manufacture  of  Gas-mask  Carbon 

BY    KIRTLAND    MARSH,*  B.    S.,    NEW    KENSINGTON,    PA. 
(Chicago  Meeting,  September,  1919) 

THE  manufacture  of  gas  masks  by  the  Chemical  Warfare  Service, 
U.  S.  A.,  required  preparation  of  the  carbon  used  in  the  canisters.  The 
largest  plant  for  the  production  of  this  carbon  was  situated  at  the  works 
of  the  Astoria  Light,  Heat  and  Power  Co.,  at  Astoria,  N.  Y.;  this  paper 
will  deal1  with  the  pyrometry  equipment  at  that  plant. 

Commercial  charcoal  was  found  unsuitable  for  use  in  gas  masks 
because  of  its  low  power  of  absorption  and  its  poor  resistance  to  abrasion. 
The  best  raw  material  for  the  production  of  carbon  was  found  to  be  coco- 
nut shells,  but  any  kind  of  nut  shells  or  fruit  pits  were  used  when  sufficient 
quantities  of  coconut  shells  could  not  be  obtained.  The  shells  were 
first  carbonized  in  retorts,  the  carbon  was  then  crushed  and  screened 
between  8  and  16  mesh,  and  finally  submitted  to  a  special  heat-treat- 
ment in  air;  steam  treatment  was  later  substituted  for  treatment  in  air. 
The  production  of  gas-mask  carbon  on  a  commercial  scale  was  begun 
at  Astoria  about  Aug.  1,  1917. 

Temperature  control  was  essential  during  the  initial  carbonization 
of  the  she  Is,  as  well  as  during  the  air  or  steam  treatment,  and  pyrometers 
were  installed  for  this;  purpose.  The  installation  and  maintenance  of  all 
pyrometry  equipment  was  done  by  the  pyrometry  department,  consisting 
of  an  officer  in  charge,  an  assistant,  and  two  men.  A  special  pyrometry 
laboratory  was  maintained  in  which  all  repairs  were  made. 

TEMPERATURE  CONTROL  IN  THE  RETORTS 

The  initial  carbonization  of  the  raw  material  was  accomplished  in 
horizontal  retorts,  formerly  used  for  the  production  of  coal  gas.  These 
retorts,  of  semi-elliptical  cross-section,  were  20  ft.  (6  m.)  long,  2  ft.  (60 
cm.)  wide  and  18  in.  (46  cm.)  high;  they  were  arranged  in  banks  of 
two  rows  of  four  retorts  each,  one  above  the  other;  four  banks  constituted 
a  bench.  Every  retort  had  a  door  at  each  end,  opening  to  the  full  size 
of  the  cross-section  of  the  retort,  which  was  clamped  shut  except  during 


*  Pyrometric  Engineer,  Aluminum  Co.  of  America;  formerly  officer  in  charge, 
Pyrometry  Department,  Astoria  Detachment,  Chemical  Warfare  Service,  U.  S.  A. 

1  For  further  details  as  to  the  manufacturing  process,  see  Contributions  from 
the  Chemical  Warfare  Service.  Ind.  and  Eng.  Chem.,  1919. 


KIRTLAND    MARSH 


653 


charging,  drawing,  or  raking  the  charge.  The  retorts  were  heated  to 
about  900°  C.  by  hot  gases  coming  from  coke  fires  at  each  end  of  the  bank 
and  passing  through  flues  around  the  retorts.  The  retort  was  charged 
by  buckets  about  10  ft.  (3  m.)  long  on  the  traveling  boom  of  a  charging 
machine,  one  at  each  end  of  the  retorts.  The  charge  was  pushed  out  by 
a  semi-elliptical  plate,  a  little  smaller  than  the  cross-section  of  the  retort, 
on  the  end  of  the  traveling  boom  of  a  drawing  machine  which  traveled 
on  tracks  at  one  end  of  the  retorts.  The  plate  Was  pushed  through  the 
retort  from  end  to  end,  forcing  the  carbon  out  into  a  chute  leading  to 
small  hopper  cars  (Fig.  1). 


HOAVING    A   RETORT  BEING  DRAWN  BY  THE  DISCHARGING  MACHINE. 


This  method  of  charging  and  drawing  obviously  prohibited  the  in- 
stallation of  any  permanent  thermocouple  in  the  retorts.  Thermocouples 
which  could  be  removed  during  the  charging  and  drawing  might  have 
been  used,  but  the  life  of  a  base-metal  couple,  even  in  a  protecting  sheath, 
would  have  been  short,  and  a  couple  long  enough  to  reach  to  the  middle 
of  the  retort  would  have  been  awkward  to  handle;  the  expense,  also,  of 
maintaining  a  thermocouple  for  each  retort  would  have  been  excessive. 
Furthermore,  it  was  necessary  to  measure  the  temperature  in  the  flues, 
and  for  that  purpose  a  base-metal  couple  could  not  have  been  used, 
the  temperature  being  about  1200°  C.  The  choice  thus  lay  between 
optical  or  radiation  pyrometers;  a  Leeds  &  Northrup  optical  pyrometer 
was  tested  and  found  to  give  very  satisfactory  results. 


654  MANUFACTURE    OF   GAS-MASK    CARBON 

Readings  of  the  temperatures  in  the  retorts  and  flues  were  taken  every 
two  hours.  More  frequent  readings  were  unnecessary  because  closer 
temperature  control  was  not  needed,  in  fact  was  almost  impossible,  be- 
cause of  the  slow  change  in  retort  temperature  after  manipulation  of  the 
fires. 

TEMPERATURE  CONTROL  IN  THE  AIR-TREATERS 

The  air-treater  consisted  of  a  set  of  five  iron  tubes,  set  one  above  the 
other,  at  a  slope  of  about  3°,  in  a  combustion  chamber;  each  tube  was 
12  ft.  (3.6  m.)  long  and  12  in.  (30  cm.)  inside  diameter.  A  10-in.  (25-cm.) 
screw  conveyor  running  the  entire  length  of  the  tube,  tangent  to  the 
bottom,  conveyed  the  carbon  through  the  tube.  The  carbon  was  fed 
into  the  back  end  of  the  upper  tube,  traveled  through  it  and  dropped  into 
the  next  tube,  and  so  on  until  it  was  discharged  into  drums  at  the  end 
of  the  fifth  tube.  The  discharge  end  of  the  air-treaters  is  shown  in  Fig. 
2.  Each  combustion  chamber  contained  two  sets  of  tubes,  heated  by  a 
gas  burner  at  the  bottom  of  the  chamber,  to  a  temperature  of  about 
400°  C. 

The  level  of  the  material  in  the  tube  was  about  4  in.  (10  cm.)  from  the 
bottom,  while  the  free  space  at  the  top  of  the  tube  was  2  in.  In  the 
center  of  this  free  space  was  placed  a  ^-in.  (12.7-mm.)  wrought-iron 
pipe  12  ft.  long,  anchored  at  each  end  in  the  head  of  the  tube;  this 
pipe  constituted  the  pyrometer  tube. 

When  these  air-treaters  were  first  built  there  was  only  a  single  com- 
bustion chamber  with  two  sets  of  tubes.  The  temperatures  were  meas- 
ured by  inserting  a  Price,  sheathed-wire,  iron-constantan  thermocouple 
in  the  pyrometer  tube  and  allowing  it  to  remain  until  temperature  equi- 
librium was  established;  the  temperature  was  then  read  from  a  Price 
indicating  pyrometer  and  the  couple  moved  to  the  next  tube. 

Later,  stationary  thermocouples,  6  ft.  (1.8  m.)  long,  made  of  No.  16 
iron  and  constantan  wires,  insulated  with  porcelain  beads,  were  inserted 
in  each  pyrometer  tube.  Similar  couples  in  similar  pyrometer  tubes  were 
placed  in  the  combustion  chambers,  between  the  two  sets  of  reaction 
tubes,  at  four  levels.  The  iron  and  the  constantan  lead  wires  from  the 
thermocouples  were  run  in  a  conduit  to  a  common  junction  box, 
where  they  were  soldered  to  the  copper  leads  connected  to  Wilson- 
Maeulen,  two-pole,  rotary  selector  switches  mounted  on  the  box.  Fig. 
2  shows  the  ends  of  the  pyrometer  tubes  projecting  from  the  heads  of  the 
reaction  tubes,  and  the  conduit  running  to  the  common  junction  box. 
In  Fig.  3  can  be  seen  the  lead  wires,  the  common  junction  box,  and  the 
selector  switches.  These  switches  were  all  connected  to  a  Leeds  & 
Northrup  indicating  potentiometer  set  in  the  top  of  the  junction  box. 

As  the  lead  wires  were  of  iron  and  constantan,  the  cold  junc- 
tions of  the  thermocouples  were  actually  at  the  end  of  the  lead  wires  in 


KIRTLAND    MARSH 


655 


the  common  junction  box.  The  first  potentiometer  used  was  equipped 
with  a  hand-operated,  cold-junction  compensator,  making  it  necessary 
to  determine  the  temperature  in  the  common  cold-junction  box  with  a 
thermometer  and  make  the  necessary  adjustment  on  the  potentiometer, 
but  as  the  temperature  in  the  junction  box  changed  very  little  from  hour 
to  hour,  the  adjustment  was  relatively  simple.  Later  a  Leeds  &  Northrup 
potentiometer  was  used  which  was  equipped  with  an  automatic  cold- 
junction  compensating  coil,  which  was  placed  with  the  cold  junctions 
in  the  junction  box. 


FIG.  2.- 


-DlSCHARGE  END  OF  AIR-THEATERS,  SHOWING  ENDS  OF  PYROMETER  TUBES  AND 
CONDUIT  FOR  THE  LEAD  WIRES. 


At  first,  readings  were  taken  on  all  the  couples  in  the  tubes  and  com- 
bustion chambers  every  half  hour,  but  it  was  soon  found  that  the  quality 
of  the  material  bore  a  close  relation  to  the  temperature  in  the  fourth 
tube,  which  was  the  hottest,  and  as  there  was  no  means  for  regulating 
the  temperature  of  each  tube  individually  it  became  a  matter  of  regulat- 
ing the  burners  to  give  the  proper  temperature  in  the  fourth  tube.  As 
there  was  a  fairly  constant  relation  between  the  temperature  of  the  fourth 
tube  and  the  other  tubes  in  the  same  set,  it  was  necessary  to  take  read- 
ings only  on  the  fourth  or  control  tube.  Similarly,  the  reading  of  tern- 


656 


MANUFACTURE   OF   GAS-MASK   CARBON 


peratures  in  the  combustion  chambers  was  reduced  to  one  couple  in  each 
chamber;  occasionally  readings  on  all  the  couples  would  be  made  to  see 
that  the  relation  had  not  changed.  The  temperature  in  the  control 
tube  was  maintained  at  about  400°  C. 


FIG.  3. — PYROMETER  FOR  AIR-TREATERS,  SHOWING  COMMON  COLD-JUNCTION  BOX, 
WlLSON-MAEULEN  SWITCHES,  AND  LEEDS  &  NORTHRUP  POTENTIOMETER. 


TEMPERATURE  CONTROL  IN  THE  STEAM-TREATERS 

Research  developed  a  method  of  treating  the  carbon  from  the  retorts 
with  steam,  which  produced  a  material  of  much  better  quality  than  was 
obtained  from  the  air-treaters.  Furnaces  for  treating  carbon  with  steam 
were  designed,  and  the  first  battery  of  10  furnaces  was  put  into  operation 
at  Astoria  early  in  March,  1918. 

The  furnace  consisted  of  a  vertical,  gas-fired  combustion  chamber,  7 
ft.  (2.1  m.)  high  and  27  in.  (68  cm.)  inside  diameter,  surrounding  a 
nichrome  reaction  tube  having  %-in..  (19-mm.)  walls  and  an  inside 
diameter  of  7  in.  (17.8  cm.).  Coal  gas  mixed  with  air  in  Premix  burners 
was  used  as  fuel.  Inside  the  nichrome  reaction  tube  was  a  2-in.  (5- 
cm.)  nichrome  pipe  extending  the  entire  height  of  the  furnace,  and  per- 


KIRTLAND    MARSH  657 

f orated  all  around  for  8  in.  (20  cm.),  at  about  the  level  of  the  center  of 
the  combustion  chamber,  with  ^-in.  (6-mm.)  holes.  This  nichrome 
pipe  was  essentially  a  steam  jet;  at  first,  steam  was  admitted  at  the 
top,  but  later  at  the  bottom. 

A  charging  valve  at  the  top  of  the  furnace  admitted  the  carbon  be- 
tween the  reaction  tube  and  the  steam  jet,  which  space  was  kept  filled, 
and  a  similar  valve  at  the  bottom  served  to  discharge  the  carbon.  Be- 
tween the  bottom  of  the  reaction  tube  and  the  discharge  valve  a  rotating 
spider  valve  maintained  the  flow  of  carbon  through  the  tube  at  a  con- 
stant rate. 

Research  had  shown  that  the  best  quality  of  carbon  was  produced 
when  it  was  treated  with  steam  at  a  temperature  of  950°  C.  To  obtain 
this  temperature  in  the  mass  of  charcoal  at  the  level  of  the  steam  orifice, 
and  at  a  point  midway  between  the  steam  pipe  and  the  wall  of  the  reac- 
tion tube,  it  was  necessary  to  maintain  a  temperature  of  about  1150°  C. 
in  the  combustion  chamber  at  a  level  about  27  in.  (68  cm.)  above  the  gas 
burner. 

At  Nela  Park,  Cleveland,  Ohio,  where  the  original  designing  and  test- 
ing of  these  steam-treaters  was  done,  the  temperature  of  the  material 
was  measured  by  a  Price  sheathed-wire,  iron-constantan  thermocouple, 
connected  to  a  Price  indicator.  The  thermocouple  was  introduced  into 
the  material  through  a  horizontal  open-end,  nichrome  pyrometer  tube 
screwed  into  the  wall  of  the  reaction  tube.  The  pyrometer  tube  had 
about  1  in.  (25  mm.)  inside  diameter  and  was  surrounded  by  an  open- 
end  iron  tube,  of  3-in.  diameter,  which  extended  from  the  reaction  tube  to 
the  outside  of  the  furnace;  this  provided  an  air  space  around  the  pyrome- 
ter tube  where  it  passed  through  the  combustion  chamber  and  kept  it 
cooler,  thereby  adding  to  the  life  of  the  thermocouple.  As  the  pyrome- 
ter tube  was  open  at  the  inner  end,  a  packing  gland  was  required  around 
the  thermocouple,  where  it  protruded  from  the  outer  end  of  the  pyrome- 
ter tube,  to  prevent  the  pressure  within  the  reaction  tube  from  blowing 
the  carbon  out  through  the  pyrometer  tube.  The  hot  junction  of  the 
thermocouple  was  in  direct  contact  with  the  carbon  and  the  steam  which, 
at  this  temperature,  would  be  decomposed  into  oxygen  and  hydrogen. 
Under  these  conditions,  the  life  of  a  thermocouple  was  about  four  days. 
To  replace  a  thermocouple  it  was  necessary  to  shut  off  the  steam  to  pre- 
vent the  carbon  from  blowing  out  through  the  pyrometer  tube  when  the 
old  thermocouple  was  removed;  even  with  the  steam  shut  down,  some 
carbon  followed  the  thermocouple  into  the  pyrometer  tube  and  rendered 
the  insertion  of  another  thermocouple  very  difficult. 

When  a  production  unit  of  ten  of  these  furnaces,  to  be  erected  at 
Astoria,  was  being  designed,  it  was  decided  that  a  pyrometer  tube  closed 
at  its  inner  end  would  be  best.  The  danger  of  burning  out  the  reaction 
tube,  if  the  flow  of  steam  was  interrupted  for  any  length  of  time,  often  made 

42 


MANUFACTURE    OF    GAS-MASK    CARBON 

it  necessary  to  shut  off  the  gas  when  a  base-metal  couple  in  an  open-end 
pyrometer  tube  was  being  changed.  The  short  life  of  a  base-metal  couple 
at  the  temperature  encountered  in  the  combustion  chamber  through 
which  it  passed  promised  to  be  a  source  of  great  expense.  A  base-metal 
couple  of  other  materials  than  iron  and  constantan,  such  as  nickel  and 
nichrome,  would  probably  have  given  good  service  even  at  these  tem- 
peratures if  used  intermittently,  but  for  continuous  use  even  such  couples 
would  undoubtedly  require  frequent  changing.  For  these  reasons  it 
was  decided  to  use  a  thermocouple  of  platinum  and  platinum  plus  10 


FIG.  4. — STEAM- THEATERS,  SHOWING  PREMIX  BURNERS,  PYROMETER  TUBES,  CONNEC- 
TION BOXES,  COMPENSATING  COUPLES,  AND  COLD-JUNCTION  WELLS.  TYPICAL  OF  FIRST 
FORTY  STEAM-TREATERS  ERECTED. 

per  cent,  rhodium,  protected  by  a  glazed,  ceramic,  pyrometer  tube  inside 
of  a  closed-end  nichrome  tube  screwed  into  the  wall  of  the  reaction  cham- 
ber. The  nichrome  pyrometer  tube  was  turned  down  to  a  wall  thickness 
of  about  j^-in.  (3  mm.)  and  was  so  placed  in  the  furnace  that  the  hot 
junction  of  the  platinum  thermocouple  would  be  midway  between  the 
steam  jet  and  the  reaction  tube. 

On  the  outer  end  of  the  pyrometer  tube  was  attached  a  stamped- 
steel  zone  box,  4  by  4  by  3  in.  (10  by  10  by  7.6  cm.)  provided  with  a 
hinged  cover  and  a  lock.  On  the  inside  of  the  zone  box  was  a  small 
asbestos  block  on  which  were  mounted  three  binding  posts.  The  cold  ends 


KIRTLAND    MARSH 


659 


of  the  thermocouple,  the  ends  of  an  auxiliary  couple,  and  the  lead  wires 
to  the  measuring  instrument  were  connected  to  these  three  binding  posts 
in  such  a  way  that  the  platinum  thermocouple,  the  auxiliary  couple,  and 
the  lead  wires  were  in  series.  The  auxiliary  couple  was  made  of  Wilson- 
Maeulen  compensating  wire,  which,  within  the  limits  of  temperature 
encountered  at  the  cold  junction  of  a  thermocouple,  has  the  same  thermo- 
electric characteristics  as  the  platinum  thermocouple.  By  using  this 
auxiliary  couple,  the  cold  junction  of  the  platinum  thermocouple  was  re- 
moved from  a  position  of  varying  temperature,  at  the  end  of  the 
pyrometer  tube,  to  one  of  constant  temperature  in  a  well  in  a  pipe 
through  which  tap  water  was  circulating,  the  soldered  ends  of  the  auxil- 
iary couple  being  buried  in  this  well.  The  general  arrangement  of  Pre- 


FIG.  5. — THERMOCOUPLE  CONNECTION  BOX  ON  STEAM-TREATERS,  SHOWING  DETAILS 
OF  CONNECTION  BOX,  NICHROME  PYROMETER  TUBE,  IMPERVITE  PYROMETER  TUBE,  PLATI- 
NUM THERMOCOUPLE,  BINDING  POSTS.  COMPENSATING  COUPLE,  AND  COLD-JUNCTION 

WELL. 

mix  burners,  zone  box,  and  cold-junction  wells  can  be  seen  in  Fig.  4,  and 
the  details  of  the  zone  box,  auxiliary  couple,  and  cold-junction  well  in 
Fig.  5.  In  each  cold-junction  well,  with  the  junction  of  the  auxiliary 
couple,  was  placed  a  small  glass  tube  into  which  a  thermometer  could 
be  put;  this  tube  and  the  auxiliary  couple  were  sealed  in  the  well  with 
paraffin.  It  was  found  that  with  a  moderate  flow  of  water  through  the 
wells,  the  cold-junction  temperature  varied  only  1°  or  2°  F.  from 
day  to  day. 

The  lead  wires  from  the  zone  box  to  the  instrument  were  of  copper  and 
were  run  in  conduit  to  a  switch  box  located  in  front  of  the  measuring 
instrument  which,  in  this  case,  was  a  Leeds  &  Northrup  recording  poten- 
tiometer. In  the  switch  box  was  a  push-button  switch  for  each  thermo- 
couple, so  arranged  that  the  thermocouple  was  normally  connected  to  the 
recorder,  but  upon  the  depression  of  the  button  the  thermocouple  would 


660 


MANUFACTURE    OF    GAS-MASK    CARBON 


be  disconnected  from  the  recorder  and  connected  to  an  indicator.  This 
was  to  permit  checking  of  the  recorder  against  an  indicator  or  reading  of 
the  furnace  temperature  at  any  time  the  recorder  was  not  in  operation. 
The  recorder  was  placed  in  the  furnace  room  so  that  the  operators 
could  follow  the  temperature  record  and  adjust  the  burners  accordingly. 
It  rested  on  a  stand  attached  to  a  turntable  made  of  two  4-in.  pipe  flanges 
connected  by  a  6-in.  nipple,  the  bottom  flange  being  bolted  down  to  a  solid 
brick  pier.  The  recorders  were  enclosed  in  a  wooden  cabinet  supported 
independently  of  the  recorders  and  their  supports.  The  outside  of  this 
case  was  flush  with  the  wall  of  the  furnace  room  and  was  provided  with  a 
window,  while  the  side  within  the  office  had  a  door,  through  which  the 


FIG.  6. — RECORDING  PYROMETER  AS  SEEN  FROM  FURNACE  ROOM,  SHOWING  LEEDS 
&  NORTHRUP  RECORDING  POTENTIOMETER  IN  THE  CASE,  AND  PUSH-BUTTON  SWITCHES 
AND  INDICATING  POTENTIOMETER  BELOW. 

recorder  could  be  cleaned;  thus  the  instrument  was  protected  from  the 
carbon-laden  atmosphere  of  the  furnace  room  while  it  was  being  cleaned 
or  adjusted.  The  exterior  of  the  case,  the  push-button  switches,  and  the 
indicating  potentiometer  for  checking  the  recorder  are  shown  in  Fig.  6. 
There  was  one  recorder  for  each  battery  of  10  furnaces  and  the  tempera- 
ture of  each  furnace  was  recorded  every  10  minutes. 

In  two  furnaces,  Price  sheathed-wire  thermocouples  were  placed  diag- 
onally opposite  the  platinum  couple,  and  at  the  same  level,  to  compare 
the  indications  by  the  two  systems.  At  times  the  two  couples  agreed 
fairly  well,  but  at  other  times  wide  variations  were  observed  Wide 
and  sudden  fluctuations  of  temperature  were  occasionally  indicated  by 
both  thermocouples ;  at  one  time  the  temperature,  which  had  been  steady 


KIRTLAND   MARSH  661 

for  some  time,  was  observed  suddenly  to. rise  100°  C.,  and  even  more 
suddenly  drop  200°,  after  which  it  slowly  rose  to  normal.  This  was 
caused  by  the  formation  of  a  gas  pocket  around  the  pyrometer  tube, 
followed  by  an  increase  of  temperature  within  the  pocket,  due  to  the 
absence  of  carbon  to  absorb  the  heat.  When  the  pocket  broke,  a  mass 
of  cooler  carbon  rilled  the  space,  and  slowly  assumed  the  normal  tem- 
perature at  that  level.  It  was  thus  seen  that  accurate  temperature 
control,  which  was  necessary  because  the  nichrome  tube  was  being  used  at 
a  temperature  very  near  its  softening  point,  could  not  be  maintained. 

A  test  to  determine  the  horizontal  temperature  gradient  between 
the  wall  of  the  reaction  tube  and  the  steam  jet,  at  the  level  of  the  pyro- 
meter tube,  showed  a  difference  in  temperature  varying  from  150°  to  200° 
C.,  the  distance  between  these  two  points  being  about  2^£  in.  (54  mm.). 
This  showed  why  the  platinum  and  the  base-metal  couples  in  the  same 
furnace  had  not  agreed  more  closely;  under  the  conditions  noted  above, 
the  possible  error  in  measuring  temperatures  could  not  safely  be  esti- 
mated at  less  than  plus  or  minus  50°  C. 

After  the  above  tests  had  been  made,  the  idea  of  measuring  tempera- 
ture within  the  reaction  tube  was  abandoned.  The  thermocouples  were 
removed  from  the  reaction  tube  and  placed  in  closed-end  nichrome  pyro- 
meter tubes  in  the  combustion  chamber,  at  the  same  level  as  before; 
the  end  of  the  tube  was  %  in.  (19  mm.)  from  the  reaction  tube,  which 
point  was  found  to  be  the  hottest  at  that  level.  To  determine 
whether  the  pyrometers  were  at  the  best  level  in  the  combustion 
chamber,  similar  thermocouples  in  protection  tubes  were  placed  at  two 
levels  above  and  at  one  level  below  the  regular  thermocouple.  These 
three  extra  couples  were  connected  to  the  recorder  for  about  12  hr. 
Examination  of  the  records  showed  that  the  nearer  the  couple  was  to  the 
burner,  the  wider  were  the  temperature  fluctuations.  The  records 
showed  little  difference  in  sensitivity  to  temperature  changes,  i.e.,  a  change 
indicated  by  the  couple  lowest  in  the  furnace  would  be  indicated  at  the 
same  time  by  the  couple  at  the  highest  level.  The  selection  of  the  best 
level  thus  became  a  compromise,  for  the  lower  the  couple  was  placed  the 
higher  was  the  temperature  to  which  the  nichrome  pyrometer  tube  was 
subjected,  and  the  higher  the  couple  the  less  marked  were  the  changes 
in  temperature.  It  was  decided  that  the  level  at  which  the  couples  were 
already  installed  was  as  satisfactory  as  any. 

This  method  of  temperature  control,  i.e.,  a  platinum  thermocouple 
in  the  combustion  chamber,  was  adopted  for  four  batteries  of  10  furnaces. 
Sixty  more  of  these  furnaces  were  built  in  batteries  of  10  furnaces  each, 
but  as  Leeds  &  Northrup  recording  potentiometers  could  not  be  delivered 
until  long  after  the  furnaces  were  scheduled  to  be  put  in  operation,  another 
system,  known  as  central-station  temperature  and  signal  control,  was 
installed. 


662 


MANUFACTURE    OF   GAS-MASK    CARBON 


The  thermocouples,  protection  tubes,  zone  boxes,  and  cold-junction 
compensating  couples  were  installed  as  on  the  original  40  furnaces.  The 
leads  from  the  10  thermocouples  in  each  battery  were  connected  to  a  cable 
through  copper  strips  mounted  in  a  terminal  box  on  the  end  of  the  battery, 
the  cable  connecting  with  a  central  switchboard  in  the  administration 
office.  The  switchboard,  with  a  capacity  for  24  thermocouples,  consisted 
of  a  horizontal  and  a  vertical  section.  The  cables  connected  to  the  ther- 
mocouples on  two  batteries  were  fanned  out  to  double-pole  switches 
mounted  on  the  horizontal  section  of  the  switchboard;  these  switches 
were  connected  to  a  Leeds  &  Northrup  indicating  potentiometer,  equipped 
with  a  hand-operated  cold-junction  compensator.  On  the  vertical  section 


FIG.  7. — STEAM-TREATERS,  SHOWING  THERMOCOUPLE   CONNECTION  BOXES,  COLD- 
JUNCTION    WELLS.    AND    SIGNAL   LIGHTS    ARRANGED    FOR    CENTRAL-STATION    CONTROL. 

TYPICAL  OF  LAST  SIXTY  STEAM-TREATERS  ERECTED. 

of  the  switchboard  were  24  sets  of  three  signal-lamp  switches,  each 
having  a  small  telephone  pilot  lamp  over  it.  The  signal-lamp  switches 
and  pilot  lamps  of  each  set  were  in  series  with  three  signal  lamps  mounted 
over  the  burner  of  each  furnace.  A  cable  connected  the  signal  switches 
and  pilot  lamps  on  the  switchboard  to  a  terminal  box  on  the  end  of  each 
battery,  from  which  circuits  were  run  to  the  three  signal  lamps  on  each 
furnace,  seen  at  the  top  of  Fig.  7.  The  signal  lamps  on  the  furnace  were 
red,  white,  and  green,  respectively,  and  the  switchboard  pilot  lamp  in 
series  with  each  of  them  was  the  same  color. 

The  temperature  of  each  of  the  20  furnaces  connected  to  one  switch- 


KIRTLAND    MARSH  663 

board  was  read  every  10  min.  and  recorded  every  20  min.  by  an  operator, 
who  set  the  colored  signals  according  to  the  following  schedule: 

1183°  or  higher white,  red  and  green 

1166°  to  1183° red 

1158°  to  1166° red  and  white 

1142°  to  1158° white 

1134°  to  1142° white  and  green 

Below  1134° green 

By  watching  the  colored  signals  over  each  furnace,  the  operator 
could  adjust  his  burners;  the  officer  in  charge  also  could'  tell  at  a  glance 
the  temperature  of  each  furnace.  As  the  pilot  lamps  on  the  switchboard 
were  in  series  with  the  signal  lamps  on  the  furnaces,  neither  lamp  could 
burn  out  without  being  observed  at  once.  Beside  each  switchboard  was 
a  telautograph  instrument  through  which  information  or  instructions 
could  be  transmitted  in  writing  from  the  switchboard  operator  to  the 
burner  operator,  or  vice  versa.  In  case  of  damage  to  the  switchboard, 
it  was  possible  to  read  the  temperature  of  the  furnaces  by  connecting  a 
potentiometer  to  the  leads  from  each  thermocouple  at  the  terminal  box 
on  the  end  of  the  battery.  The  success  of  this  installation  was  never 
demonstrated  because  the  furnaces  were  operated  for  only  a  short  time 
after  the  signing  of  the  armistice. 

TEMPERATURE  CONTROL  IN  TECHNICAL  DEVELOPMENT 
LABORATORY 

In  the  technical  development  laboratory  were  full-size  steam-treaters 
for  experimental  purposes,  small  electric-furnace  steam-treaters,  several 
sizes  of  experimental  retorts,  and  other  miscellaneous  furnaces  for  research 
work.  In  most  of  these  furnaces,  control  was  maintained  by  measure- 
ments of  temperature  in  the  combustion  chamber,  by  platinum  couples 
connected  to  a  Leeds  &  Northrup  indicating  potentiometer  through 
selector  switches. 

For  measuring  temperature  in  the  mass  of  material  in  a  steam-treater, 
retort,  or  other  furnace,  Wilson-Maeulen  sheathed-wire  iron-constantan 
thermocouples  (known  by  the  trade  name  "Pyod")  were  used,  in  connec- 
tion with  a  Wilson-Maeulen  or  a  Price  indicating  pyrometer  (millivolt- 
meter),  or  a  Leeds  &  Northrup  potentiometer.  In  the  case  of  permanent 
installations,  iron  and  constantan  lead  wires  transferred  the  cold  junction 
of  the  thermocouple  to  a  water-cooled  well  similar  to  those  used  for  the 
platinum  couples.  For  temporary  installations,  as  in  short  tests  of 
temperature  gradient,  the  compensating  lead  wires  were  connected 
directly  to  the  binding  posts  of  the  instrument,  at  which  point  the  cold- 
junction  temperature  was  read  with  a  mercury  thermometer. 


664  MANUFACTURE    OF   GAS-MASK   CARBON 

These  pyods  were  used  in  direct  contact  with  carbon  at  temperatures 
as  high  as  950°  C.,  and  although  the  life  of  the  couples  was  short,  they 
gave  very  satisfactory  results  and  maintained  their  calibration  closely 
until  burned  out;  this  fact  was  established  by  repeated  recalibration. 
On  several  occasions  these  couples  satisfactorily  measured  the  tempera- 
ture of  a  mass  of  carbon  as  high  as  1200°  C. 

TYPES  OF  PYROMETER  TUBES  EMPLOYED 

For  the  primary  protection  tubes  of  platinum  thermocouples,  alundum, 
impervite,  and  usalite  were  all  used;  the  choice  depended  principally  on 
the  deliveries  that  could  be  obtained  and  on  the  dimensions  of  the  avail- 
able tubes,  for  all  three  materials  gave  good  service.  For  the  secondary 
protection  tubes,  nichrome  was  most  frequently  used,  but  some  alundum 
and  some  impervite  tubes  were  employed.  In  the  first  steam-treater 
installation,  in  which  the  secondary  protection  tube  was  screwed  into 
the  reaction  tube,  nichrome  was  used  from  necessity.  After  measurements 
of  combustion-chamber  temperature  were  adopted  for  control,  nichrome 
protection  tubes  were  still  employed  because  they  were  not  fragile,  but 
some  alundum  and  some  impervite  tubes  were  tested  and  were  found  to 
protect  the  primary  protection  tube  and  the  platinum  thermocouple 
better  than  the  nichrome  tube.  A  primary  tube  protected  by  an  alundum 
or  impervite  secondary  tube,  in  use  for  two  months,  remained  in  better 
condition  than  one  protected  by  a  nichrome  tube,  used  for  the  same 
length  of  time.  It  is  probable  that  when  it  became  necessary  to  replace 
the  nichrome  protection  tubes,  alundum  or  impervite  would  have  been 
selected.  Tubes  of  these  materials  were  never  known  to  soften  at  the 
temperatures  encountered  in  the  furnaces,  as  the  nichrome  tubes  some- 
times did. 


Thermoelectric  Pyrometers 

Recording  pyrometers  were  inspected  and  checked  against  an  indi- 
cating potentiometer  once  or  twice  a  day.  All  platinum  thermocouples, 
ceramic  protection  tubes,  and  pyrometer  connections  were  inspected 
daily.  All  base-metal  couples,  their  connections,  and  the  indicators  and 
switches  in  circuit  with  them,  were  inspected  daily.  Optical  pyrometers 
were  inspected  two  or  three  times  a  week;  more  frequent  inspection  was 
unnecessary  because  when  anything  was  out  of  order  in  these  instruments, 
it  was  immediately  apparent. 

The  iron-constantan  thermocouples  in  the  air-treaters  were  checked 
by  recalibration  in  an  electric  furnace  in  the  laboratory  or,  in  place,  by 


KIRTLAND   MARSH  665 

a  standard  base-metal  thermocouple  inserted  in  the  same  furnace  tube 
and  to  the  same  depth  as  the  couple  to  be  calibrated.  Readings  of  the 
temperature  indicated  by  the  standard  couple  on  a  laboratory  potentio- 
meter were  compared  with  the  readings  on  the  plant  instrument  of  the 
temperature  indicated  by  the  couple  being  checked. 

The  platinum  thermocouples  used  in  the  steam-treaters  were  removed 
from  the  furnaces  periodically  and  taken  to  the  laboratory  for  checking. 
The  thermocouple,  stripped  of  its  porcelain  two-bore  insulation,  was  first 
annealed  at  a  temperature  of  about  1400°  C.  for  10  min.  by  passing  electric 
current  through  it.  The  couple  was  then  tested  for  homogeneity  by 
exploring  the  platinum  and  the  platinum-rhodium  elements  with  un- 
contaminated  wires  of  the  same  materials.  In  this  operation  one  element 
of  the  thermocouple  and  a  pure  wire  of  the  same  material  were  con- 
nected to  a  galvanometer,  and  the  two  wires  were  brought  into  contact 
in  the  oxidizing  flame  of  a  Bunsen  burner;  any  inhomogeneity  was  in- 
dicated by  a  deflection  of  the  galvanometer.  After  the  entire  length  of 
one  element  of  the  couple  had  been  tested  in  this -manner  the  other  ele- 
ment was  tested  in  the  same  way.  Any  inhomogeneous  spots  in  the 
thermocouple,  located  by  this  test  for  homogeneity,  were  removed  by 
further  annealing;  or  if,  after  reannealing  for  30  to  40  min.,  the  spots 
were  not  rendered  homogeneous  the  affected  section  of  the  couple  was 
cut  out  and  the  two  ends  welded  together  in  an  oxy-gas  flame.  In  only 
a  very  few  cases  was  it  necessary  to  cut  out  any  part  of  the  couples. 
When  it  was  ascertained  that  the  elements  of  the  couple  were  in  good  con- 
dition, it  was  threaded  in  four-bore  insulation,  together  with  a  standard 
platinum  couple,  and  placed  in  a  pyrometer  tube  suspended  in  a  vertical, 
electric,  checking  furnace.  The  cold  ends  of  both  couples  were  immersed 
in  mercury  wells  in  a  cold-junction  box,  with  a  thermometer.  Copper 
leads  connected  the  mercury  wells  with  a  Leeds  &  Northrup  indicating 
potentiometer,  through  a  Wilson-Maeulen  selector  switch.  Readings  at 
equal  intervals  of  time,  generally  every  30  sec.,  were  taken  alternately 
on  the  standard  couple  and  on  the  couple  being  checked  until  a  constant 
difference  between  the  readings  of  the  two  couples  was  obtained.  From 
the  time-temperature  curves  plotted  from  these  readings  the  actual  differ- 
ence in  the  indications  of  the  two  couples  could  be  determined.  This 
method  of  taking  readings  at  equal  intervals  of  time  eliminated  the  neces- 
sity of  waiting  for  an  absolutely  constant  temperature  in  the  furnace, 
although  the  more  nearly  constant  the  temperature  the  better  the  results. 
A  steady  rising  or  falling  temperature  is  more  satisfactory  than  a  fluctuat- 
ing temperature. 

So  long  as  the  couples  remained  in  good  condition  and  showed  no 
indications  of  inhomogeneity,  the  above  method  of  calibration  was  satis- 
factory, but  as  the  couples  deteriorated  and  the  calibration  changed  it 
was  planned  to  calibrate  them  under  working  conditions.  For  this  pur- 


666  MANUFACTURE    OF    GAS-MASK    CARBON 

pose,  alundum  secondary  and  primary  protection  tubes  were  mounted  in 
one  of  the  steam-treaters,  at  the  same  level  and  inserted  to  the  same 
depth  as  the  regular  control  couple.  The  standard  couple  and  the 
couple  to  be  calibrated  would  then  be  inserted  in  the  tube,  and  readings 
would  be  taken  as  just  described.  In  this  method  of  calibration,  the 
effects  of  any  deterioration  in  the  couple  would  be  the  same  as  when 
the  couple  was  in  use,  because  the  temperature  gradient  along  the-calibrat- 
ing  tube  was  the  same.  The  need  of  such  a  method  of  calibration  had 
not  been  felt  up  to  the  time  that  the  furnaces  were  shut  down  on  Dec.  31, 
1918. 

Of  98  platinum  thermocouples  installed,  only  three  were  destroyed  in 
use.  The  readings  from  these  three  couples  indicated  that  something 
was  wrong,  and  upon  investigation  it  was  found  that  for  a  distance  of 
10  in.  (25  cm.)  from  the  hot  end  both  elements  of  the  couple  were  crystal- 
lized to  such  an  extent  that  they  crumbled  when  handled.  A  superficial 
examination  of  the  alundum  protection  tube  disclosed  nothing  unusual, 
and  the  cause  of  crystallization  was  never  ascertained.  This  occurred 
about  three  weeks  after  they  were  put  in  service. 

A  complete  record  of  each  platinum  thermocouple  was  kept  on  a 
card  showing  the  date  of  purchase,  order  number,  from  whom  purchased, 
original  weight  and  length  of  each  element,  original  weight  of  the  couple, 
and  the  original  calibration  readings.  Every  time  its  couple  was  cali- 
brated, a  record  was  made  of  the  date,  weight  of  the  couple,  length  of  the 
elements,  condition  when  removed,  calibration  data,  and  notes  on  any 
alterations.  The  following  data  are  quoted  from  a  report  submitted  by 
P.  H.  Walker,  assistant  to  the  officer  in  charge  of  the  pyrometry  depart- 
ment, after  all  the  furnaces  were  shut  down  and  the  couples  assembled  in 
the  laboratory. 

•  GRAMS 

Total  stock  received 830. 1134 

Platinum  and  platinum-rhodium  inventoried: 

Thermocouples 806.6714 

Scraps 11 .7981 

Unused  platinum  wire 3 . 1080 

Unused  platinum-rhodium  wire.  .f 5. 1400 

Total  accounted  for . .  826 . 7175 


Loss  in  use 3 . 3959 

The  average  length  of  time  that  these  couples  were  in  use  was  proba- 
bly about  six  months,  which  places  the  platinum  loss  at  about  0.82 
per  cent,  per  year. 

Optical  Pyrometers 

Optical  pyrometers  were  checked  in  the  laboratory,  sometimes  by 
comparison  with  one  another,  but  generally  by  observations  on  a  heated 


DISCUSSION  667 

body,  in  an  electric  furnace,  the  temperature  of  which  was  measured  by 
means  of  a  platinum  thermocouple. 

When  comparing  one  optical  pyrometer  with  another,  the  milli- 
ammeters  were  first  tested  by  connecting  them  in  series  and  observing 
whether  both  indicated  the  same  current ;  it  was  found  that  these  instru- 
ments retained  their  calibration  very  well.  The  two  telescopes  were  then 
mounted  horizontally  on  a  rack  so  arranged  that  after  one  telescope  had 
been  sighted  on  the  object  the  second  telescope  could  be  sighted  on  the 
same  object  by  moving  the  rack  through  a  small  horizontal  angle.  By 
making  observations  with  alternate  instruments  at  equal  intervals  of 
time,  readings  could  be  obtained  for  time-temperature  curves,  from  which 
the  relation  of  the  two  instruments  could  be  determined. 

A  precise  calibration  could  be  made  by  measuring  the  temperature 
of  the  object  sighted  upon,  with  a  platinum  thermocouple  connected  to  a 
potentiometer.  In  this  operation,  care  should  be  taken  to  secure  as 
nearly  black-body  conditions  as  possible  or  conditions  similar  to  those 
under  which  the  instrument  is  regularly  employed. 

DISCUSSION 

R.  W.  NEWCOMB,  New  York,  N.  Y.  (written  discussion*). — This 
paper  is  particularly  interesting  to  me,  because  it  gives  data  on  a  much 
mooted  question,  viz.,  the  serviceability  of  Le  Chatelier  (platinum- 
platinum-rhodium)  thermocouples  under  severe  industrial  conditions. 
It  has  always  been  supposed  that  platinum-platinum-rhodium  thermo- 
couples, used  at  high  temperatures  under  strong  reducing  conditions, 
would  not  give  long  service  and  were  subject  to  rapid  volatilization. 
The  fact,  however,  that  the  carefully  kept  records  indicate  that  a 
little  more  than  3  gm.  out  of  more  than  830  gm.  was  lost  over  this 
considerable  period,  shows  what  kind  of  service  can  be  obtained  from 
platinum-platinum-rhodium  thermocouples,  when  properly  installed, 
and  well  cared  for. 

While  the  writer  has  no  actual  figures  on  the  loss  of  weight,  etc.  of 
platinum-platinum-rhodium  thermocouples  under  service,  he  was  told, 
by  the  man  in  charge  of  pyrometers  at  a  large  industrial  plant  in  which 
over  400  platinum-platinum-rhodium  thermocouples  were  installed  in 
furnaces  doing  various  kinds  of  heat-treating  work,  that  the  average 
maintenance  cost,  for  materials  alone,  including  platinum-platinum- 
rhodium  wires  and  protecting  tubes,  was  27  c.  per  furnace,  per  month. 

CARLETON  W.  HUBBARD,  Greenwich,  Conn,  (written  discussionf). — 
It  has  been  my  experience  that  there  is  a  considerable  difference  in  the 
serviceability  and  accuracy  of  platinum  thermocouples.  I  believe  it  is 


Received  Oct.  15,  1919.  f  Received  Oct.  21,  1919. 


668  MANUFACTURE   OF   GAS-MASK   CARBON 

said  that  this  serviceability  depends  on  several  factors  aside  from  the 
matter  of  the  protection  of  the  thermoelement  wires.  In  view  of  the 
unusually  good  results  described  in  the  paper,  from  the  use  of  thermo- 
couples under  conditions  that  can  be  considered  as  severe,  it  would  be  of 
value  if  the  author  could  identify  the  manufacturer  supplying  the  thermo- 
couples he  used  while  collecting  his  data. 

KIRTLAND  MARSH  (author's  reply  to  discussion*). — All  the  platinum- 
platinum  plus  10  per  cent,  rhodium  thermocouples  used  at  the  Astoria 
Detachment  C.  W.  S.,  U.  S.  A.,  were  purchased  from  Charles  Engelhard 
in  the  form  of  wire  0.5  mm.  in  diameter,  at  different  times  from  Nov 
5,  1917,  to  Aug.  13,  1918.  After  the  wire  was  received  at  the  laboratory, 
it  was  cut  into  the  required  lengths  and  the  different  elements  were 
welded  in  an  oxy-gas  flame.  After  the  thermocouples  were  made  up, 
they  were  annealed  and  calibrated  by  comparison  with  a  standard  to 
see  that  they  agreed  with  the  standard.  All  the  platinum-rhodium 
thermocouples  made  up  at  this  laboratory  from  the  Engelhard  wire 
agreed  with  the  standard  within  plus  or  minus  3°  C.  at  1000°  C. 

*  Received  Dec.  26,  1919. 


TEACHING  PYROMETRY  IN  OUR  TECHNICAL  SCHOOLS        669 


Teaching  Pyrometry  in  Our  Technical  Schools 

BY   GEORGE  V.    WENDELL,*   PH.    D.,    NEW   YORK,    N.    Y. 
(Chicago  Meeting,  September.  1919) 

THE  fact  that  a  symposium  on  pyrometry  is  being  held  under  the 
auspices  of  the  American  Institute  of  Mining  and  Metallurgical  Engineers 
may  very  properly  be  taken  as  a  recognition  of  the  importance  of 
temperature  measurements  and  control  in.  production  and  in  the  indus- 
tries, and  of  the  need  for  instruction  in  pyrometry  in  our  schools  of  engi- 
neering. Though  there  may  be  general  agreement  as  to  the  need  of 
some  instruction  along  this  line,  there  will  undoubtedly  be  divergence  of 
opinion  as  to  the  amount  of  time  that  can  be  allowed  to  such  instruction, 
as  well  as  to  its  character.  Those  who  have  given  little  attention  to  the 
complexity  of  high-temperature  measurements  will  be  apt  to  think  that  a 
few  simple  experiments  are  all  that  is  required.  They  fail  to  distinguish 
between  the  artisan,  who  merely  has  to  learn  how  to  use  any  piece  of 
apparatus  that  is  installed,  and  the  technically  trained  man,  who  must 
be  prepared  to  assume  the  responsibility  for  the  supervision  and  main- 
tenance of  the  pyrometric  equipment,  for  the  calibration  of  the  instruments, 
and  for  the  solution  of  any  temperature  problems  that  arise. 

In  approaching  this  subject  two  questions  present  themselves:  What 
should  be  the  character  of  the  instruction  in  pyrometry  in  an  engineer- 
ing school,  and  what  can  be  expected  of  the  students  who  have  had  this 
instruction?  In  brief,  it  may  be  said  that  the  aims  of  a  course  in  pyrome- 
try should  be  to  give:  (1)  A  thorough  grounding  in  the  fundamental 
principles  of  thermoelectric,  resistance,  radiation,  and  optical  pyrometry, 
and  of  liquid  and  gas  thermometry;  (2)  practical  instruction  in  the 
calibration  of  pyrometers;  (3)  acquaintance  with  the  errors  that  are 
likely  to  arise  and  the  precautions  that  must  be  observed  in  practice; 
(4)  information  regarding  the  limitations  of  the  various  classes  of  instru- 
ments, their  relative  reliability,  and  the  accuracy  attainable  with  them  at 
different  temperatures;  (5)  acquaintance  with  the  commercial  instr^i- 
ments  on  the  market,  their  construction  and  relative  merits;  (6)  a  knowl- 
edge of  the  chief  sources  of  information  relating  to  the  measurement  of 
high  and  low  temperatures.  To  sum  up,  the  aim  should  be  to  turn  out 
resourceful  young  men  familiar  with  the  leading  instruments  on  the 
market,  capable  of  handling  them  with  skill  and  qualified  to  meet,  with 
intelligence  and  confidence,  any  emergency  that  may  arise  in  the  meas- 
urement and  control  of  temperatures.  This  can  be  realized  only  by 


Professor  of  Physics,  Columbia  University. 


670  TEACHING    PYROMETRY    IN    OUR   TECHNICAL    SCHOOLS 

laying  an  adequate  theoretical  foundation  and  by  insisting  on  the  "  why  " 
for  everything  done  in  the  laboratory. 

An  examination  of  the  textbooks  on  pyrometry  and  of  the  scientific 
papers  and  pamphlets  relating  to  temperature  measurements  indicates 
very  clearly  that  any  instruction  in  this  subject  should  be  given  by  men 
possessing  a  thorough  knowledge  of  physics.  It  would,  therefore,  seem 
natural  and  proper  to  develop  a  laboratory  for  instruction  in  the  measure- 
ment of  temperature  and  for  standardization  of  pyrometers  under  the 
jurisdiction  of  the  department  of  physics  rather  than  under  one  of  the 
engineering  departments.  Such  a  laboratory  should  be  expected  to  turn 
out  students  thoroughly  trained  in  the  methods  and  principles  of 
pyrometry,  and  capable  without  further  instruction  of  using  pyrometers 
effectively.  It  should,  however,  leave  strictly  technical  matters,  such  as 
the  heat  treatment  of  metals,  to  the  proper  engineering  laboratories. 

Besides  its  first  purpose  of  serving  as  a  laboratory  for  the  instruction  of 
students  in  the  underlying  principles  of  temperature  measurements,  this 
laboratory  should  be  provided  with  standardized  instruments  covering 
the  range  of  temperature  —  200°  C.  to  the  highest,  so  that  all  departments 
can  find  here  opportunity  for  the  calibration  and  checking  of  various 
types  of  pyrometers.  In  addition,  provision  should  be  made  to  assist, 
by  means  of  advice,  those  in  the  university  engaged  in  researches  in- 
volving the  measurement  of  temperatures.  In  other  words,  this  labora- 
tory should  serve  as  a  general  clearing  house  for  all  questions  relating  to 
temperature  measurements. 


Admission  to  the  course  in  pyrometry  must  presuppose  a  satis- 
factory completion  of  at  least  an  excellent  college  course  in  general  and 
experimental  physics.  Evidently  the  character  of  instruction  in  pyro- 
metry and  the  results  that  can  be  obtained  will  depend  on  the  thor- 
oughness of  the  student's  previous  training. 

ALLOTMENT  OF  TIME 

The  amount  of  time  required  for  the  course  and  its  subdivision  will 
necessarily  depend  on  the  character  of  the  preparation  prescribed  for 
admission.  If  the  students  have  already  received  a  thorough  grounding 
in  the  principles  of  heat,  including  a  discussion  of  temperature  scales  and 
gas  thermometry,  in  the  laws  of  radiation  of  the  black  body  and  in 
electricity,  there  will  be  less  need  for  formal  lectures,  and  the  hours 
assigned  to  the  course  can  be  devoted  to  laboratory  work  with  provision 
for  conferences  or  quizzes.  Without  adequate  preparation,  however, 
provision  must  be  made  for  lectures  on  the  theory  and  on  the  physical 
principles  underlying  temperature  measurements,  in  order  that  the 
laboratory  work  may  be  performed  with  intelligence  and  effectiveness. 

Assuming  a  satisfactory  preparation,  the  course  should  extend  through 


GEORGE  V.    WENDELL  671 

at  least  one  term  (half  the  college  year)  and  consist  of  a  weekly  laboratory 
period  of  not  less  than  three  consecutive  hours  and  preferably  four,  with  a 
weekly  conference  hour  assigned  for  informal  discussions,  for  the  cross- 
examination  of  the  students,  and  as  an  opportunity  for  talks  by  specialists 
invited  from  outside  the  university.  More  time  may  be  advantageously 
allowed  the  course,  but  it  is  doubtful  whether  this  will  prove  feasible 
owing  to  the  congested  nature  of  the  curricula  of  our  engineering  schools. 
In  any  case,  opportunity  for  additional  work  should  be  given  to  any 
students  who  desire  it  and  have  available  time. 

Besides  the  assignment  of  a  definite  number  of  hours  weekly  for 
laboratory  and  conference  work,  provision  must  be  made  in  the  curricu- 
lum for  at  least  6  hours  weekly  for  home  preparation  if  the  instruction 
is  to  be  made  effective.  This  time  is  needed  to  complete  the  assigned 
reading  of  textbook  matter,  of  the  scientific  papers,  and  of  manufacturers' 
catalogs  relating  to  the  experiments  to  be  performed,  and  for  the  prepa- 
ration of  a  satisfactory  report  on  the  test  made  in  the  laboratory.  Should 
the  school  curriculum  be  too  full  to  permit  this  home  work,  the  course 
on  pyrometry  should  extend  over  a  period  of  1  year  in  order  that  the 
necessary  reading  and  preparation  of  reports  may  be  done  in  a  regular 
laboratory  period  without  causing  a  reduction  in  the  actual  amount  of  the 
experimental  work  originally  proposed. 

TEXTBOOKS  AND  REFERENCE  MATERIAL 

Each  student  should  own,  in  addition  to  a  textbook  on  pyrometry  and 
any  special  notes  that  may  be  provided  by  the  department,  certain 
circulars  of  the  Bureau  of  Standards  and  the  catalogs  of  the  leading 
makers  of  pyrometric  apparatus.  In  the  laboratory  there  should  always 
be  kept  for  ready  reference  complete  files  of  all  textbooks,  catalogs, 
circulars,  and  scientific  papers  that  may  have  a  bearing  on  the  subject- 
matter  taught.  Among  textbooks  may  be  suggested:  Burgess, 
"  Measurements  of  High  Temperature;  "Griffiths,  "Methods  of  Measuring 
Temperature;"  Ferry,  "Practical  Pyrometry;"  Darling,  "Pyrometry." 
One  cannot  afford  to  be  without  the  reprints  of  two  articles  by  Foote, 
Harrison  and  Fair  child,  entitled,  "Standardization  of  Rare-metal 
Thermocouples"  and  "Standardization  of  Base-metal  Thermocouples" 
which  appeared  in  the  issues  of  Metallurgical  and  Chemical  Engineering  of 
Apr.  1  and  15,  1918. 

Experience  has  shown  conclusively  that  in  such  a  specialized  labora- 
tory publications  and  pamphlets  of  the  Bureau  of  Standards  like  the 
following  are  indispensable  and  should  be  in  the  possession  of  the  students : 
Circular  7,  "Pyrometer  Testing  and  Heat  Measurements";  Circular  8, 
"Testing  of  Thermometers;"  Circular  35,  "Melting  Points  of  Chemical 
Elements  and  other  Standard  Temperatures;"  Circular  66,  "Standard 
Samples  for  Thermometric  Fixed  Points;"  Scientific  Paper  250,  "Charac- 
teristics of  Radiation  Pyrometers;"  Scientific  Paper  11,  "Optical  Pyrome- 


672       I  TEACHING  PYROMETRY  IN  OUR  TECHNICAL  SCHOOLS 

try,J'  Scientific  Paper  202,  "Note  on  Cold-junction  Corrections  for 
Thermocouples;"  Scientific  Paper  124,  "Platinum-resistance  Thermome- 
try  at  High  Temperatures." 

For  additional  publications  of  the  Bureau  of  Standards  see  Circular 
24,  issued  Apr.  14,  1919. 

In  addition,  valuable  contributions  have  been  made  from  the 
Geophysical  Laboratory,  the  Nela  Research  Laboratory,  and  other 
sources.  Besides,  there  is  much  excellent  matter  in  many  catalogs 
and  bulletins  issued  by  instrument  makers,  which  they  willingly  dis- 
tribute to  students  at  the  request  of  a  school.  These  publications  arouse 
the  interest  of  the  students  in  temperature  measurements  and  methods  of 
temperature  control  and  afford  them  some  familiarity  with  the  types  of 
apparatus  and  instruments  in  actual  use  and  are  frequently  of  real  serv- 
ice in  connection  with  the  actual  experimental  work  in  the  laboratory. 

SUBJECT  MATTER  TO  BE  TREATED 

In  such  a  short  article  as  this  it  is  possible  to  give  only  a  brief  outline 
of  the  physical  principles  that  must  be  included. 

In  the  first  place,  the  conception  of  the  thermodynamic  scale  of  tem- 
perature, the  gas  scale,  the  international  hydrogen  scale,  black-body 
temperatures,  and  the  practical  realization  of  these  scales  should  be 
presented  and  an  effort  should  be  made  to  acquaint  the  students  with 
the  accuracy  and  reproducibility  of  known  freezing  and  boiling  points 
and  the  accuracy  attainable  in  the  measurement  of  high  and  low 
temperatures. 

In  thermoelectric  pyrometry,  starting  with  a  knowledge  of  thermo- 
electricity, a  study  should  be  made  of  rare-  and  base-metal  couples. 
This  should  include  such  questions  as:  the  annealing  of  rare-metal 
couples,  welding  of  hot-junctions  and  making  of  cold-junctions,  choice 
of  metals  for  couples,  discussion  of  formulas  for  the  representation  of  the 
temperature-electromotive  force  relationship  of  couples,  cold-junction 
corrections,  interchangeability  of  base-metal  couples,  relative  advantages 
of  indicators  versus  service  potentiometer  or  deflection  potentiometer, 
high-  versus  low-resistance  indicators,  effects  of  depth  of  immersion, 
protecting  tubes,  the  selection  of  couples,  and  some  mention  of  recording 
thermoelectric  pyrometers. 

In  resistance  pyrometry,  the  relation  of  resistance  to  temperature 
should  be  taken  up,  followed  by  a  discussion  of  Callendar's  work  on  the 
resistance  of  platinum  and  of  his  formula  giving  the  relation  between  the 
true  temperature  and  the  platinum  temperature  and  the  range  for  which 
this  relationship  holds;  the  construction  of  platinum  thermometers, 
their  calibration  and  use  in  connection  with  the  Wheatstone  bridge;  the 
calibration  and  use  of  platinum  and  nickel  thermometers  with  direct- 
reading  instruments;  range  of  the  nickel  thermometer;  precision  of  re- 


GEORGE   V.    WENDELL  673 

sistance  thermometers  and  their  availability;  and  something  on  recording 
resistance  pyrometers. 

As  a  basis  for  intelligent  experimental  work  with  radiation  and  op- 
tical pyrometers,  considerable  attention  must  be  paid  to  the  laws  of 
radiation  and  of  their  bearing  on  the  construction,  theory,  and  use  of  such 
radiation  pyrometers  as  the  Fe"ry  (Taylor  Instrument  Co.,  or  Cambridge 
Scientific  Co.),  Thwing  and  Brown,  and  of  such  optical  pyrometers 
as  the  Wanner  or  Scimatco  and  the  Leeds  &  Northrup.  The  discus- 
sion of  radiation  pyrometers  should  deal  with  the  Stefan-Boltzmann 
"fourth  power'"  law  for  the  total  radiation  of  a  black-body  radiator, 
with  the  question  of  the  experimental  realization  of  black-body  condi- 
tions, with  the  conception  of  black-body  temperature,  and  with  the  total 
radiation  from  oxide  and  metallic  surfaces  in  its  bearing  upon  the  tem- 
perature of  such  bodies  when  measured  with  a  radiation  pyrometer  cali- 
brated to  read  black-body  temperatures.  Likewise,  the  laws  of  Wien 
and  of  Planck  on  the  distribution  of  energy  in  the  spectrum  of  a  "full 
or  black-body  radiator"  in  their  bearing  on  the  measurement  of  tempera- 
ture by  optical  pyrometers  must  be  treated  and  attention  given  to  the 
emissivity  of  metals  and  oxides  as  they  influence  the  "apparent  tempera- 
tures," as  found  by  a  calibrated  optical  pyrometer.  Also,  the  distinction 
between  brightness  temperature  and  color  temperature  should  not  be 
overlooked.  The  construction,  theory,  calibration,  range,  and  limita- 
tions of  the  various  types  of  optical  instruments  and  the  precautions  to  be 
observed  in  their  use  should  receive  attention,  particularly  in  connection 
with  the  laboratory  tests. 

If  the  time  permits,  a  general  study  of  liquid  and  gas  thermometers, 
especially  of  the  high-temperature  mercury  thermometer,  can  be  made  to 
advantage.  Such  a  study  would  naturally  include  the  discussion  of  the 
errors  and  limitations  of  this  class  of  thermometers,  their  calibration  and 
advantages.  Any  experimental  procedure  should  follow  the  general 
plan  outlined  in  the  Bureau  of  Standards  Circular  8  and  Dickinson's 
paper  on  Heat  Treatment  of  High  Temperature  Mercurial  Thermometers, 
Reprint  No.  32  of  the  Bureau  of  Standards. 

Should  this  theoretical  material  be  presented  by  lectures?  Prefer- 
ably not.  What  is  desired  is  to  teach  the  students  how  to  get  at  the 
truth  and  how  to  handle  the  information  when  found.  They  will  achieve 
this  ability  most  quickly  if  they  are  required  to  work  up  the  material  by 
themselves.  This  method  leads  to  independence  of  thought  and  confi- 
dence in  one's  self,  valuable  traits  that  should  be  encouraged.  Naturally 
the  guidance  of  the  instructor  will  be  needed,  but  this  can  be  given  effect- 
ively at  the  weekly  conference,  during  the  laboratory  tests  when  the 
theoretical  points  arise,  and  by  personal  interviews.  Informal  discus- 
sion of  theoretical  and  practical  matters  by  students  and  instructors  is 
bound  to  arouse  keen  interest  and  enthusiasm. 


674  TEACHING  PYROMETRY  IN  OUR  TECHNICAL  SCHOOLS 

SUGGESTIONS  RELATIVE  TO  CONDUCT  OF  LABORATORY 

In  conducting  a  laboratory  of  pyrometry,  it  should  be  constantly  borne 
in  mind  that  the  success  of  the  instruction  will  in  a  large  degree  be  meas- 
ured by  the  skill  and  resourcefulness  shown  in  the  solution  of  unexpected 
and  puzzling  temperature  problems  arising  in  later  professional  work. 
Consequently,  throughout  the  course  emphasis  should  be  laid  on  the 
fact  that  laboratory  conditions  are  not  industrial  conditions,  that  the 
methods,  of  the  laboratory  often  cannot  be  followed  in  the  works  and  that 
common  sense  must  always  be  exercised  in  applying  to  industrial  con- 
ditions the  knowledge  acquired  in  the  laboratory.  To  this  end  a  few 
suggestions  may  be  offered: 

The  apparatus  for  any  experiment  or  test  should  not  be  set  up  ready 
for  mere  "press  the  button"  or  "turn,  the  crank"  observations.  On  the 
contrary,  the  students  should  be  required  to  assemble  and  adjust  all 
apparatus  that  is  needed  in  any  test  or  calibration.  An  effort  should  be 
made  to  require  in  the  experimental  work  speed  as  well  as  quality,  in 
order  that  there  may  always  be  present  in  the  mind  of  the  student  the 
importance  of  the  "time  factor"  in  any  undertaking.  The  report  of 
tests  should  include  original  data,  calculations,  plots,  a  brief  discussion 
of  the  experimental  test,  and  a  summary  of  conclusions. 

There  should  be  prepared  a  set  of  questions  for  distribution  among 
the  students.  These  questions  should  be  chosen:  (1)  to  emphasize 
vital  points  that  arise  in  industrial  measurements  of  temperatures  but 
which  do  not  come  up  in  the  university  laboratory  and  are,  therefore,  often 
overlooked  by  even  the  keenest  students,  and  (2)  to  test  the  student's 
ability  to  use  the  information  that  has  been  given  him  in  the  course. 
They  should  also  bring  out  common  errors  made  by  users  of  pyrometric 
apparatus,  who  often  express  dissatisfaction  with  an  installation  when  the 
real  trouble  lies  in  their  incorrect  use  of  such  apparatus. 

A  few  examples  may  suffice  as  illustrations: 

1.  Should  a  single  thermocouple  installed  in  a  furnace  be  relied  upon 
for  the  control  of  the  furnace?     Does  such  a  couple  measure  the  actual 
temperature  of  the  furnace  and  of  a  charge  in  the  furnace? 

This  question  should  be  used  to  emphasize  the  important  but  fre- 
quently overlooked  fact  that  the  pyrometer  tells  only  the  temperature 
of  the  hot  .end  of  the  thermocouple  and,  when  in  a  furnace,  may  fail  for 
various  reasons  to  indicate  the  temperature  of  the  charge  in  the  furnace. 

2.  Will  the  correct  temperature  of  a  mass  of  molten  metal  be  obtained 
if  a  4-ft.  base-metal  couple,  made  of  wires  of  large  cross-section  and  in  a 
suitable  protecting  tube,  is  thrust  into  the  metal  to  a  depth  of  only  3  or 
4  in.? 

3.  Give  a  diagram  of  the  connections  by  which  a  single  indicator  may 
be  used  for  reading  temperatures  given  by  three  separate  thermocouples. 

4.  Will  the  temperature  of  a  flue  gas  be  correctly  measured  by  a 


GEORGE  V.   WENDELL  675 

thermometer  or  pyrometer  if  the  instrument  is  exposed  to  the  direct 
radiation  of  hot  material? 

5.  Can  the  true  temperature  of  hot  gases  be  obtained  by  the  use  of  a 
base-metal  couple  made  of  wires  of  large  diameter? 

6.  In  measuring  the  temperature  of  the  wall  of  a  furnace  that  is  full 
of  cooler  gases  and  fumes,  would  there  be  any  advantage  in  selecting  an 
optical  pyrometer  such  as  the  Leeds  &  Northrup  or  the  Wanner,  in  pref- 
erence to  a  radiation  pyrometer  of  the  Fe"ry  or  Thwing  type? 

7.  State  what  type  of  instrument  you  would  select  to  measure  tempera- 
ture over  the  following  ranges  with  the  degree  of  accuracy  that  can  be 
expected : 

-  100°  to  0°  C.;  100°  to  500°  C.;  500°  to  1200°  C.;  1200°  to  2400°  C. 

8.  With  a  Leeds  &  Northrup  resistance  indicator,  can  any  platinum 
resistance  thermometer  be  substituted  for  the  one  furnished  with  the 
instrument  without  altering  the  calibration? 

In  the  compilation  of  such  a  set  of  questions  it  is  advisable  to  obtain 
suggestions  from  professional  men  who  have  had  experience  in  the  actual 
use  and  installation  of  pyrometers. 

TYPICAL  EXPERIMENTS 

Since  it  is  in  the  laboratory  that  the  student  becomes  acquainted 
with  the  actual  apparatus  and  with  experimental  methods,  it  is  very 
important  that  a  wise  selection  of  experiments  be  planned.  In  this 
connection  it  may  be  advisable  to  offer  a  word  of  caution  with  reference 
to  the  use  of  automatic  instruments,  such  as  thermoelectric  or  resistance 
recorders,  for  instruction  purposes.  While  it  is  desirable  that  the 
students  should  handle  such  instruments  and  be  familiar  with  their  con- 
struction, operation  and  adjustment,  it  should  not  be  at  the  sacrifice  of 
more  fundamental  matters,  for  a  competent  student  of  pyrometry  can 
master  the  chief  features  of  these  instruments  quickly.  As  an  illustra- 
tion, take  the  case  of  the  determination  of  the  transformation  points  of 
a  specimen  of  steel.  It  may  be  much  more  instructive  for  the  student 
to  "obtain  the  data  for  the  plotting  of  the  desired  transformation  point 
curve  by  two  galvanometers  rather  than  by  means  of  the  Leeds  &  Northrup 
Transformation  Point  Indicator,  which  registers  automatically  such  curves. 

The  following  experiments  are  offered  as  a  suggestion  of  the  type  of 
experiment  that  seems  adapted  to  give  a  sound  training  in  the  theoretical 
and  experimental  basis  of  temperature  measurements  and  in  approved 
methods  of  calibration,  while  serving  at  the  same  time  to  familiarize  the 
students  with  the  pyrometers  on  the  market  and  the  precautions  required 
in  their  use. 

EXPERIMENT  1. — Precision  Calibration  of  Rare-metal  Thermocouples. — 
To  calibrate  a  platinum-rhodium  couple  for  the  range  300°  to  1100°  C. 
by  measuring,  with  a  precision  potentiometer,  the  electromotive  force  at 
the  freezing  point  of  zinc,  antimony,  and  copper  when  the  cold  junctions 


676  TEACHING    PYROMETRY   IN   OUR  TECHNICAL   SCHOOLS 

are  at  0°  C  .,and  to  determine  the  constants  in  the  thermoelectric  formulas 
of  Holman  and  of  Holborn  and  Day  connecting  the  temperature  and  elec- 
tromotive force  for  interpolation  from  300°  to  1200°  C.  The  carrying 
out  of  the  experimental  work  will  involve  such  matters  as  the  setting  up 
and  adjustment  of  the  precision  potentiometers,  the  test  for  any  thermo- 
electromotive  forces  in  the  potentiometer  when  all  junctions  of  the 
rare-metal  couple  are  at  0°  C.,  and  the  experimental  determination  of 
typical  freezing-point  curves  of  the  metals  zinc,  antimony,  and  copper. 
Other  important  points  that  can  be  brought  out  through  such  a  funda- 
mental experiment  will  occur  to  the  reader. 

EXPERIMENT  2. — Comparison  Method  of  Calibration  of  Thermocouples. 
This  experiment  should  serve  to  bring  out  the  correct  procedure  to  be 
followed  in  the  comparison  method  of  calibrating  thermocouples  and 
the  precautions  to  be  observed.  If  the  calibration  is  of  base-metal 
couples  against  a  standard  rare-metal  couple,  the  furnace  method  may 
be  used  or  the  molten-metal  bath.  (See  Foote,  Harrison  and  Fairchild, 
Metallurgical  &  Chemical  Engineering,  April,  1918.)  A  critical  study  of 
methods  for  making  cold-junction  corrections  may  be  included  to  advantage 
EXPERIMENT  3. — A  Study  of  Indicators. — This  exercise  should  be 
planned  to  give  a  fairly  comprehensive  insight  into  the  theory,  construc- 
tion, operation,  and  limitations  of  millivoltmeter  indicators  and  of 
service  potentiometers.  It  should  include  the  effect  of  any  appreciable 
change  in  the  resistance  of  the  circuit  due  to  faulty  contacts,  change  in 
lead  resistance  and  change  in  resistance  of  the  couple  on  the  millivolt 
temperature  indications  of  high-  and  low-resistance  millivoltmeter  indica- 
tors and  on  the  service  potentiometer.  Likewise,  such  questions  as  the 
possibility  of  the  interchangeability  of  couples  with  any  given  indicator, 
the  effect  of  size  of  thermocouple  wires  and  depth  of  immersion  may  be 
brought  out. 

EXPERIMENT  4. — Electrical  Resistance  Thermometers. — This  exercise 
may  be  devoted  to  a  fairly  comprehensive  study  of  the  theory  of  the 
resistance  thermometer,  its  precision  and  limitations.  It  should  natu- 
rally include  such  matters  as:  (1)  the  calibration  of  a  platinum  bulb  for 
the  range  —  40°  to  1 100°  C.  by  use  of  the  melting  point  of  ice  and  the  boiling 
points  of  steam  and  sulfur  and  the  reduction  of  platinum  temperatures 
to  true  temperatures  by  the  Callendar  formula;  and  (2)  the  measurement 
of  temperatures  with  commercial  types  of  direct-reading  indicators 
designed  for  use  with  platinum  and  nickel  thermometers,  and  the  test 
of  their  calibration.  Other  points  that  could  be  brought  out  are  the 
three  versus  four  compensation  leads,  the  interchangeability  of  resistance 
bulbs  with  a  given  indicator,  and  the  availability  of  resistance  thermome- 
ters versus  the  .thermoelectric  pyrometers  for  industrial  work. 

EXPERIMENT  5. — Radiation  Pyrometers. — The  following  suggestions 
maybe  offered  as  possible  objects  of  such  an  experiment:  (1)  To  familiar- 
ize the  student  with  the  construction  and  technique  of  such  instruments 


GEORGE  V.    WENDELL  677 

as  the  F6ry,  Thwing,  and  Brown.  (2)  To  emphasize  certain  theoretical 
considerations  involved  in  the  construction  and  use  of  these  instruments. 
(3)  To  compare  the  readings  of  these  instruments  with  each  other  and 
with  the  temperature  given  by  a  calibrated  platinum-rhodium  couple  using 
as  the  radiating  source  a  furnace  possessing  closely  black-body  conditions. 

EXPERIMENT  6.— Optical  Pyrometers. — This  experiment  should  in- 
clude at  least  these  two  objects:  (1)  To  bring  out  the  fundamental  laws 
of  radiation  on  which  optical  pyrometry  is  based  and  to  emphasize  the 
relative  advantages  of  optical  versus  radiation  pyrometers.  (2)  To 
acquaint  the  students  with  the  construction,  theory,  operation,  calibra- 
tion, errors,  range  and  limitations  of  the  Leeds  &  Northrup  or  Holborn- 
Kurlbaum  and  the  Wanner  or  Scimatco  types  of  instruments. 

The  foregoing  experiments  are  offered  merely  as  suggestions  and 
should  be  modified  to  meet  the  needs  and  ideals  of  the  individual  labora- 
tories. They  are  not  intended  to  be  specific  or  complete.  Many  other 
experiments  are  possible,  such  as  the  study  of  Seger  cones,  the  specific 
heat  pyrometer,  high-temperature  mercury  thermometer,  the  location 
of  transformation  points,  etc.  Also  no  mention  has  been  made  of  the 
length  of  the  exercises,  whether  they  should  cover  one,  two,  or  more 
laboratory  periods.  These  are  matters  for  individual  solution. 

As  an  important  feature  of  the  instruction  is  to  develop  initiative  and 
independence  of  thought,  it  is  advisable  to  give  the  student  a  few  experi- 
mental problems  for  which  he  is  required  to  plan  the  procedure.  For 
example,  the  student  might  be  furnished  with  a  precision  potentiometer 
assumed  to  read  correctly  or  for  which  there  is  a  calibration  curve,  a 
Weston  standard  cell,  sensitive  galvanometer,  storage  battery,  and  rheo- 
stat and  then  be  asked  to  calibrate  a  service  potentiometer  and  a  pyro- 
volter,  submitting  his  plan  to  the  instructor  for  criticism  and  approval 
before  beginning  the  calibration. 

CONCLUSIONS 

Mere  acquaintance  with  constructional  details  of  pyrometers  and 
their  operation  is  readily  acquired.  Such  information  is,  however,  not 
sufficient  for  the  intelligent  application  of  pyrometers  to  the  scientific 
and  industrial  measurement  of  temperature.  What  a  course  in  pyrome- 
try must  do  is  to  lay  such  a  sound  foundation  that  any  subsequent  heat 
problems  can  be  attacked  with  confidence  and  good  judgment.  It 
should  also  make  impossible  ridiculous  claims  of  accuracy  that  are  patently 
unwarranted. 

As  to  the  attitude  of  the  student  body  toward  such  a  course,  there 
can  be  no  question.  The  nature  of  the  experimental  work  and  its  practi- 
cal bearing  offer  a  strong  appeal  to  engineering  students  and  to  those  in 
pure  science.  The  main  difficulty  is  likely  to  be  that  the  students  will 
become  absorbed  in  the  course  to  the  detriment  of  some  of  their  other 
university  studies. 


678          TEACHING  PYROMETRY  IN  TECHNICAL  SCHOOLS 


Teaching  Pyrometry  in  Technical  Schools 

BY   C.    E.    MENDENHALL,  *   PH.   D.,    MADISON,    WIS. 
(Chicago  Meeting,  September,  1919) 

FOR  the  purpose  in  hand,  pyrometry  may  be  taken  to  include  all 
temperature  measurements  from,  say,  200°  C.  to  the  highest  attainable, 
especially  when  considered  from  the  technical  or  applied  side.  It  will 
be  convenient  first  to  consider  the  entire  content  of  a  course  of  study 
grouped  under  the  headings  of  the  various  methods  of  measurement, 
which  are  quite  distinct  in  principle  and  involve  different  equipment. 
Brief  suggestions  will  be  given  under  each  heading,  and  the  summary 
will  be  followed  by  comments  on  the  purpose  of  such  a  course  and  methods 
of  conducting  it. 

1.  Expansion  and  pressure  methods:  mercury  thermometers  (glass 
and  quartz),  liquid,  gas,  and  vapor-pressure  thermometers,  especially 
recording  instruments.     Especial  attention  should  be  given  to  the  limi- 
tations of  these  methods,  which,  particularly  in  high-temperature  mercury 
thermometry,  .are  often  overlooked.     Range  approximately  to  700°  C. 

2.  Resistance  thermometers:  both  bridge  and  potentiometer  methods, 
of  indicating  and  recording.     Calibration;  lead  compensation;  design  of 
thermometers  for  special  purposes.     Range  to  1200°  C. 

3.  Thermoelectric  pyrometers :  potentiometer  and  deflection  methods 
indicating  and  recording.     Calibration,  leakage  errors,   contamination 
errors,  base  and  noble  couples.     Range  to  1500°  C. 

4.  Total  radiation  methods:  thermoelectric,  resistance,  and  expan- 
sion indicators,  mirror  and  lens  collectors,  calibration,  permanence  of 
characteristics,  absorption  errors.     Range,  to  highest  attainable  tem- 
peratures.    Methods  of  producing  perfect  radiators — true,  and  virtual  or 
"black  body"  temperatures. 

5.  Partial  radiation  methods:  absorption  and  spectroscopic  methods 
of  getting  partial  radiation,  various  comparison  sources,  polarization,  elec- 
tric, sector,  and  absorption  methods  of  controlling  intensity   (Morse, 
Lummer,  and  Wanner  types).     Calibration,  permanence  of  calibration, 
absorption  errors.     Methods  of  producing  perfect  radiators — improvised 
methods;  true,  and  virtual  or  black-body  temperatures. 

It  is  not  necessary  to  go  into  the  detailed  working  out  of  a  course  to 
cover  the  ground  outlined,  but  certain  general  questions  present  them- 


*  Chairman  of  Section  on  Mathematics  and  Physics  of  the  National  Research 
Council. 


C.    E.    MENDENHALL  679 

selves  and  must  be  answered.  For  example,  should  the  course  be  planned 
as  one  strictly  dealing  with  high-temperature  measurement,  or  should  it 
deal  more  broadly  with  the  measurement  and  production  of  high  tem- 
peratures? The  latter  arrangement  adds  to  the  interest  of  the  course, 
and  the  combination  is  quite  logical.  If  this  choice  is  made,  we  can  pro- 
ceed to  consider  the  distribution  of  time.  The  total  time  may  be  taken 
at  from  twelve  to  eighteen  periods  of  3  or  4  hr.  each,  and  the  content 
must  be  varied  somewhat  to  suit  the  time  available  and  the  special 
conditions. 

During  the  first  half  of  the  course,  attention  should  be  concentrated 
rather  on  the  methods  of  measurement,  the  simplest  and  most 
reliable  means  being  provided  for  giving  steady  temperatures  just 
sufficiently  high  for  the  purpose,  as  uncertainties  are  apt  to  increase 
greatly  at  higher  ranges.  For  certain  parts  of  the  work,  these  devices 
can  be  of  low  thermal  capacity  (such  as  heated  strips,  lamps,  etc.)  that 
will  reach  a  steady  state  very  quickly,  so  that  no  time  will  be  lost  in  waiting 
for  things  to  "settle  down."  In  other  cases,  it  will  be  necessary  to  use 
furnaces  or  boiling  tubes,  and  arrangements  should  be  made  so  that  these 
can  be  started  before  the  regular  laboratory  period.  With  proper 
care,  it  will  be  possible 'during  this  part  of  the  course  to  do  effective  work 
in  the  laboratory  during  2  or  3  hr.  of  the  total  period;  the  remaining 
time  should  be  devoted  to  classroom  discussions  on  topics  such  as  these: 
Fundamental  ideas  of  temperature  and  the  temperature  scale,  standard 
fixed  points,  theory  of  bridge  and  potentiometer  measurements,  laws  of 
radiation,  perfect  and  ordinary  radiators,  ideas  of  thermal  conductivity 
and  thermal  capacity  applied  to  furnace  construction. 

The  latter  half  of  the  course  may  be  devoted  to  the  application  of  the 
methods,  the  technique  of  which  has  been  briefly  studied,  and  to  the 
measurement  of  temperatures  under  conditions  closely  approximating 
those  actually  found  in  practice.  More  attention  should  also  be  paid  to 
the  methods  used  in  the  production  and  maintenance  of  high  temperatures, 
and  the  range  of  temperatures  used  should  be  extended.  The  exact 
content  of  this  part  of  the  course  can  be  greatly  varied,  depending  on  the 
equipment  available,  but  the  various  furnaces  should  follow  standard 
practice,  on  a  reduced  scale,  and,  if  possible,  should  be  designed  to  bring 
out  the  different  limitations  and  advantages  of  the  several  methods  of 
measurement.  Interest  would  be  increased  if  the  temperatures  measured* 
were  those  concerned  with  or  controlling  important  high-temperature 
phenomena;  that  is,  melting  points  (for  standardization),  recalescence 
points,  reactions,  crystalline  transformation,  increase  of  conductivity 
in  substances  normally  insulators,  etc.  The  reason  for  dividing  the  work 
rather  sharply  into  two  parts,  as  has  been  suggested,  is  obvious. 

The  difficulties  involved  in  the  mere  production  and  control  of  high 
temoeratures  are  in  themselves  considerable,  and  increase  rapidly  at  the 
higher  ranges.  If  the  student  is  confronted  at  the  same  time  by  these 


680  TEACHING    PYROMETBY   IN   TECHNICAL   SCHOOLS 

difficulties  and  those  inherent  in  the  various  methods  of  measurement, 
the  result  is  sure  to  be  confusion  and  discouragement.  A  large  part  of 
this  can  be  avoided  by  the  scheme  proposed. 

Another  general  question  that  arises  is,  should  the  apparatus  be  so 
designed  and  arranged  as  to  be  "fool-proof"  or  "fool-killing"?  The 
difference  is  clear.  According  to  the  first,  every  effort  would  be  made  by 
choice  of  equipment,  arrangement,  and  instructions  to  insure  that  all  the 
experiments  proceed  smoothly  and  without  interruption  or  mishap,  very 
little  of  the  assembly  of  apparatus  being  left  to  the  student,  who  is  not 
made  to  feel  much  responsibility  for  its  successful  operation.  In  this 
case  the  thoughtful  student  will  get  an  excellent  idea  of  the  maximum 
possible  accuracy  of  the  methods  used,  while  the  poor  student  will  get  an 
entirely  erroneous  idea  of  the  ease  with  which  the  work  may  be  done,  and 
neither  will  appreciate  fully  the  effort  and  thought  expended  on  the  design 
and  arrangement,  in  order  to  produce  the  result.  By  the  second  method, 
in  which  the  equipment  is  intentionally  chosen  in  more  disconnected 
elements  which  must  be  assembled  or  connected  up  by  the  student,  and 
in  which  the  "eternal  cussedness  of  inanimate  things"  is  allowed  to 
display  itself  in  more  normal  fashion,  the  thoughtful  student  will  get  more 
insight  into  the  difficulties  of  the  situation  and  be  stimulated  to  overcome 
them  by  his  own  initiative,  but  the  poor  student  will  be  well-nigh  hope- 
lessly muddled  and  discouraged  and  ultimately  dropped.  Which  of 
these  is  chosen  must  depend  on  circumstances,  on  the  relative  importance 
attached  to  numbers  as  compared  to  quality  of  students,  and  on  the 
extent  of  equipment  available.  In  most  cases  a  compromise  is  necessary 
and  probably  desirable. 

Finally  comes  the  question  as  to  where  the  course  should  be  given; 
that  is,  in  what  department.  The  situation  demands  a  combination  to 
insure  the  best  results.  On  the  one  hand  is  needed  that  interest  in  pre- 
cision, in  method,  and  in  working  out  new  methods,  which  is  more  apt 
to  be  found  in  departments  of  physics;  on  the  other  hand,  it  is  essential  to 
have  an  immediate  contact  with  real  problems  and  real  conditions,  such 
as  would  exist  in  departments  of  metallurgy,  electrochemistry,  or  similar 
engineering  groups.  If  the  course  could  be  given  by  a  combination 
of  a  physics  and  an  engineering  department,  not  only  would  the  proper 
balance  of  the  course  be  maintained,  but  an  important  step  taken  to 
insure  that  close  cooperation  of  departments  of  pure  and  applied  science 
which  everyone  believes  to  be  most  stimulating  and  wholesome  for  both. 
Unfortunately,  such  cooperation  in  course  giving,  while  possible  at  smaller 
institutions,  becomes  very  difficult  at  the  larger  and  more  elaborately 
organized  universities,  where  the  dividing  line  between  departments  and 
between  groups  of  departments  unfortunately  tends  to  become  more 
marked.  Though  this  situation  frequently  involves  high-temperature 
phenomena,  they  are  not  measurable  by  our  methods  and  therefore  do  not 
concern  us. 


TEACHING    PYROMETRY  681 


Teaching  Pyrometry 

BY    O.    L.    KOWALKE,*   MADISON,    WIS. 
(Chicago  Meeting,  September,   1919) 

THE  measurement  and  control  of  temperatures  have  assumed  posi- 
tions of  great  importance  in  many  industries.  The  manufacturers  of  by- 
product coke  and  carbureted  water  gas  find  that  proper  temperature 
control  helps  to  produce  a  better  product  and  economizes  in  the  use  of  raw 
materials.  In  the  manufacture  of  glass,  enameled  ware,  brass,  and 
high-temperature  refractories,  temperature  control  is  now  regarded  as 
being  coordinate  in  importance  with  the  control  of  materials  used.  To 
control  such  processes  as  malleablizing  cast  iron,  and  hardening,  temper- 
ing, and  annealing  of  steel  in  a  manner  to  meet  exacting  market  require- 
ments has,  within  recent  years,  involved  the  installation  of  large  and 
expensive  pyrometer  equipments.  In  many  of  these  installations,  it  was 
necessary  not  merely  to  measure  but  also  to  record  the  temperatures  over 
a  period  of  time  and  thus  obtain  a  record  of  the  entire  heat  treatment  and 
an  effective  check  on  the  workmen  in  charge. 

In  view  of  the  importance  of  temperature  measurement  and  control 
in  such  a  wide  variety  of  industries  and  the  necessity  for  proper  super- 
vision of  the  installation  and  operation  of  the  outfits,  the  following  ques- 
tions may  be  pertinent:  What  instruction  is  offered  by  the  engineering 
colleges  and  what  departments  in  the  colleges  are  responsible  therefor? 

The  latest  available  catalogs  of  ten  prominent  engineering  colleges 
east  of  the  Mississippi  River  showed  courses  in  high-temperature  measure- 
ments given  by  the  departments  of  physics.  These  courses,  in  general, 
were  described  as  comprising  classroom  and  laboratory  instruction  in  the 
theory  of  high-temperature  measurements,  together  with  exercises  in  the 
calibration  of  the  various  devices  used.  In  some  cases  it  was  stated  that 
consideration  would  be  given,  in  the  courses,  to  the  practical  applications 
in  the  industries.  In  only  one  college  was  the  course  required  of  all 
engineering  students;  in  most  colleges  it  was  optional;  in  a  few  colleges, 
it  was  required  only  of  certain  groups.  In  colleges  offering  a  course  in 
metallurgical  engineering,  instruction  in  pyrometry  was  always  given  in 
connection  with  metallurgical  laboratory  work.  The  students  in  ceramic, 
chemical,  and  metallurgical  engineering  in  three  colleges  were  given  some- 
what formal  instruction  in  pyrometry  by  their  respective  faculties.  It 
did  not  appear  that  mechanical  or  electrical  engineering  students  in  any  of 

*  Professor  of  Chemical  Engineering,  University  of  Wisconsin. 


682  TEACHING    PYROMETRY 

the  colleges,  except  one,  were  scheduled  for  instruction  in  pyrometry, 
although  such  students,  after  graduation,  are  frequently  engaged  in  work 
requiring  heat  treatment  of  metal  or  temperature  control. 

Is  a  special  course  in  pyrometry  in  the  curricula  for  ceramic,  chemical, 
mechanical,  and  metallurgical  engineers  justifiable?  This  may  be  a 
debatable  question.  Local  conditions  and  plan  of  organization  of  the 
college  usually  govern  the  feasibility  of  such  a  requirement.  Considerable 
time,  however,  is  given  to  instruction  of  students  in  the  above  courses 
in  the  determination  of  the  heating  values  of  coal  and  gas,  the  quality  of 
steam,  the  composition  of  gases,  and  the  measurement  of  electrical 
energy.  Is  it  probable  that  all  these  students,  after  graduation,  will 
make  use  of  the  instruction  in  any  of  the  above  determinations  more 
frequently  than  of  a  proportionate  amount  of  instruction  in  pyrometry? 
The  field  of  usefulness  of  pyrometers  will  be  extended  in  keeping  with  the 
improvements  in  the  instruments  and  a  realization  of  the  necessity  for 
accurate  temperature  measurements  and  control.  In  view  of  the  present 
wide  application  and  importance  of  pyrometry,  instruction  in  it  should 
receive  at  least  the  same  emphasis  in  the  curricula  for  ceramic,  chemical, 
and  mechanical  engineering  that  it  apparently  receives  in  the  curriculum 
for  metallurgical  engineering  in  most  colleges. 

It  would  be  difficult  to  prescribe  the  content  of  a  course  in  pyrometry 
to  meet  the  conditions  in  all  colleges.  Since  most  engineers  will  have  to 
do  with  the  measurement  and  control  of  temperature  in  industrial  opera- 
tion, it  seems  desirable  to  confine  the  scope  of  this  discussion  to  industrial 
needs.  The  operation  of  the  law  of  the  survival  of  the  fittest  seems  to 
have  left  the  thermocouple,  the  radiation,  and  the  optical  pyrometers  in 
possession  of  the  field  of  high-temperature  operations.  Thus,  a  mini- 
mum content  of  course  ought  to  include  instruction  in  the  principles  of 
operation  and  the  calibration  and  applications  of  each  of  these  types. 
Historical  matter  and  development  of  the  fundamental  temperature  scales 
need  be  only  briefly  considered;  but  some  time  can  profitably  be  devoted 
to  the  study  of  the  construction  of  auxiliary  equipment,  such  as  furnaces, 
heat  regulating  and  insulating  devices. 

The  thermocouple  is  no  doubt  more  widely  used  than  any  other  device 
for  measuring  high  temperatures,  and  it  is  also  much  abused.  The  in- 
struction concerning  it  might  well  include:  method  of  making,  calibra- 
tion by  determining  the  electromotive  force  at  the  melting  points  of  pure 
metals,  calibration  against  a  standard  couple,  effect  of  depth  of  immersion 
on  resultant  electromotive  force  due  to  heterogeneity,  and  protection 
against  contamination.  The  measurement  of  the  electromotive  forces  of 
couples  by  millivoltmeters  and  potentiometers  may  be  studied  with 
special  reference  tb  low  against  high  resistance  millivoltmeters,  milli- 
voltmeters against  potentiometers  with  different  depths  of  immersions, 
and  varying  temperatures  in  the  lead  wires. 


O.    L.    KOWALKE  683 

Radiation  pyrometers  of  the  fixed  and  movable  focus  type  and  optical 
pyrometers  of  the  Wanner  and  Morse  types  should  be  studied  with  refer- 
ence to  the  principles  involved  in  each,  the  construction,  the  methods  of 
calibration,  the  limitations  of  each  for  various  kinds  of  work,  and  the  ease 
with  which  they  may  be  manipulated.  Special  optical  pyrometers,  in- 
volving the  matching  of  colored  screens  in  the  instrument  against  the 
field  of  vision,  can  be  given  some  consideration. 

Due  to  the  limited  use  of  resistance  thermometers  in  the  measure- 
ment of  furnace  temperatures,  less  emphasis  may  well  be  put  on  them  than 
on  thermocouples.  It  is  worth  while  to  point  out  the  principles  involved, 
the  construction  of  the  apparatus,  and  the  methods  of  calibration.  If 
the  time  can  be  spared,  actual  calibrations  from  fixed  points  are  greatly 
worth  while. 

Since  check  calibrations  on  pyrometer  installations  are  always  nec- 
essary and  since  the  engineer  in  charge  will  many  times  have  to  do  such 
work  with  meager  equipment,  it  will  be  worth  the  time  spent  to  teach  the 
student  how  to  construct  furnaces  and  heat-regulating  devices.  It  may 
not  be  advisable  to  require  the  student  to  construct  the  furnaces  used  in 
the  co'urse,  but  it  is  desirable,  wherever  possible,  for  him  to  make  the 
repairs. 

The  interest  of  the  average  student  is  not  sustained  in  a  course  in 
pyrometry  if  it  is  devoted  exclusively  to  problems  in  calibration,  such 
problems  being  long  and  somewhat  tedious.  He  takes  a  much  greater 
interest,  however,  if  he  is  given  exercises  that  bring  out  the  limitations  and 
sources  of  error  in  the  particular  pyrometer  employed.  Among  the  prob- 
lems that  have  been  found  stimulating  to  the  student  are :  the  determi- 
nation of  the  temperature  gradient  in  various  kinds  of  firebricks;  the 
temperature  of  decomposition  of  limestone ;  comparison  of  thermocouples 
with  Seger  cones,  or  with  "  Sentinel  pyrometers  "  frequently  used  in  the  heat 
treatment  of  steel ;  and  dehydration  temperature  of  clay.  The  determina- 
tion of  the  transformation  temperatures  in  steels  of  various  carbon  con- 
tents by  the  differential  couple  and  the  simple  cooling,  curve  seldom  fails 
to  arouse  interest;  it  also  ties  the  principles  of  metallography  and  py- 
rometry together.  The  measurement  of  the  temperature  of  molten  copper 
in  a  ladle,  with  and  without  the  oxide  film,  simultaneously  with  a  ther- 
mocouple, a  radiation  pyrometer,  and  optical  pyrometers  of  the  Wanner 
and  Morse  types,  is  an  excellent  exercise  to  show  the  effect  of  emissivity. 
Many  other  problems  can  be  devised;  these  are  suggestions.  Such  cor- 
relation has  been  found  to  work  well  here  and  in  other  colleges  because 
it  gives  an  opportunity  to  link  together  a  number  of  lines  of  study  through 
the  teaching  of  pyrometry. 


INDEX 


NOTE.  —In  this  Index  the  names  of  authors  of  papers  are  printed  in  small 
capitals,  and  the  titles  of  papers  in  italics. 


Absolute  temperature,  40. 
Absolute  therm odynamic  scale,  63. 
Absolute  zero,  40. 
value,  42,  66. 

Absorbing  screens  for  optical  pyrometer,  313. 
effect  of  temperature  change,  316. 
spectral  transmission,  313. 
total  transmission,  313. 
Accuracy  of  Wien's  Equation,  293. 

Accuracy  tests  with  optical  pyrometer:  different  laboratories,  311. 
experienced  observers,  310. 
inexperienced  observers,  309. 

ADAMS,  L.  H. :  Tables  and  Curves  for  Use  in  Measuring  Temperatures  with  Thermo- 
couples, 165. 

Discussions:  on  Standard  Scale  of  Temperature,  59; 
on  Potentiometers  for  Thermoelement  Work,  147. 

Alloys  Suitable  for  Thermocouples  and  Base-metal  Thermoelectric  Practice  (LoHK),  181. 
Alundum  tubes,  241,  252,  256. 
AMES,  J.  S. :  Temperature,  37. 

Annealing  of  Glass  (TOOL  and  VALASEK),  475;  Discussion:  (WILLIAMSON),  482. 
Annealing  of  glass,  472. 
cooling  procedure,  480 
relaxation  time,  478. 
temperature  range,  476. 
Apparent  temperature,  337. 

correction  to  true,  337,  339. 

Application  of  Pyrometers  to  the  Ceramic  Industry  (GOHEEN),  535. 
Application   of  Pyrometry   to  the  Ceramic   Industries    (THWING),   516;    Discussion: 

(OWENS),  519;  (PURDY),  520. 
Application  of  Pyrometry  to  the  Manufacture  of  Gas-mask  Carbon   (MARSH),   652; 

Discussion:  (NEWCOMB),  667;  (HUBBARD),  667;  (AUTHOR),  668. 
Application  of  Pyrometry  to  Problems  of  Lamp  Design  and  Performance  (VAN  HORN), 

638;  Discussion:  (WORTHING),  644. 
Arsem  furnace,  277. 

ASHMAN,  A.  O. :  Discussions:  on  High-temperature  Thermometers,  239; 
on  Pyrometer  Porcelains  and  Refractories,  253 ; 
on  Recent  Improvements  in  Pyrometry,  204; 
on  Thermoelectric  Pyrometry,  128. 

Automatic  Compensation  for  Cold- junction  Temperatures  of  Thermocouple  Pyrometers 
(WUNSCH),  206;  Discussion:  (BRISTOL),  212. 

685 


686  INDEX 

Base-metal  thermocouples,  78,  181,  183,  191. 

calibration  data,  170. 

constancy,  156. 

electrical  resistance,  108. 

factors  affecting  usefulness,  154. 

homogeneity,  156. 

insulation  of  wire,  194. 

resistance  to  oxidation,  160. 
Bases:  optical  pyrometry,  70,  325. 

pyrometer  calibration,  325. 
BASH,  F.  E. :  Electric,  Open-hearth  and  Bessemer  Steel  Temperatures,  578. 

Forging  Temperatures  and  Rate  of  Heating  and  Cooling  of  Large  Ingots,  614. 

Industrial  Applications  of  Disappearing- filament  Optical  Pyrometer,  352. 

Report  of  Sub-Committee  of  Pyrometer  Committee,  32. 
BEHK,  L. :  Recording  Thermocouple  Pyrometers,  400. 
Bessemer  steel  furnace  temperature,  578. 
Bisque  burn,  248. 
Black  body:  condition,  335,  348. 

definition,  42,  66,  285,  293,  352,  367,  496. 

Lummer-Kurlbaum,  56. 

practical,  293,  324. 

radiation,  42. 

wedge  method,  385. 
Black-body  temperature,  43,  71,  286,  288. 

brightness  temperature,  288,  304. 

color  temperature,  288,  304. 
Blast-furnace  work,  pyrometry  in,  544 

BONIS,  N.  E. :  Reference  Standard  for  Base-metal  Thermocouples,  179. 
Bottle-glass  manufacture,  pyrometry  in,  483. 
Brightness  copper  surface,  350. 
Brightness  temperature,  288,  304. 

corrected  to  constant  wave-length,  306. 

true  temperature  relation,  304. 

wave-length  ascribed  to,  304,  389. 

BRISTOL,  W.  H. :  Discussions:  on  Recent  Improvements  in  Pyrometry,  205; 
on  Automatic  Compensation  for  Cold-junction  Temperatures,  2l2; 
on  Pyrometry  and  Steel  Manufacture,  577. 
BROWN,  R.  P.:  Recent  Improvements  in  Pyrometry,  188. 

Discussion  on  Pyrometry  and  Steel  Manufacture,  576. 
Brown  heatmeter,  90,  198. 

BRUSH,  C.  F. :  Some  Thermal  Relations  in  the  Treatment  of  Steel,  590. 
Bulb  temperature,  gas-filled  lamps,  643. 
Bureau's  method  of  calibrating  thermocouples,  147. 

BURGESS,  G.  K. :  Report  of  Pyrometer  Committee  of  National  Research  Council,    3; 
Discussion,  36. 

Discussion  on  Optical  and  Radiation  Pyrometry,  350. 
BURGESS,  G.  K.  and  WAIDNER,  C.  W. :  Metals  for  Pyrometer  Standardization,  61. 

c2,  value  of,  52,  53,  73,  286. 

Calibration :  optical  pyrometer,  300,  324,  334,  503. 

thermocouples,  76,  147,  164,  168,  170,  617,  675. 
Carbon  steel,  602. 
Carbon  vs.  tungsten  pyrometer  filaments,  303. 


INDEX  687 

Carborundum  tubes,  241,  253,  262. 

Camot'a  principle,  41,  63. 

Cement  kilns,  pyrometer  in,  522. 

Cement  temperature,  365. 

Centigrade  thermodynamic  scale,  46. 

Centigrade  system,  40. 

Ceramic  industry,  516,  535. 

Chromel-alumel  couples,  76,  182. 

Chromel-iron  couple,  calibration  of,  617. 

CLARK,  W.  M.  and  SPENCER,  C.  D. :  Pyrometer  Shortcomings  in  Glass-hottse  Practice, 

509. 
Clay  wares :  drying  of,  543. 

pyrometry  in  manufacture,  513. 

COBLENTZ,  W.  W. :  Present  Status  of  Radiation  Constants,  72. 
Cold  junction:  burying  of,  113,  204. 

compensation,  95,  96,  99,  103,  175,  184,  194,  195,  206,  401,  510. 

correction  factors,  96. 

method  using:  carbon  disks,  208. 
mercury,  206! 
potentiometer,  98. 
resistance  coils,  209. 
shunt,  100,  103. 
wheatstone  bridge,  103,  209. 

temperature  of,  96,  122. 
Color  temperature,"  288,  304. 
Committee,  International,  46,  58. 

Committee,  Pyrometer,  of  National  Research  Council,  names,  3. 
Compensated  leads,  97,  205. 
Concept  of  temperature,  37. 
Constant-pressure  gas  scale,  65. 
Constant-volume  gas  scale,  65. 
Contamination  of  thermocouples,  76. 
Corning  red  glass,  296,  313. 
Couples:  see  thermocouples. 

DANA,  L.  I.:  Melting  Point  of  Refractory  Materials,  267;  Discussion,  283. 

DANA,  L.  I.  and  FAIRCHILD,  C.  O. :  Pyrometry  in  Rotary  Portland  Cement  Kilns,  522. 

Deflection  potentiometer:  method,  90,  148,  301. 

theory,  91. 

Degree,  measure  of,  39. 
Depth  of  immersion :  thermocouples,  123.. 

thermometer,  586. 
Diffraction  around  pyrometer  filament,  306. 

test  of,  318. 
Disappearing-filament  pyrometer,  291,  319,  324,. 352. 

polarization  in,  319. 
Dressier  tunnel  kiln,  542. 
Drinker  method,  7. 

Effective  wave-length,  54,  297. 

determination  of,  297. 

variation  of,  298. 
Electric  charge,  unit,  Millikan's  value,  73. 


688 


INDEX 


Electric  furnace:  temperature,  578. 

tapping  temperature,  579. 
Electric,    Open-hearth   and   Bessemer   Steel    Temperatures    (BASH),    578;    Discussion: 

(TAYLERSON),  589. 

Emissive  Powers  and  Temperatures  of  Non-black  Bodies  (WORTHING),  367. 
Emissive  powers,  287,  363,  367,  368,  384. 

pseudo,  397. 

spectral,  372,  381. 

total,  371,  373. 

variation  of,  393. 
Emissivity:  steel,  slag,  13,  356. 

temperature  correction  for,  13. 

tungsten,  390. 

various  metals,  337. 

EMMONS,  J.  V.:  Pyrometry  in  the  Tool-manufacturing  Industry,  610. 
Englehard  indicator,  83. 
Errors:  line  resistance,  82. 

in  radiation  pyrometry,  347,  377. 

temperature  of  steel  furnaces,  36. 
Expansion  pyrometers,  189. 

FAIRCHILD,  C.  O. :  Discussion  on  Theory  and  Accuracy  in  Optical  Pyrometry,  322. 
FAIRCHILD,  C.  O.  and  DANA,  L.  I. :  Pyrometry  in  Rotary  Portland  Cement  Kilns,  522. 
FAIRCHILD,  C.  O.  and  FOOTE,  P.  D.:  Optical  and  Radiation  Pyrometry,  324. 
High-temperature  Control,  435. 
Recording  Pyrometry,  406. 
FAIRCHILD,  C.  O.,  FOOTE,  P.  D.  and  HARRISON,  T.  R.:  Thermoelectric  Pyrometry,  74; 

Discussion,  134. 
FEILD,  A.  L. :  Discussions:  on  Report  of  Pyrometer  Committee,  36. 

on  Pyrometry  in  Blast-furnace  Work,  558. 
FENNER,  C.  N.:  Use  of  Optical  Pyrometers  for  Control  of  Optical-glass  Furnaces,  495; 

Discussion,  505. 
Fery :  optical  pyrometer,  326. 

radiation  pyrometer,  345,  374. 
Field  of  view,  optical  pyrometer,  333,  354. 
Firebrick,  282. 

Fixed  points,  thermometry,  50,  51,  167. 
Fixed  junction  correction,  174. 

FOOTE,  P.  D. :  Discussions:  on  Standard  Scale  of  Temperature,  60. 
on  Melting  Point  of  Refractory  Materials,  282 ; 
on  Optical  and  Radiation  Pyrometry,  351; 
o/i  Tin:    An  Ideal  Pyrometric  Substance,  465. 

FOOTE,  P.  D.  and  FAIRCHILD,  C.  O. :  Optical  and  Radiation  Pyrometry,  324. 
High-temperature  Control,  435. 
Recording  Pyrometry,  406. 
Foote  &  Fisher  pyrometer,  331.       - 

FOOTE,  P.  D.  and  HARRISON,  T.  R. :  Discussion  on  Self-checking  Galvanometer  Pyro- 
meter, 151. 
FOOTE,  P.  D.,  HARRISON,  T.  R.  and  FAIRCHILD,  C.  0.:  Thermoelectric  Pyrometry,  74; 

Discussion,  134. 
FOOTE,  P.  D.,  WAIDNER,  C.  W,  and  MUELLER,  E.  F.:  Standard  Scale  of  Temperature, 

46. 
Forging  temperatures,  5,  357,  570,  614,  623. 


INDEX  689 

Forging  Temperatures  and  Rate  of  Heating  and  Cooling  of  Large  Ingots   (BASH),   614; 

Discussion:  (FRY;,  626. 
Formula  for  thermocouple,  51. 

FORSYTHE,  W.  E. :  Theory  and  Accuracy  in  Optical  Pyrometry  with  Particular  Ref- 
erence to  the  Disappearing-filament  Type,  291;  Discussion,  323. 

Discussions:  on  Potentiometers  for  Thermoelement  Work,  148; 

.  on  Pyrometer  Protection  Tubes,  257. 
Foster,  radiation  pyrometer,  344. 
Frazil  ice,  462. 
FREEMAN,  J.  R.,  Jr.  ^nd  Scott,  H. :  Use  of  Modified  Rosenhain  Furnace  for  Thermal 

Analysis,  214. 

FREY,  C.  P. :  Resistance  Thermometry  for  Industrial  Use,  458. 
FRINK,  R.  L. :  Pyrometry  Applied  to  Bottle-glass  Manufacture,  483. 
FRY,  L.  H. :  Discussion  on  Forging  Temperatures  of  Large  Ingots,  626. 
Fundamental  Laws  of  Pyrometry  (Mendenhall),  63. 
Furnaces:  Arsem,  277. 

black-body,  293. 

glass-tank,  483. 

Heroult,  588. 

iridium-tube,  53,  277. 

Owens  revolving  pot,  488. 

refractory  melting  points.  276. 

Rosenhain,  214. 

tungsten  and  molybdenum  wound,  279. 

vacuum,  277. 

Galvanometer:  direct  reading,  138. 

high  resistance,  80. 

lack  of  precision  in,  139. 

mounting  of,  113. 

sensitivity  of,  138. 

swamping  resistance,  80. 
Gas  loss,  straight  coiled  filament,  640. 
Gas-mask  carbon :  application  of  pyrometer  in  manufacture,  652. 

air  treater,  654. 

steam  treater,  656 

thermocouple  used,  661. 

pyrometer  tubes  for,  664. 
Gas  pressure,  64. 
Gas  scale,  46,  63. 
Gas  thermometers,  189. 
Gas  thermometry,  limits  of,  64. 
Glass:  annealing  of,  475. 

cooling  procedure,  478. 

relaxation  time,  478. 

thermocouple  installation  for  annealing  kiln,  466. 

transformation  temperature,  478. 

Glass  industry,  optical  pyrometer  in,  130,  358,  491,  509. 
Glass  pot,  measurement  of  temperature  in,  502. 
Glass-tank  furnace,  483. 
Glazing  of  tubes,  248. 
Gloat  burn,  250. 
GOHEEN,  J.  P.:  Application  of  Pyrometers  to  the  Ceramic  Industry,  535. 


C90  INDEX 

Gold,  melting  point,  51,  2K6.- 

Gold-point  palladium-point  brightness  ratio,  52. 

Graphite  tip,  16. 

GUILLAUME,  C.  E. :  Discussion  on  Standard  Scale  of  Temperature,  58. 

HARRISON,  T.  R. :  Discussions:  on  Potentiometers  for  Thermoelement  Work,  147; 
on  Thermoelectric  Pyrometry,  136;  . 

on  Some  Factors  Affecting  Usefulness  of  Base-metal  Thermocouples,  164. 
HARRISON,  T.  R.   AND  FOOTE,   P.  D. :  Discussion  on  Self-checking  Galvanometer  Py- 
rometer, 151. 
HARRISON,  T.  R.,  FOOTE,  P.  D.  and  FAIRCHILD,  C.  O. :  Thermoelectric  Pyrometry,  74; 

Discussion,  134. 

Harrison  &  Foote  indicator,  84,  141. 
HARVEY,  F.  A.:  Pyrometer  Protection  Tubes,  255;  Discussion,  257. 

Discussion  on  Optical  and  Radiation  Pyrometry,  351. 
Heat  generated  after  tempering,  590. 
carbon  steel,  602. 
manganese  steel,  604. 
nickel  chromium,  595. 
Heatmeter,  Brown,  90. 
Heat  treatment,  steel,  358. 

High-temperature  Control  (FAIRCHILD  and  FOOTE),  435;  Discussion:  (NEWCOMB),  449. 
High-temperature  control:  automatic  alarms,  441. 
field  of,  435. 
manual  alarms,  441. 

High  temperature,  measurement  of,  368. 
High-temperature  Scale  and  its  Application  in  the  Measurement  of  True,    Bright  HCXN 

and  Color  Temperatures  (HYDE),  285. 
High-temperature  scale,  51. 

reproduction  of,  57. 
High-temperature     Thermometers    (WILHELM),     225;     Discussion:     (ASHMAN),     239; 

(AUTHOR),  239. 

High-temperature  thermometers,  testing  of,  237. 
High-resistance  galvanometers,  80. 
Hot  junctions,  making,  77. 

Hot-wire  Anemometer  with  Thermocouple  (TAYLOR),  221. 
HUBBARD,    C.    W. :  Discussions:  on   Application   of  Pyrometry   to   the    Manufacture 

of  Gas-mask  Carbon,  667 ; 
on  Pyrometer  Porcelains  and  Refractories,  254. 
HUTCHINS,  O. :  Pyrometer  Protection  Tubes,  262. 
HYDE,  E.  P.:  High-temperature  Scale  and  its  Application  in  Measurement  of  True, 

Brightness  and  Color  Temperature,  285. 
Discussion  on  Standard  Scale  of  Temperature,  60. 
Hydrogen  scale,  41. 
international.  46. 

Ice,  frazil,  462. 

Ice  point -on  Kelvin's  scale,  66. 

Incandescent  gas  mantles,  temperature,  632. 

Incandescent  lamp  filaments,  temperature  of,  627. 

Industrial  Applications  of  Disappearing- filament  Optical  Pyrometer  ^BASH),  352. 

Ingots,  rate  of  heating  and  cooling,  614,  618. 

International  hydrogen-scale  temperature,  46. 


INDEX  691 

International  temperature  scale,  49,  450. 

Indium  tube  furnace,  53,  277. 

Iron-constantan  in  couples,  260. 

Iron  oxide,  radiation  temperature  and  true  temperature  relation,  378. 

IVES,  H.  E. :  Temperature  Measurements  of  Incandescent  Gas  Mantles,  632. 

Japan  p'rotecting  tubes,  256. 

JOSEPH,  T.  L.  and  ROYSTER,  P.  H. :  Pyrometry  in  Blast-furnace  Work,  544;  Discussion, 

563. 
Joule-Kelvin  coefficient,  41. 

Kelvin's  temperature  scale,  41,  46,  63,  288. 

KEUFFEL,  C.  W. :  Pyrometry  as  Applied  to  Manufacture  of  Optical  Glass,  506. 

Kirchhoff's  black  body,  42,  66. 

Kirchhoff's  law,  368. 

KOWALKE,  O.  L.:  Some  Factors  Affecting  the  Usefulness  of  Base-metal  Thermocouples, 

154. 

Teaching  Pyrometry,  681. 
KRAYBILL,  H.  R.  and  SLIGH,  T.  S.,.Jr. :  Temperature  of  a  Burning  Cigar,  645. 

Laboratory  form  of  pyrometer,  295. 
Lambert's  cosine  law,  302,  371,  396. 
Lamps:  temperature  of  incandescent  filament,  627,  638. 

temperature  and  efficiency  of,  628. 

temperature  and  size,  630. 
Laws  of  pyrometry,  63. 
Le  Chatelier  pyrometer,  292. 

LINCOLN,  R.  B.:  Protecting  Tubes  for  Thermocouples,  258. 
LINVILLE,  C.  P. :  Discussion  on  Pyromelry  in  Blast-furnace  Work,  560,  562. 
Liquids  in  thermometers,  234. 

LITTLETON,   J.  T.,  Jr. :  Discussion  on  Thermoelectric  Pyromelry,  130. 
LOHR,  J.  M.:  Alloys  Suitable  for  Thermocouples  and  Base-metal  Thermoelectric  Practice, 

181. 
Lummer-Kurlbaum  black  body,  56. 

Manganese  steel,  magnetic  effects  of,  604. 
Manufacture  of  porcelain  tubes,  244. 
Marquardt  tubes,  255. 

methods  of  making,  241. 
MARSH,  K. :  Application  of  Pyrometry  to  the  Manufacture  of  Gas-mask  Carbon,  652; 

Discussion,  668. 
Melting  Point  of  Refractory  Materials  (DANA),  267;  Discussion:  (UNGER),  282;  (FOOTE) 

282;  (PURDY),  283;  (AUTHOR),  283. 
Melting  points,  gold,  51,  286. 

palladium,  52,  54,  55,  286. 

platinum,  56. 

refractory  materials,  267. 

tungsten,  57,  392. 
Melting  point  standardization,  61. 

MENDENHALL,  C.   E. :  Fundamental  Laws  of  Pyromeiry,  63. 
Teaching  Pyrometri/  in  Technical  Schools,  678. 


692  INDEX 

Mercurial  thermometers,  225. 

emergent  stem  error,  229. 

error  in,  232. 

industrial  type,  228. 

inert  gas  in,  232. 

stem  correction  data,  231. 

testing  of,  237. 

Metals  for  Pyrometer  Standardization  (WAIDNER  and  BURGESS),  61. 
MILLER,  A.  H. :  Pyrometry  and  Steel  Manufacture,  567. 
Millikan's  value,  unit  electric  charge,  73. 
Monochromatic  screens,  295. 

Corning  red  glass,  296. 

effective  wave-length,  297. 

transmission  of,  296. 

Morse  thermogage  or  pyrometer,  611,  627. 
MUELLER,  E.  F.,  WAIDNER,  C.  W.,  and  Foote,  P.  D. :  Standard  Scale  of  Temperature, 

46. 
Multiple  point  recorders,  420. 

NEWCOMB,  R.  W. :  Pyrometer  Porcelains  and  Refractories,  251. 

Discussions:  on  Recording  Thermocouple  Pyrometers,  405, 

on  Application  of  Pyrometry  to  Manufacture  of  Gas-mask  Carbon,  667. 
on  High-temperature  Control,  449; 
on  Recording  Pyrometry,  433 ; 
Nickel-chromium  steel,  595. 
Nitrogen  scale:  correction  to  constant  pressure,  66. 

correction  to  constant  volume,  65. 
Non-black  bodies,  304,  367. 
conditions,  335,  348. 
temperature  of,  3Q4. 
Non-ferrous  foundry,  pyrometer  in,  360. 
NORTHRUP,  E.  F:  Tin:  An  Ideal  Pyr-ometric  Substance,  464. 
Discussions:  on  Recent  Improvements  in  Pyrometry,  205; 

on  Optical  and  Radiation  Pyrometry,  349. 
Northrup  pyrovolter,  89. 

Open  hearth,  temperature  in,  578. 

Optical  .  and  Radiation  Pyromelry  (FOOTE  and  FAIRCHILD),  324;  Discussion: 
(NORTHRUP),  349;  (BURGESS),  350;  (WORTHING),  350;  (FOOTE),  351; 
(HARVEY),  351. 

Optical  glass,  pyrometer  in  manufacture  of,  491,  495,  506,  509. 
Optical  pyrometer  (see  also  pyrometers),  677. 

as  photometer,  350. 

basis  of,  70,  325. 

calibration  of,  300,  324,  334,  503. 

Fery,  366. 

field  of  view,  333,  354. 

theory  of,  291. 

OWENS,  F.  T. :  Discussion  on  Application  of  Pyrometry  to  the  Ceramic  Industries,  519. 
OWENS  revolving  pot,  488. 

Palladium,  melting  point,  52,  54,  55,  286. 
Peltier  effect,  74. 


INDEX  693 

PENCE,  F.  K. :  Pyrometry  in  the  Manufacture  of  Clay  Wares,  513. 

Planck,  69,  285. 

Planck's  constant,  53,  73. 

Platinum,  melting  point,  56. 

Platinum-platinum-rhodium  thermocouples:  167,  191,  254,  257,  262. 

Polarization,  disappearing-filament  pyrometer,  319. 

Porcelain  for  Pyrometric  Purposes  (RIDDLE),  240. 

Porcelain  protecting  tubes:  manufacture  of,  244. 

Marquardt,  241,  255. 

primary,  251. 

secondary,  252. 

Porous-plug  experiment,  41,  44. 
Portable  potentiometer,  88,  142. 
Portable  test  sets,  82. 

PORTER,  H.  F. :  Self-checking  Galvanometer  Pyrometer,  149;  Discussion,  153. 
Position  rotating  sector,  319. 
Potentiometer:  deflection,  90,  91,  144,  148,  301. 

double,  145. 

portable,  88,  142. 

precision,  142. 

split  circuit,  403. 

standard  cells,  methods  to  avoid,  140. 

recording,  404. 

thermocouple  work,  137. 

White,  144. 

Wulf,  144. 
Potentiometers  for  Thermoelement  Work  (White),  137;  Discussion:  (HARRISON),  147; 

I         (ADAMS),  147;  (FORSYTHE),  148. 
Present  Status  of  Radiation  Constants  (CoBLENTz),  72. 
Pressure,  gas,  64. 
Protecting  tubes :  alundum,  256. 

carborundum,  241,  253,  262. 

for  thermocouples,  124,  192,  258. 

Japan,  256. 

nichrome,  358. 

pyrometer,  255,  262. 

quartz,  251,  258. 

qualities  of,  258. 
-  rate  of  heating,  271. 

Silfrax,  256. 

Usalite,  256. 

Worcester,  256. 

Protecting  Tubes  for  Thermocouples  (LINCOLN),  258. 
Pseudo-emissive  powers,  397. 
PTJRDY,  R.  C. :  Discussions:  on  Application  of  Pyrometry  to  the  Ceramic  Industries, 

520; 

on  Melting  Point  of  Refractory  Materials,  282. 
Pyod  thermocouple,  78. 
Pyrometer  (see  also  optical  pyrometer,  radiation  pyrometer) : 

basis  of  calibration,  325. 

Committee  on,  3,  5. 

disappearing-filament  type,  291,  319,  324,  352. 

expansion,  189. 


694  INDEX 

Pyrometer  (continued),  Fery:  374. 
optical,  326. 
radiation,  345. 
filaments:  302. 

constancy  of,  303. 

diffraction  around,  306. 

test  of  diffraction  around,  318. 

time  of  heating,  303. 

tungsten  vs.  carbon,  303. 
Foote  &  Fisher,  331. 
Foster,  344. 
galvanometer,  149. 
improvements,  recent,  188. 
in  various  industries:  see  pyrometers. 
LeChatelier,  292. 
precautions,  working,  292. 
protecting  tubes,  255,. 262. 
recording,  406. 

samples  for  standardization  of,  62. 
Scimatco,  330 
signaling,  200. 
Wanner,  328. 
Pyrometer  Porcelains  and  Refractories  (NEWCOMB),  251;  Discussion:  (ASHMAN),  253; 

(HUBBARD),  254. 

Pyrometer  Protection  Tubes  (HARVEY),  255;  Discussion:  (RIDDLE),  257;  (AUTHOR), 

257;  (FORSYTHE),  257;  (SOSMAN),  257. 
Pyrometer  Protection  Tubes  (HUTCHINS;,  262. 

Pyrometer  Shortcomings  in  Glass-house  Practice  (CLAKK  and  SPENCER),  509. 
Pyrometers  in  various  industries:  bottle-glass  manufacturing,  482. 

optical,  487. 

radiation,  487. 

thermoelectric,  485. 
blast-furnace  work,  544. 
ceramic  industry,  516,  535. 
clay  ware  manufacturing,  513. 
glass  manufacturing,   130,  358,  475,  491,  495,  506. 

results  of  introduction,  501. 

shortcomings  of  pyrometer,  509. 

thermocouples,  491. 
Portland  cement  kilns,  522. 

clinkering  zone,  523. 

errors  in,  522. 

methods,  524. 

temperature  in  kilns  and  stacks,  531. 
steel  industry,  555,  567,  578. 

arbitrary  standard,  569. 

calibration  and  checking,  575. 

forging  temperature,  570. 

heat  treatment,  358,  571. 

tapping,  579. 

thermocouples,  568,  571. 
tool  manufacture,  606. 


INDEX  695 

Pyrometry    and    Steel    Manufacture    (MILLER),    567;    Discussion:  (BROWN),    576; 

(BRISTOL),  577. 

Pyrometry  Applied  to  Bottle-glass  Manufacture  (FRINK),  483. 
Pyrometry  as  Applied  to  Manufacture  of  Optical  Glass  (KEUFFEL),  506. 
Pyrometry  in  Blast-furnace  Work  (ROYSTER  and  JOSEPH),  544:  Discussion:  (FEILD), 

558;  (LINVILLE),  560,  562;  (AUTHORS),  562,  563. 
Pyrometry  in  the  Manufacture  of  Clay  Wares  (PENCE),  513. 
Pyrometry  in  the  Manufacture  of  Optical  Glass  (WALCOTT),  491. 
Pyrometry  in  Rotary  Portland  Cement  Kilns  (DANA  and  FAIRCHILD),  522. 
Pyrometry  in  the  Tool-manufacturing  Industry  (EMMONS),  610. 
Pyrovoltmeter,  89,  140,  149. 

Quartz,  259. 

with  platinum  thermometer,  251,  254. 
Quartz  protecting  tubes,  251. 

Radiation  constants:  c2,  52,  53,  73,  286. 

present  status  of,  72. 

total  radiation,  72. 
Radiant  flux,  287. 
Radiation  engine,  68. 

pressure,  67. 
Radiation  pyrometer,  324,  374. 

advantage  and  disadvantage  of,  348. 

calibration  of,  324. 

errors,  347,  377. 

Fery,  345. 

Foster,  344. 

Thwing,  342. 
Radiation  selective,  393. 
Radiation  temperature,  43,  289,  374,  379. 

corrections  to,  349. 
Range  control  board,  95. 
Rare-metal  thermocouples,  675. 
Recent    Improvements   in   Pyrometry    (BROWN),    188;    Discussion:  (TILLYER),    203; 

(ASHMAN),  204;  (BRISTOL;,  205;  (NORTHRUP),  205;  (ZELENY),  205. 
Recording  potentiometers,  404. 

Recording  Pyrometry  (FAIRCHILD  and  FOOTE),  406;  Discussion:  (NEWCOMB),  433. 
Recording  thermoelectric  pyrometers,  199. 

Recording  Thermocouple  Pyrometers  (BEHR),  400;  Discussion:  (NEWCOMB),  405. 
Recording  thermometers,  233. 
Red  glass,  Corning,  296. 

Reference  Standard  for  Base-metal  Thermocouples  (BONN),  179. 
Refractory  tube  in  steel  bath,  6. 
Refractory  materials :  factors  affecting,  269. 

meaning  of  melting  point,  267,  282. 

melting  points,  267. 
Report  of  Pyrometer  Committee  of  National  Research  Council  (BURGESS),  3;  Discussion: 

(SCOTT),  34;  (FEILD),  36;  (BURGESS),  36. 
Reports  of  subcommittees:  pyrometer  in  open-hearth  furnace:  First  report,  13. 

second  report,  20. 

third  report,  23. 

on  crucible  steel,  32. 


696  INDEX 

Resistance  base-metal  thermocouples,  108. 
Resistance  of  indicating  instruments,  79. 
Resistance  thermometers,   190,  450,  458. 

accuracy,  459. 

construction  of  spirals,  452. 

industrial  uses,  458. 

limitations,  452. 

material,  460. 

Resistance  Thermometry  (ROBINSON),  450. 

Resistance  Thermometry  for  Industrial  Use  (FREY,,  458;  Discussion:  (Reman),  462. 
Resistance  to  oxidation  of  thermocouples,  160. 
RIDDLE,  F.  H. :  Porcelain  for  Pyrometric  Purposes,  240. 
Discussion  on  Pyrometer  Protection  Tubes,  257. 
ROBERTS,  H.'-S.pand  WILLIAMSON,  E.  D. :  Thermocouple  Installation  in  Annealing 

Kilns  for  Optical  Glass,  ^66. 
ROBINSON,  F.  W. :  Resistance  Thermometry,  450. 

ROUSH,  G.  A. :  Discussion  on  Resistance  Thermomelry  for  Industrial  Use,  462. 
Royal  Worcester  tubes,  256. 

ROYSTER,  P.  H. ;  Discussion  on  Pyrometry  in  Blast-furnace  Work,  562. 
ROYSTER,  P.  H.  and  JOSEPH,  T.  L. :  Pyrometry  in  Blast-furnace  Work,  544;  Discussion, 
563, 

Scimatco  pyrometer,  330. 

SCOTT,  H. :  Discussion  on  Report  of  Pyrometer  Committee,  34. 

SCOTT,  H.  and  FREEMAN,  J.  R.,  JR.  :  Use  of  Modified  Rosenhain  Furnace  for  Thermal 

Analysis,  214. 

Second  law  of  thermodynamics,  64. 
Sector  for  pyrometry,  303,  313. 
Sector,  position  of,  319. 
Seebeck  effect,  74 
Self-checking    Galvanometer    Pyrometer    (PORTER),     149;    Discussioji:    (FOOTE    and 

HARRISON),  151;  (AUTHOR),  153. 
Seger  cones,  517. 
Semi-potentiometer  method,  88. 

SHACKELFORD,  B.  E. :  Temperatures  of  Incandescent-lamp  Filaments,  627. 
Shore  pyroscope,  327. 
Shrinkage :  carbon  steel,  602. 

manganese  steel,  604. 

nickel  chromium  steel,  595. 

steel,  592. 

Signaling  pyrometer,  200. 
Silfrax  tubes  and  sheaths,  256. 
Sillimanite  tubes,  243. 

SLIGH,  T.  S.,  JR.:  Discussion  on  Temperature  of  a  Burning  Cigar,  651. 
SIIGH,  T.  S.,  JR.  and  KRAYBILL,  H.  R. ;  Temperature  of  a  Burning  Cigar,  645. 
Slit-width  corrections,  56. 
Solar  radiation  temperature,  379. 
Some  Factors  Affecting   Usefulness  of  Base-metal   Thermocouples    (KOWALKE),    154; 

Discussion:  (HARRISON),  164. 

Some  Thermal  Relations  in  the  Treatment  of  Steel  (BRUSH),  590. 
SOSMAN,  R.  B. :  Discussion  on  Pyrometer  Protection  Tubes,  257. 
Spectral  emissive  powers,  372,  381. 
Spectral  transmission  absorbing  screens,  313. 


INDEX  697 

SPENCER,  C.  D.  and  CLARK,  W.  M. :  Pyrometer  Shortcomings  in  Glass-house  Practice, 

509. 

Standard  cells,  devices  to  avoid,  140. 
Standard  fixed  points,  51. 
Standard  lamp,  calibration  of,  302. 
Standard  Scale  of  Temperature  (WAIDNER,  MuELLER^and  FOOTE),  46;  Discussion: 

(GuiLLAUME),  58;  (ADAMS),  59;  (HYDE),  60;  (FOOTE),  60. 
Steel,  thermal  relation  in  treatment,  590. 
Steel  manufacture:  heat  generated  after  tempering,  590. 

heat  treatment,  358. 

pyrometer  in,  355,  567,  578. 

Stefan-Boltzmann  Law,  43,  72,  285,  325,  450,  496. 
Sulfur  boiling  point,  49,  58. 
Symbols,  temperature,  289. 
Symposia,  previous,  5. 

Tables  and  Curves  for  Use  in  Measuring  Temperatures  with  Thermocouples  ( ADAMS), 

165. 

Tapalog,  422. 

TAYLOR,  T.  S. :  A  Hot-wire  Anemometer  with  Thermocouple,  221. 
TAYLERSON,  E.  S. :  Discussions:  on  Thermoelectric  Pyromelry,  133. 

on  Electric,  Open-hearth  and  Bessemer  Steel  Tempeiatures,  589. 
Teaching  pyrometry:  reference  material,  text-book,  671. 
Teaching  Pyrometry  (KOWALKE),  681. 

Teaching  Pyrometry  in  Technical  Schools  (MENDENHALL),  678. 
Teaching  Pyrometry  in  Our  Technical  Schools  (WENDELL),  669. 
Teeming  temperature,  34,  356. 
Temperature  (AMES),  37. 
Temperature:  absolute,  40. 

annealing,  range,  476. 

apparent,  337. 

apparent,  corrections  to  true,  337,  339. 

brightness,  288,  304. 

color,  288,  304. 

concept  of,  37. 

condition  for  discussion,  38 

control  pyrometers,  201. 

forging,  353,  570,  614,  623. 

high-temperature  scale,  285 

hydrogen  scale  of,  41,  46. 

international  scale  of,  45. 

Kelvin's  scale,  41,  63,  288. 

measurement  of  high,  368. 

radiation,  43,  243,  289. 

rolling,  570. 

scales  in  intervals:   -40°C  to  450°C,  48. 
450°C  to  1100°C,  50. 
above  1100°C,  51. 

teeming,  34,  356. 

theory  of,  44. 

true  temperature,  304,  337,  385. 

true  temperature,  black-body  relation,  287. 

true,  brightness,  color  temperature  relation,  289. 

two  bodies  at  same,  37. 


698  INDEX 

Temperature  and:  manganese  constant,  555. 

silicon  constant,  546. 

sulfur  constant,  546. 
Temperature  of:  basic  electric  furnace,  24,  30. 

burning  cigar,  645. 

cement,  365. 

cold  end  of  thermocouple,  184. 

electric  furnace,  578. 

glass  pot,  448,  502,  507. 

incandescent  lamp  filament,  627,  638. 

incandescent  gas  mantle,  632. 

manganese  steel,  588. 

nickel  ordinance  steel,  580. 

non-black  bodies,  304. 

single  molecule,  45. 

slag  pot,  265. 

steel  in  open-hearth  furnace,  6,  17,  19,  20. 
method  of  measuring,  7. 
Drinker's  method,  7,  9. 

various  metals,  364. 
Temperature  of  a  Burning  Cigar  (SLIGH  and  KRAYBILI.),  645;  Discussion:  (WHITE),  650; 

(SLIOH),  651.  • 

Temperature  Measurements  of  Incandescent  Gas  Mantles  (Jvss),  632. 
Temperature  recorders,  405. 

Beighlee,  413. 

Brown,  414.  ..        • 

Hoskins,  417. 

industrial  types,  412. 

Leeds  and  Northrup,  428. 

record  chart,  408. 

roll  chart,  415. 

thread,  418. 

Thwing,  422. 

Wilson-Maeulen,  423. 

Temperatures  of  Incandescent-lamp  Filaments  (SHACKELFORD),  627. 
Test  of  pyrometer:  different  laboratories,  311. 

experienced  observers,  310. 

inexperienced  observers,  309. 

Theory  and  Accuracy  in  Optical  Pyrometry  with  Particular  Reference  to  the  Disappearing.- 
filament  Type  (FORSYTHE),  291;  Discussion:  (FAIRCHILD),  322;  (AUTHOR;, 
323. 

Thermal  equilibrium,  38. 
Thermocouple  Installation  in  Annealing  Kilns  for  Optical  Glass   (Wiu.iAMsox  and 

ROBERTS),  466. 

Thermocouple  pyrometers,  400. 
Thermocouples:  alloys  for,  181. 

annealing  of,  472. 

base-metal,  78,  181,  183,  191. 

calibration,  76,  147,  164,  617,  675. 
tables,  168,  170. 

calorized  iron  constant  in,  163. 

care  of,  187. 

choice  of  wire,  466. 


INDEX  699 

Thermocouples  (continued):  constancy,  156. 
contamination  of,  76. 
copper-constantan,  168. 
depth  of  immersion,  123. 
desirable  properties,  74. 
deviation  curves,  173. 
fixed  junction  correction,  174. 
formula  for,  51. 
homogeneity  of,  156. 
Hoskins  couple,  171. 
installations,  112. 

cold-junction  burying,  1J3,  194,  204. 

common  return,  113. 

commutating  switches,  119. 

contact  resistance,  1 12. 

for  annealing  kilns,  466. 

indicator,  location  of,  113. 

junction   box   and  zone  box,  119. 

primary  protection,  251. 

secondary  protection,  251. 

Wilson- Maeulen  zone  box,  122. 

wiring  diagram,  114. 
insulation  of,  159,  194. 
irreproducibility  of  couples,  correction  for:  104. 

by  series  resistance,  105. 

by  shunt  resistance,  108. 

by  shunt  and  series  resistance,  109. 
life  of  platinum  thermocouples,  255,  257,  262. 
platinum-platinum-rhodium,  167,  191. 
protection  of,  77,  113,  255,  259,  263,  361. 
resistance  to  oxidation,  160,  167,  191. 
resistance  of  indicating  instruments,  79. 
reproducibility  of  couples,  111. 
tables  for  calibration,  168,  170. 
Thermodynamic  scale,  63. 

Thermoelectric    Fyrometry    (FooTE,    HARRISON^  and    FAIRCHILD),    74;     Discussion: 
(THWING),    128;    (ASHMAN),    128;    (WILSON),    129;    (LITTLETON),    130; 
(TAYLERSON),  133;  (AUTHORS),  134;  (WHITE),  135;  (HARRISON),  136. 
Thermometers,  39. 

fixed  points,  50,  51,  167. 

gas,  189. 

high-temperature  mercurial,  225. 

liquids  in,  234. 

pressure,  234. 

accuracy  of,  236. 

principle  of,  235. 
quartz  with  platinum,  251.  254. 
recording.  233. 
resistance,   190,  250,  258,  489. 

accuracy  of,  459. 

materials,  460. 
Thermoelectric  power,  167. 


700 


INDEX 


Thermoelectric  pyrometer,  74,  181,  190. 

Brown  heatmeter,  90. 

deflection  potentiometer,  90. 

galvanometer,  79. 

Harrison-Foote,  84. 

potentiometer,  88. 

pyrovolter,  89. 

recording,  199. 

semi-potentiometer,  88. 
Thermogage,  613. 

Thermos  bottle  for  cold  junction,  185,  205. 
Theory  of  optical  pyrometer,  291. 
Thomson  effect,  74. 
THWING,  C.  B. :  Application  of  Pyrometry  to  the  Ceramic  Industries,  516. 

.Discussion  on  Thermoelectric  Pyrametry,  128. 
Thwing  radiation  pyrometer,  342. 

TILLYER,  E.  D. :  Discussion  on  Recent  Improvements  in  Pyrometry,  203. 
Tin:  An  Ideal  Pyrometric  Substance  (NORTHRUP),  464;  Discussion:  (FooTE),  465. 
TOOL,  A.  Q.  and  VALASEK,  J. :  Annealing  of  Glass.  475. 
Tool  manufacture,  pyrometer  in,  606. 
Total  emissive  power,  371,373 
Total  radiation  constant,  72. 
Transformation  point  recorders,  430. 
Transformation  temperature  of  glasses,  478. 
Transmission  absorbing  screens:  Corning  red  glass,  296. 
constancy  of,  296. 
change  in,  299. 
spectral,  313. 
total,  314. 

effect  temperature  change,  316. 
Tungsten:  emissivity  of,  390. 

melting  point  of,  57,  392. 

vs.  carbon  pyrometer  filament,  303. 

UNGER,  J.  S. :  Discussion  on  Melting  Point  of  Refractory  Materials,  282. 
Unipivotal  instruments,  79. 
Usalite  tubes,  256. 

Use  of  Modified  Rosenhain  Furnace  for  Thermal  Analysis  (Scorr  and  FREEMAN),  214. 
Use  of  Optical  Pyrometers  for  Control  of  Optical-glass  Furnaces  (FENNER,)  495;  Discus- 
sion: (AUTHOR),  505. 

VALASEK,  J.  and  TOOL,  A.  Q. :  AnneaUng  of  Glass,  475. 
Value  of:  c»,  52,  53,  73,  286. 

"e",  73. 

VAN  HORN,  I.  H. :  Application  of  Pyrometry  to  Problems  of  Lamp  Design  and  Per- 
formance, 638. 

WAIDNER,  C.  W.  and  BURGESS,  G.  K.:  Metals  for  Pyrometer  Standardization,  61. 
WAIDNER,  C.  W.,  MUELLER,  E.  F.  and  FOOTE,  P.  D.:  Standard  Scale  of  Temperature, 

46. 

WALCOTT,  A.  J.:  Pyrometry  in  the  Manufacture  of  Optical  Glass,  491. 
Wanner  pyrometer,  328. 
Wave-length,  effective,  54,  297,  304,  389. 


INDEX  701 

Wedge  method  for  black  body,  385. 

Wedgewood  pyrometer,  517. 

WENDELL,  G.  V. :  Teaching  Pyrometry  in  Our  Technical  Schools,  669. 

Wheatstone  bridge-cold  junction  compensation,  103. 

WHITE,  W.  P.:  Potentiometers  for  Thermoelement  Work,  137. 

Discussions:  on  Thermoelectric  Pyrometry,  135. 

on  Temperature  of  a  Burmng  Cigar,  650. 
Wien's  law:  44,  68, 

accuracy  of,  293. 

WILHELM,  R.  M.:  High-temperature  Thermometers,  225;  Discussion,  239. 
WILLIAMSON,  E.  D. :  Discussion  on  Annealing  of  Glass,  482. 
WILLIAMSON,  E.  D.  and  ROBERTS,  H.  S.:  Thermocouple  Installation   in  Annealing 

Kilns  for  Optical  Glass,  466. 

WILSON,  C.  H. :  Discussion  on  Thermoelectric  Pyrometry,  129. 
Window,  transmission  corrections  for,  340. 
WORTHING,  A.  G. :  Emissive  Powers  and  Temperatures  of  Non-black  Bodies,  367. 

Discussions:  on  Optical  and  Radiation  Pyrometry,  350; 

on  Application  of  Pyrometry  to  Problems  of  Lamp  Design  and  Performance,  644. 
WUNSCH,   F. :  Automatic  Compensation  for  Cold-junction   Temperatures    of  Thermo- 
couple Pyrometers,  206. 

ZELENY,  A. :  Discussion  on  Recent  Improvements  in  Pyrometry,  205. 
Zero,  absolute,  40,  66. 
Zirconium  oxide  tubes,  250. 


THE  BROWN  INSTRUMENT  CO. 


PHILADELPHIA,  PA. 


NEW  YORK 

DENVER 


PITTSBURGH 

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Manufacturers  of  Pyrometers,  Thermometers,  Tachometers,  Recording  Gauges,  Time 
and  Operation  Recorders  and  Other  Scientific  Instruments 


The  Brown  High  Resistance 
Indicating  Pyrometer 


BROWN  PYROMETERS 

The  Brown  Thermo-Electric  Pyrometer  is  designed  for 
Indicating  or  Recording  temperatures  from  SOOT,  to 
3000°F.,  or  equivalent  Centigrade.  It  consists  of  a 
millivoltmeter,  with  scale  graduated  in  Fahrenheit  or 
Centigrade  degrees  and  a  Thermo-Couple,  suitably 
protected,  inserted  in  the  heat  and  wired  to  the 
instrument.  The  Thermo-Couple  formed  of  two 
units  of  different  alloys,  joined  at  the  end  and  in- 
serted in  the  heat,  generates  a  small  current  of  electricity 
dependent  on  the  heat,  and  this  is  indicated  or  recorded  by 
the  instrument. 

For  temperatures  below  300°F.,  Brown  Resistance  Ther- 
mometers are  recommended.  For  temperatures  above 
3000°F.  the  Brown  Radiation  Pyrometer  is  extensively  used. 


BROWN  RECORDING  PYROMETERS 

Make  a  permanent  record  of  temperatures.  Positive  in 
action,  sturdy  in  construction,  accurate  and  with  clear 
readings.  The  circular  type  gives  a  24-hour  record.  The 
continuous  types  give  an  unbroken  record  over  a  period  of 
approximately  two  months,  and  make  from  1  to  10  records 
on  one  chart. 

Brown  Pyrometers  are  also  made  to  regulate  or  control 
automatically  the  temperatures  of  electric,  gas  or  oil 
furnaces. 


Circular  Chart,  Brown 
Recording    Pyrometer 


The  Brown  Single  Record 

Continuous  Recording 

Pyrometer 


The  Brown  Precision 
Portable  Potentiometer 


BROWN  LONG  DISTANCE  INDICATING 

AND  RECORDING  THERMOMETERS 
Brown  Recording  Thermometers  have  a  bulb  of 
copper,  usually  about  10  inches  long,  connected  by  a 
capillary  tube  to  a  helical  expansive  tube  in  a  record- 
ing gauge.  The  bulb  is  filled  with  nitrogen  gas  under 
pressure  for  high  ranges  to  1000°F.  and  with  alcohol 
for  ranges  below  200°F.  In  accordance  with  the  law 
of  expansion  of  gases,  the  gas  expands  at  a  uniform 
ratio  with  increase  in  temperatures.  This  means  an 
evenly  divided  scale  throughout  the  range  of  the 
instrument. 

Brown  Long  Distance  Recording  Thermometers  may 
have  tubing -as  long  as   100  feet  if  required.     This 
tubing   is   protected   by   a    flexible   bronze   armored 
tubing  which  can  be  stepped  on  without  injury  antl 
serves  as  a  perfect  protection  to  the  inside  tube. 
Bulbs  are  supplied  to  meet  every  requirement. 
Also  made  in  continous  recording  type. 

BROWN  RECORDING  PRESSURE  GAUGES 

For  recording  all  ranges  of  vacuum  and  pressure  from 
a  few  ounces  of  water  to  5000  pounds,  Brown  Re- 
cording Pressure  Gauges  are  guaranteed  completely 
as  to  dependability  and  accuracy. 
Also  made  in  a  recording  type. 

BROWN  TIME  AND  OPERATION  RECORDERS 

Extensively  used  for  recording  the  time  of  operation  of 
machinery,  switches,  valves,  pumps,  and  for  recording 
the  reversals  of  glass  melting  tanks,  open  hearth  furn- 
aces and  annealing  furnaces,  also  for  recording  the 
time  of  starting  and  stopping  of  paper  machines  and 
other  devices. 

BROWN  TACHOMETERS 

Indicating  and  Recording  types  for  measuring  and  count- 
ing revolutions  per  minute.  The  Electric  type  records 
machine  operations  hundreds  of  feet  away.  The  Mercurial 
types  operate  by  the  unvarying  law  of  centrifugal  force. 

OTHER  SCIENTIFIC  INSTRUMENTS 
Resistance  Thermometers,  Thermometers  of  the  Mercurial 
type,  Draft  and  Vacuum  Gauges  are  among  the  other 
scientific  instruments  produced  in  the  Brown  Laboratory, 
Wayne  Junction,  Philadelphia,  where  visitors  are  always 
most  welcome. 


The  Brown  Dial  Indicating 
Thermometer 


The  Brown  Long  Distance 
Recording  Thermometer 


The  Brown  Recording 
Pressure  Gauge 


The  Brown 
2"  Draft  Gauge 


Brown  Electric  Tachometer 


The  Brown  Mercurial 
Tachometer 


The  Brown  Time  and 
Operation  Recorder 


ENGELHARD 

The  Original 

Ze]  Chatelier  Pyrometers 


WHY   ENGELHARD 

PYROMETERS   Excel  in 

Service :' 

For  twenty-five  years  we  have  specialized 
in     Rare     Metal     Pyrometer    equipment. 
Engelhard  Pyrometers  have,  during  all  of 
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care  taken  in  refining,  alloying  and  manu- 
facturing Engelhard  Thermo-elements,  the 
ample  factor  of  safety  allowed  in  the  size 
and  length  of f the   Thermo-element  wires, 
the  grade  and  types  of  protecting 
tubes  used,  the  accuracy  and  un- 
usual durability  of  our  indicators 
and  recorders. 

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Our  New  Model  Indicators  and 
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Because  we  are  not  dependent  on 
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are  in  position  to  supply  promptly, 
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We  guarantee  interchangeability  and  calibration  of  Englehard  Pyrometers 
within  Y\  of  1%.  As  to  the  service  they  give,  let  us  refer  you  to  some  of  those 
who  have  used  them  for  upwards  of  ten  years. 

The  exceptional  service  given  by  Engelhard  Pyrometers,  is  not  alone  due 
to  a  superiority  of  the  parts  entering  into  them,  but  in  the  selection  of  those 
parts  that  will  best  meet  the  needs  of  the  user.  A  Pyrometer  installation,  be 
it  for  one  furnace  or  for  many,  should  be  made  exactly  suitable  for  the  partic- 
ular conditions  under  which  it  is  to  be  used.  A  Protecting  Tube  that  would 
be  the  best  for  a  Cyanide  Bath,  would  not  be  best  for  a  galvanizing  pot;  nor 
would  one  that  should  be  used  in  a  Brick  Kiln  be  suitable  for  a  High  Speed 
Steel  Furnace.  We  offer  the  services  of  our  engineers  in  making  the  proper 
selection,  in  specifying  the  right  instruments,  and  in  designing  and  laying  out 
complete  installations. 

Our  facilities  for  supplying  complete  Pyrometer  installations  suitable  for 
the  individual  needs  of  the  user,  the  individual  attention  given  each  problem, 
as  well  as  the  constant  service  we  give  to  those  who  install  our  equipment, 
taken  into  consideration  with  the  superiority  of  Engelhard  Pyrometers,  should 
influence  those  who  might  otherwise  be  attracted  by  a  lower  price,  to  equip 
their  plant  with  instruments  in  which  confidence  can  be  placed. 

CHARLES  ENGELHARD 

30  Church  St. 

Hudson  Terminal  Building  NEW  YORK 


LEEDS  &  NORTHRUP  PYROMETERS 


Curve  Drawing 
Potentiometer  Pyrometer 


THE  POTENTIOMETER  SYSTEM 
OF  PYROMETRY  uses  a  balance  method, 
no  current  flowing  through  the  thermo- 
couple or  leads  at  the  moment  of  meas- 
urement, thus  eliminating  effects  of  length 
and  leads,  thermocouple  resistance  and 
galvanometer  resistance. 

THIS  permits  the  use  of  base  metal 
thermocouples  and  of  leads  of  the  same 
materials  as  the  thermocouples,  thus  bring- 
ing the  cold  junction  back  to  the  potentio- 
meter, where  its  effects  are  compensated  for 
automatically. 

POTENTIOMETER  INDICATORS  are 
balanced  by  hand  and  Potentiometer 
Recorders  are  balanced  automatically  by 
external  power,  the  galvanometer  acting  merely  as  a  current  detector.  The 
recorder  may  be  of  the  single-point  curve-drawing  type  or  of  the  multiple- 
point  printing  type  for  any  number  of  thermocouples  up  to  16. 

THE  CURVE-DRAWING  RECORDER  is  frequently  used  to  control 
signal  lamps  and  an  indicator  at  the  furnace,  showing  when  and  by  how 
many  degrees  the  temperature  has  departed  from  the  desired  temperature. 
This  instrument  is  also  arranged  to  regulate  the  temperature  automatically 
by  controlling  valves,  rheostats,  etc. 

THE  LEEDS  &  NORTHRUP  TRANSFORMATION  POINT  AP- 
PARATUS determines  the  correct  temperatures  for  working,  hardening, 
quenching,  annealing,  etc.,  by  locating  the  transformation  or  critical 
point.  A  differential  thermocouple  is  used  for  detecting  the  transformation 
point,  and  the  potentiometer  method  for  measuring  the  temperature  at 
which  the  transformation  occurs.  The  instrument  produces  a  diagram 
upon  which  the  points  are  indicated  in  a  pronounced  and  unmistakable 
manner. 

TEMPERATURES    of    in- 
candescent or  glowing  bodies 
are  read  with  great  accuracy 
by  inexperienced  operators  us- 
ing the  LEEDS  &   NORTH- 
RUP    OPTICAL      PYROM- 
ETER.    The     hot     body     is 
viewed  through  a  small  tele- 
scope in  which  the  filament  of  a  small 
tungsten  lamp  is  seen  against  a  back- 
ground formed  by  the  hot  object.     The 
current  through  the  lamp  is  regulated 
until  the  filament  merges  with  this  back-  Optical  Pyrometer 

ground,  and  the  temperature  is  then  determined  directly  from  the  reading 
of  a  milliammeter.  A  large  surface  to  sight  upon  is  not  required  and 
distance  from  the  hot  object  does  not  matter.  The  ability  to  match  colors, 
and  color  blindness  have  no  effect  upon  the  reading,  and  different  observers 
agree  within  3°C. 

THE  LEEDS  &  NORTHRUP  COMPANY 

Makers  of  Electric  Measuring  Instruments,  including  Indicating  and  Recording 

Thermocouple  and  Resistance  Pyrometers,  Condensers,  Galvanometers, 

Wheatstone  Bridges,  Testing  Sets,  etc. 


4920  STENTON  AVENUE 


PHILADELPHIA,  PENN. 


MONOPIVOT 

Pyrometer  Indicator 

Scale  7"  long 

Calibrated  for  base  or  rare 
metal  couples  or  double 
range  for  use  with  both. 
The  monopivot  construc- 
tion permits  very  high  re- 
sistance windings  without 
any  sacrifice  of  robustness, 
giving  sensitivity  without 
delicacy. 

The  fume-proof,  dust-proof 
switch  is  a  modern  essen- 
tial. 


WILSON-MAEULEN  COMPANY 

381   Concord  Ave.,  New  York 

Makers  of  Pyrometers — Specializing  on  High  Grade, 
Precision  Apparatus  only, 

Complete  Catalog  will  be  sent  on  request. 


The  TAPALOG 

A  very  high  resistance 
Pyrometer  Recorder,  with 
automatic  commutator, 
taking  up  to  six  records 
in  six  colors  on  one  record 
sheet.  Usable  with  base 
or  rare  metal  thermo- 
couples. 


UNIVERSITY  OF  CALIFORNIA  LIBRARY 
BERKELEY 

Return  to  desk  from  which  borrowed. 
This  book  is  DUE  on  the  last  date  stamped  below. 


LD  21-100m-9('47(A5702sl6)476 


4:17684 


UNIVERSITY  OF  CALIFORNIA  LIBRARY 


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V. . 


